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It's all in the gearing

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NCWayne

04-25-2008 15:07:15

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I keep reading posts talking about gas mileage being so much better at X MPH vs Y MPH and how they were running at X MPH when someone passed the 'wasting gas' going Y MPH. Well, that might very well be true if we all drove the same vehicles with the same engine and gearing, all with the same tires and amount of tread wear, etc etc etc. There are so many variables to factor in so in reality all you can do is speak for your own vehicle. My reasoning behind this statement you ask. Well 55 seems to be a well liked number for saving gas so I'll use it and my work truck for an example. If you are cruising down the interstate at 55 MPH and I come up behind you in my service truck (FL-106 Freightliner weighing 33,000+ full time) and I attempted to maintain 55 MPH behind you instead of passing, well it's gonna cost me. Why, because my truck has an Allison Automatic with double overdrive (ie 5th and 6th gear are both overdrive). At 55MPH, in 5th gear, the engine is turning in the neighborhood of 2000 RPM which is out of it's optimal power band. Now let me hit 60 and get into 6th gear and the RPM's drop back down to around 1600-1800, depending on the grade, which is right where she likes to be. Let me run around town on the interstates where you can barely reach cruising speed (ie 55)and I'm gonna get maybe 4 to 5 MPG but I can hit the intertstate running 60 to 70 and get 6 to 7. Those figures do include running a fuel additive which I found did in fact increase my mileage. Sad thing is before the ULSD came along I was getting nearly 9 MPG when I had to make a long run from home to fix a machine. Less power in the fuel, more fuel to get there, and more $$$$ for the fuel. Just doesn't add up to me, especially at $4.25 a gallon.

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Roy Suomi

04-27-2008 04:15:31

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Re: It's all in the gearing in reply to NCWayne, 04-25-2008 15:07:15

My F-600 service truck with boom , welder, loaded utiity body weighs in at 19,300 lbs..It originally came with 330 gas , 4 speed , single speed rear, 750-20 rubber..Yikes on the mileage..I installed a 5 speed trans , an auxiliary trans , 10.00-22.5 rubber, Mileage is up to 6 or 7 mpg..Now it falls on its face without creative shifting..

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jose bagge

04-26-2008 08:18:05

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Re: It's all in the gearing in reply to NCWayne, 04-25-2008 15:07:15

go with a shorter tire.....

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Goose

04-25-2008 19:34:37

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Re: It's all in the gearing in reply to NCWayne, 04-25-2008 15:07:15

How much does the octane boost additive cost now? When we were racing stock cars 20 years ago, we used an additive, I can't recall the name. The recommended mix was a pint to ten gallons of gasoline. At $8.00 for a pint can, it effectively increased our gasoline cost by 80 cents per gallon.

It was simply an operating expense at the time, but if someone used it all the time in a street vehicle, it could be a significant factor.

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buickanddeere

04-26-2008 11:54:41

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it's the laws of physics, drag squared It's all in the gear in reply to Goose, 04-25-2008 19:34:37

If the engine isn't suffering from detonation. Adding octane improver will decrease the mileage a slight amount. If the engine was suffering from detonation, then yes efficiency would improve when detonation was stopped. As for mileage at on speed or another. It's an irrefutable law of physics that air drag increases with a square of the speed. That is going twice as fast takes four times the energy. Going 3 times faster takes 9 times the energy. Most reciprocating engines are most efficient at the rpms where max torque can be developed. A few hundred rpms either side of peak on a gasser isn't going make beans for difference in efficiency.

