Mileage question

I got thinking about this yesterday when I was raking hay and going a pretty good speed with the wheel rake.

With a car or truck, you get better mileage by driving a little slower because of wind resistance and lower RPMs don't you? Is there a point when you go so slow that mileage decreases?

For example, if you drive 50 miles at 50 miles per hour, you're driving that distance in an hour. If you drive 50 miles at 20 miles per hour, it will take two and a half hours to drive that distance. Wouldn't you burn more gas because the engine is running that much longer?

If that's the case, what's the point where mileage decreases by driving slower?

Using the car's computer, I get best overall mpg around 60 mph.
I get better mpg my not using brakes unless necessary.
That means look ahead, if light is red, get off gas and coast.
Coasting improves overall mpg.

My best 25 mile average mpg at 60 mph is 37. Very rare to get 37.
At 75 mph I'm lucky to get 27-28 mpg.
Car's computer will best answer your question.

Kubota's computer says I average 0.6 gallons per hour mowing grass.

Engines and automobiles have a certain speed where they are most efficient, probably somewhere between 20 and 50. I have heard that the effect of wind resistance is squared as you go faster, that's why driving 80 gives most vehicles poor mileage. I know those that like to speed will dispute this!

Interesting question. I guess the answer would be comparing miles per gallon vs. gallons per hour. At some point there would be a sweet spot where you would get the most efficient use of fuel . Just where that sweet spot is I dont know. I do remember my dad claiming he got better gas mileage running maybe 60 mph than 65-70 mph on highway driving. This would have been several years ago in a 4 cyl. Toyota Camry. YMMV lol.

My E-co boost ford f150 the sweet spot is around 55/58 miles per hr. The loss curve is pretty consistent on up to 67 or so then anything after that it los in mileage shows up fast.

Gas or diesel? Diesel is usually more efficient at partial loads than gasoline; because diesel engines take in a full charge of air regardless of load.

The highest mileage should be where accessory load is lowest (lower rpms) because 5HP accessory load over time offsets the gain of low air resistance.

Good question though.

A little off topic, but in 1915, the British government did a mileage test on a Model T Ford.
It got 25 miles to the gallon.
Of course it was very light in weight, but still, I think we could have done better in 106 years.
Richard in NW SC and owner of a 104 year old Model T

With respect to internal combustion or any type of heat engine, the maximum work (highest efficiency) is achieved when all strokes occur under reversible conditions under which each step of the stroke takes place with external pressure and internal pressure equal during each step of the process. This is a requirement that also minimizes the power output (energy per unit time) but does allow one to calculate the maximum efficiency for an engine of any design. So as rpm decreases the engine efficiency (work out divided by energy consumed) will approach the theoretical reversible limit at the sacrifice of power. This discussion is not much use in the hayfield, haha. But I'd project that going at lowest rpm that allows for enough power to sufficiently work your rake should be most efficient but make for a long day.

For semi trucks wind resistance starts to become an issue at 50mph. After 50 it takes double the hp for every ten mph approximately. Cummins used to have a graph showing the hp requirements for a truck, don't know if you can google it or not.

I was in a class in college during the original energy crisis in the 70's where we checked the mileage at 35, 50, and 70 mph. I used my 1970 Ford F250 two wheel drive with a 360 cid and four speed transmission. At 70 mph I got 12.25 mpg. At 50 mph it tested at 17 mpg. At 35 mph it jumped to 22 mpg. Back then it was generally accepted that the best mileage was at 35 mph. That could have changed as body styles and drivetrains have changed.

My Dodge pickup gets the best mileage when I go 60 mph on cruise control. It drops from 8 to 4 cylinders. And without me sticking my foot on the gas pedal, it goes around 28 mpg

yes. we could have improved on that milage, however, 'Big Oil' didn't want that !!!

I remember reading an article that addressed this years ago. It was in a trucking magazine, and talked about momentum, tires heating up, and many things pertaining to trucks. They said the most efficient speed was around 40 MPH.

As a rule of thumb, the most efficient speed is that at which the engine turns it's lowest rpm in the highest gear. Most trucks with an automatic trans will downshift or unlock the torque converter as speed drops below about 35 to 40 mph. But then, engine design will have a factor in this, as in rpms at peak torque etc. Your son who builds engines, Randy, could educate all of us on that.