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doug in illinois

04-25-2008 19:20:33

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Re: It's all in the gearing in reply to NCWayne, 04-25-2008 15:07:15

I have posted several times on MPG, and have spent the money for a Superchips tuner. It helps some, but even it can't compensate for a too low geared, meaning high speed, gearing. IF I can't get in to overdrive empty, which is about 63 MPH now, was about 68 before retuning, my MPG stinks. Going across Missouri on Hwy 136 if you try to stay speed limit legal, which I do, truck will shift down to second and work hard in the hills. My best MPG is around 75 MPH loaded or empty, I can go to overdrive then without the tranny hunting and trying to shift back down. Just can't run that fast on 136, and KILLS my MPG. DOUG

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trucker40

04-26-2008 07:50:46

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Re: It's all in the gearing in reply to doug in illinois, 04-25-2008 19:20:33

Two solutions come to mind,try and find a flatter road like highway 36 in Missouri,or find a way to get into overdrive at a lower speed.If you had a manual transmission that you could shift into overdrive whenever possible.Automatics arent really made so much for economy as for making it easier to drive something.I think I would rather shift my own gears if I could get better economy.

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wdTom

04-25-2008 19:10:04

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Re: It's all in the gearing in reply to NCWayne, 04-25-2008 15:07:15

Wind resistance goes up with the square of speed. Means that if you double the speed from say 30 to 60 you will have 4 times the wind resistance. It is mainly wind resistance that eats fuel on level ground. The faster you want to go the more power it takes to do it. That is is why you just can't change the gears in some vehicle and break the land speed record, that takes power, not just gears, power to pull the gears is required. I admit that some engines and gear combinations may be better suited to slightly different speeds, but in general the faster you want to go the more power/fuel you need. Try an expirment. Get on a bicycle and see how fast you can go. Why can't you go faster? You need more power to overcome the wind resistance, if this wasn't the case you could just shift into a higher gear and keep on going faster and faster. But a good bike rider has a tough time averageing more than 20 for long unless he is in a group to draft with. You just can't pull a higher gear without more power, and the faster you go the more power it takes. Facts.

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trucker40

04-25-2008 19:37:38

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Re: It's all in the gearing in reply to wdTom, 04-25-2008 19:10:04

More power is better as long as you dont give up something else to get it.If say you wanted to go from 500 horsepower nowdays to 1000 horsepower in the big rigs.Well if you had 2 motors with 500 horsepwer hooked together that would do it,however the extra weight would do you in over time.What you need is 1000 horsepower with the same motor,and less or the same amount of fuel,which is what companies that make big diesels have done,more power,less fuel,higher gears.At some point lower and higher it starts to loose fuel mileage.35 miles per hour and you are wasteing your time.You would be broke in 2 weeks and burn 2 times the fuel,100 mph and 330 gears might be the top of 1000 horsepower economy,105 mph and you start loosing economy.Good aerodynamics is good for 1/2 to 1 mpg,but it costs you more to replace your brakes more often.Everything has its limits,but the higher the gears you have power for,the better the fuel economy.Also the most power you can get without pushing the limits of a motor say with hot water,or the verge of blowing a head gasket,or anything that would ruin the motor,so that the motor performs better and makes more power,the higher the gears you can run.Even 2 speed axles.That would be ceramic coated pistons,double turbos,header exhaust,straight pipe and synthetic oil in all boxes among other things would all make a difference.

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RobMD

04-25-2008 19:09:25

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Sometimes... it is. in reply to NCWayne, 04-25-2008 15:07:15

Basically, Wind resistance increases as you try to go faster and faster. If you're going 65 and you want to go 80, you'll be using more gas because the truck is trying to push more and more air out of the way all at once.

60 seems to be a nice speed, **at least for me and my other family's cars and trucks** and people who go 75-80 really burn up that fuel, except for when it's all downhill.

My ford ranger 4 cylinder with 4.10 gears and a 5 speed manual gets 26 mpg hiway at 65. Anything above 65 and my gas mileage goes way down because i'd be running upwards of 2800 rpm's or so.

**IFFFFF... I had the same vehicle with 3.73 gears.... I CAN expect to run 70 mph and still get 25-26 mpg. HOWEVER... my torque at the wheels will be LOST with the higher gearset... therefore making my engine work HARDER AT LOWER SPEEDS... using more fuel as a result.

***OVERALL- I do NOT believe that there is ANY set formula for each vehicle to get good gas mileage. Road conditions, gross vehicle weight, number of engine cylinders, car or truck, auto or manual tranny, high gears or low gears, and even the person driving ALL change that variable gas mileage.

There is no real argument here other than that anything over 75 mph and drag-race-acceleration is just downright ridiculous for saving gas.

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Bill(Wis)

04-25-2008 18:38:23

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Re: It's all in the gearing in reply to NCWayne, 04-25-2008 15:07:15

We heard that same hogwash when the first fuel crisis occurred (1974). Nixon administration wanted 55 mph but truckers said their trucks were more efficient at 60 mph. So, Nixon backed down and decreed a national speed limit for cars at 55 mph and trucks at 60 mph because "of their special operating characteristics". Years later, some testing was done and guess what? Trucks were found to be most efficient at around 35mph. History does indeed repeat itself.

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Bill(Wis)

04-26-2008 05:18:44

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Re: It's all in the gearing in reply to Bill(Wis), 04-25-2008 18:38:23

I want to make a correction for the historical record. Nixon actually wanted a 50 mph national speed limit. The truckers told him that their trucks were more fuel efficient at 60 mph. He backed down and established a national speed limit for cars at 50 mph and trucks at 60 mph. It soon became obvious to almost everyone that that combination held enormous potential for disaster so they compromised on 55 mph for everyone.

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trucker40

04-25-2008 19:58:13

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Re: It's all in the gearing in reply to Bill(Wis), 04-25-2008 18:38:23

I can tell you from experience that a truck at 35 mph is way less efficient.You might find off road trucks that work good at 35 mph,but thats about it.Maybe an old 5 speed Mack,but I doubt it.You have to get a run at hills,especially if you dont have any horsepower.High horsepower trucks do better where you arent lugging or downshifting.Every little rise you come to at 35 you would be shifting.Trucks or nothing else does very good if you have to keep your foot to the floor all the time.You want enough power to climb hills with reserve.There are lots of lies about stuff like the 35 mph is better for trucks,obviously they have never driven a truck.
The quicker you can get to operating speed,and the longer you can keep it there without stopping and starting the better economy you have.A 13 speed transmission,in big hills will get better fuel mileage than a 10 speed because you can split gears and get ahead of a truck that has to drop full gears.Over a day that adds up to say 10 miles further down the road,maybe more.Gears,horsepower,drafting,and other things are way more involved than a certain speed.Every vehicle out there is not made to run 35,55,or 100 mph.

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dr sportster

04-25-2008 17:38:30

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Re: It's all in the gearing in reply to NCWayne, 04-25-2008 15:07:15

Would be kinda cool if cars came with a quick change rearend.Let me see Im driving to Florida let me put in the highway set.Unless it was a push button change nobody would want that.Ok nevermind stupid idea.If a motor is lugging too much it does kill the lower end.An editorial in todays paper stated the ethanol is turning out a bad idea and may cause a food shortage.

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Gene Davis (Ga)

04-26-2008 06:03:39

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Re: It's all in the gearing in reply to dr sportster, 04-25-2008 17:38:30

The Columbia rear axle sold for Fords in the 1930's-40's was a 2 speed rear end.

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old

04-25-2008 17:33:49

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Re: It's all in the gearing in reply to NCWayne, 04-25-2008 15:07:15

I agree but disagree also. If you have to small an engine and to high a gearing you loose MPG or if you have to big an engine for the gearing you loose again. Its all in matching things up right or you still loose MPG. But slower driving does save MPG because of wind drag etc etc etc.

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Bill(Wis)

04-25-2008 18:37:14

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Re: It's all in the gearing in reply to old, 04-25-2008 17:33:49

Also, if he keeps too much loose change in his pocket, he might lose his change.

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jdemaris

04-25-2008 17:14:36

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Re: It's all in the gearing in reply to NCWayne, 04-25-2008 15:07:15

I agree there are many variables - but with fuel mileage it isn't "all' in the gearing. For best efficiency - the vehicle needs to be set up to perform a certain targetted task. Maybe a certain load at a certain speed. No setup is going to be the best at all loads and speeds. That why some of the most modern and efficient farm tractors are dogs when used in low power situations. A modern 120 horsepower turbo-diesel tractor worked at 20 horsepower uses more fuel than a 25 horsepower old gas tractor that is more suited for the small job. From what you're saying - it sounds like some (maybe all) of the big-rigs are designed to be most efficient pulling a certain load at high speeds. That being said - any vehicle on the highway encounters more wind-resistance and rolling resistance and needs more energy to pull X amount of weight the faster it goes. The faster the speed - the more energy needed. If your truck gets the best mileage at those faster speeds - it could do even better at slower speeds IF it was set up to do so. I understand - it would not make sense to do so - since time is money.

The reason why I'm saying it is NOT all in the gearing is this. Components need to be matched for a certain job. If someone wants a pickup truck to be most efficient at a highway speed -then - the amount of power needed to maintain that speed needs to be caculated. Then - an engine chosen that will be running at the peak of it's torque and efficiency curve while making that required power. And whatever RPM that is occurring at- it needs to be geared so it matches. Some pickup trucks get worse fuel mileage with overdrive engaged if they are underpowered, fall out of their curve, and lug. And, some truck-conversions with tiny diesels stuck into then work so hard they wind up eating fuel like a bigger engine. And, in those cases - no amount of gearing can fix it.

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trucker40

04-25-2008 18:01:06

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Re: It's all in the gearing in reply to jdemaris, 04-25-2008 17:14:36

Well lets see,a 1979 International cabover with a 290 Cummins.Lightweight truck,gross weight was 73,280 in 1979.290 wasnt the biggest motor but it was common.Old cabovers were narrower by a few inches so should have a little less wind resisitance.No air cooler,old style motor,high flow water,high flow oil pressure,would get about 4 miles to the gallon on a good day.No computer.290 meant 290 horsepower.If you put an air cooler and 350 injectors and pump,turbo,pistons and liners,you could get 4.5 mpg,maybe 5 mpg on flat ground.
Jump up to mid 1990s.A N14 Cummins with 450-500 horsepower.It has an air cooler,air to air,computer,better tires,low flow cooling,more efficient oil cooler,low flow oil pressure,better heads,better sleeves,better pistons,overdrive transmission(which was available in 1979,but not seen a lot stock in cabovers with 10 speeds,newer 10 speeds have it right from the factory)and it gets 6.5 to 7 miles per gallon.A lot of the reason it gets better fuel mileage is that you can go up a hill,mountain,faster than an old 290,and the overdrive,plus the higher horse power lets you run 3.90 or even higher rear end gears.Just that right there is a lot of it,then trucks are more aerodynamic nowdays,plus the newer trucks are lighter than old ones,maybe not a cabover,but not much heavier,and todays trucks pull 80,000 lbs
Gears arent every bit of it,but a lot of it.Most vehicles,even tractors have a area that they pull,perform better at.Like that 100 horsepower tractor being used for a 20 horsepower one.You dont want to drive your Semi Tractor around town either,might not get 2 miles to the gallon.
Most vehicles have overdrive nowdays.If you can keep it in overdrive is as good as you can do.Driving at a slower speed out of overdrive,or pulling its guts out in overdrive,is going to waste fuel I am thinking.

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trucker40

04-25-2008 16:48:03

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Re: It's all in the gearing in reply to NCWayne, 04-25-2008 15:07:15

I know you are right.Its all in the gears.The more you can keep something in overdrive,which most transmissions are nowdays,even manual big rigs,the better fuel mileage you get,and the more you downshift for hills,or for 55 mph 4 wheelers you cant get around,the lower your fuel mileage gets.ULSD seems like it doesnt get as good of mileage,like a 1/2 mile to 1 mile per gallon less in my truck.
Another way to look at it is like this,if the speed limit is 70mph and you drive 55 mph,in an hour drive at 70 mph you will drive for 1 hour and 15 minutes at 55,which means you will run your engine for 25% more minutes,or 15 more minutes,and that will be in direct maybe instead of overdrive,so it could cost you money depending on your gears.Tires,air pressure,front end and total alignment,rolling resistance of your tires,how many hills you pull,the wind,all have lots to do with fuel mileage,even the temperature,if its raining,or ice on the road,octane of the gas,and I like to find a truck thats boogieing if I am on the Interstate and follow them back a ways,just enough where it pulls you along,that helps a lot on a long trip even if I am in another truck.Drafting works better if the conditions are right for it than anything else other than Georgia Overdrive which is kick it out of gear going down hill.Thats a dangerous and illegal thing to do in case you didnt know in a big truck.So is following too close.If you try to draft and get too close it hurts your mileage too. If it was better to drive slow to save fuel,in a vehicle thats built to say run 65,and you drive 55 instead,you will hurt your mileage.The whole drive 55 thing was a scam.For the most part nobody drove 55 anyway,and wont now either.Lots of people dont care what fuel costs,they have places to go,and things to do,and no time to waste. About the only way you are going to save any fuel by going slow is coast down every hill you come to and not push very much on the gas going up the other side,or higher gears,or an overdrive trans mission.You cant drive that way on the Interstate without attracting the cops and making everybody else mad.The way things are nowdays,and getting worse daily,you could get into a situation you dont want in because you think its smart to drive slow,which for the most part is not true.Your best bet is to go with the flow,draft if you can,coast if its not going to cause a wreck.If you dont like your gas mileage,look for something that gets better mileage,dont cause road rage or a possible wreck because you think it helps to run 50 on the Interstate.Ive seen plenty of wrecks caused by people running too slow.Its not a pretty sight when a car or small truck is wadded up underneath of a semi.

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trucker40

04-25-2008 17:21:29

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Re: It's all in the gearing in reply to trucker40, 04-25-2008 16:48:03

On the additive,lets say it costs 8 dollars to treat 100 gallons of fuel.Lets say I can go 50 miles further with my truck with the additive.100 gallons equals 600 miles without additive,650 with additive,50 miles divided by 6 miles to the gallon is about 8 gallons of diesel at 4.00 a gallon is 32 dollars minus 8 dollars equals 24 dollars saved.Additive costs different prices,and conditions help or make worse the amount of miles you can go.I think it could save 10 dollars on 100 gallons most of the time.For me its having the 8 or 10 dollars to buy it some days,or being able to find it if you run out.
One time I was having a day where everything was going good for me,Got unloaded,went about 5 miles got reloaded,just kept driving,looking for low priced fuel,poured the additive in while I was getting loaded,and ended up driving 1610 miles on 200 gallons of fuel.Wasnt much left in the tanks but thats 8 miles to the gallon and truck averaged 6.5.Wind seemed like it was helping me,and the additive worked good that time.Never found any cheaper fuel until I got to Missouri,1610 miles from where I filled up before.Even different kinds of fuel get different mileages.Your mileage may vary is right.I seem to not get as good of mileage with Phillips 66 as with about anybody elses.I also like to drive at night,less traffic,cool air,and if the weathers good you can go a long ways.If you just drive in the daylight,more traffic,more wrecks,more road construction,just plain more stuff in your way.Every time you slow down you burn fuel to speed back up.

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TomH in PA

04-25-2008 16:18:14

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Re: It's all in the gearing in reply to NCWayne, 04-25-2008 15:07:15

I don't doubt what you say. But I do have to observations:

1) As you say, individual fuel economy depends on the vehicle; but the goal with reduced speed limits is both safety and to optimize the average economy across all vehicles (although I have no clue what that speed is, especially after all the claims that daylight savings time saves energy were proven false).

2) When you calculate fuel economy with the additive, are you accounting for the additional cost/fuel you are adding when you pour the additive into the tank? In other words, would you go farther if you just spent that money on more fuel?

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