Ben

He's all about burning up alcohol and going real fast. LOL

Ive heard several people say their newer outfits get better mileage going a little faster than slower

What i have found over the years is finding the sweet spot of said engine . NOT ALL ARE THE SAME . Case in point here . People always said that the 460 sucked gas . Well i Bought a well used and abused 88 Ford F 350 4X4 cab and chassis , five speed , 4.10 gear and ex Oil Field refugee that had ISSUES big time . Being and ex master Ford tech and the mind se that i can FIX it . Got it all fixed up built a flat bed for it and ended up with a truck that weighed in at 7500 on the button , a rolling INGOT . I know all the tricks to make a 460 run and shell we say i enhanced it . I used this truck to drag my 28 foot goose neck to all the sales i went to not all the time as sometimes my buying habits exceeded my hauling capability . I kept track of mileage and fuel empty truck i could get 14 out of it empty trailer 12.5 13 full load 10-11 . she held right around 39 gallon of gas with dual tanks . Lets say i was just going to tri greens sale and i ran 55-60 out on the open road once i got to the four lane and i had basic non stop driving other then two traffic light on the four lane i had stop and go from the house for about 32 miles then once i got off the four lane till i got to the sale i had Delaware and Plain City to run thru . It would take almost one tank of gas for the 187.4 miles between my drive way to Tri Greens gate i had maybe two or three gallons left . Now say i was going to Stilesville In. that was 389.7 miles . On that ride i would run the same way as going to Green's but then jump on I70 and head west , usually about a bit west of the farm sincense review i would switch tanks , at Mt. Comfort IN. i would stop and fill up and this way i could run to the sale then make it back to London O. and refuel there for the ride home . One morning i over slept as i was hammered and shot out of bed no time to run th 7 miles out to the shop and get the trailer got to Go , That day we were NOT running the speed limits I knew that if i ran out 62 and thru Alliance that at that time of the morning only one traffic light was not blinking and little to no traffic the 460 was hauling mail . Out to I && down to US 30 west bound hammer down out to 71 down to US 42 down past Greens on to I 70 and i am flat up and flyen left lane just ah getting it still half asleep not thinking that i had not flipped the switch to switch tanks . I was half way across In. almost to Mt. Comfort when i looked down at the gas gauge and she was bumping empty and i happened to look at the switch and it was still on the ft tank , i thought this can't be right i must have been running on the back tank so i flipped the switch and the gauge went back up to full So i flipped back to ft and kept on going i saw a Bronco II up ahead of me and blew past it like it was parked and it was Dick Green that is who i thought is was when i saw it as we were coming into the east side of indy. as i was going past the airport on the west side the 460 coughed and sputtered as the ft. tank ran out of gas and i made the switch . That day is when i figured out how to run a 460 and get super gas milage out of it IF and again i say if i could keep it Buzzen above 3200 Rpm and up to 3800 it used less . While pulling the trailer empty or loaded i never ran fifth gear and the engine was always working in that 32-3800 rpm range , that put it between the peak torque range and peak horse power of the cam . One of the things i did with that engine was set the cam back to PRE 1971 cam specs as the cam was the same cam as when they first came out just the timing gears were changed and a new Cloyes double roller set took care of that along with a computer out of a 92 460 five speed . With that truck it was the faster ya drove it the BETTER the mileage , loafing along in fifth at 55-60 it was struggling , but when ya had the trailer on and you were running 32- 3400 it ran somuch better and my mileage went up . Only time it really sucked was the night i drug home a Gleaner K combine with both heads and made the mistake of Pulling the combine on forward . From Plymouth in to Ft. Wayne east side it sucked 37.5 gallon and would not go over 55 flat on the floor . At the little fuel stop i filled up and was setting there thinking on where i was going to have to stop for gas as at night out that way gas stops are few and FAR between and i really hate walking . So i backed it off and then backed it on . WOW what a difference it made , the long pull out of Ft Wane to Van Wert i did not know it was back there and made it home with 3/4 of a tank left. and once east of I 71 you get into HILL country with the first big one about two miles east of the I 71 Us 30 interchange that will even suck a big horse down and i have five more to the house .I ran that truck just shy of 300000 miles on the same engine with one clutch and one rearend .

Interesting topic. I usually run at 70-80 mph on the highway.! Getting 18.2 miles per gallon in my Chevy van. Not great buts its a brick in shape. My car does better I think. Dont really worry about that to much.

Vito

There are several factors, wind being the biggest.

But the engine design and gearing are also critical.

Modern engines use every available technology to get the best mileage under the widest range of conditions. Variable valve timing is one advantage, cylinder canceling is another. Fuel mixture is the big one.

Generally engineers shoot for max efficiency at around 60 MPH, because that is the most practical highway speed.

So varying from that speed will change the mileage results. Going slower will reduce wind drag, but the engine will not be running in it's optimum power band. Going a little faster will put the engine in a more efficient RPM range, but the wind drag will increase. Going a lot faster will put the engine beyond it's best RPM range, wasting energy trying to breathe, and huge increases in wind resistance!

Lower engine RPM also does not equal better mileage. Correct final gear ratio is essential for optimal efficiency. Problem is loads and conditions vary greatly. Gear it too tall, it will take more throttle opening. Gear it too low, still more throttle opening to feed the wasted effort. Just like riding a bicycle, it has to be right to be comfortable.

Personal experience and strictly anecdotal evidence.

My normal mileage for my normal driving - mostly around town is usually around 14 mpg. That is about what I expect for the type of driving. Lots of start and stop.

A bit ago, I took a more extended trip to visit my sister. Mostly turnpike driving with 70 mph speed limits, traffic moving at more like 85. At the end of the trip, I figured my mileage mostly for curiosity. It came out to about 18 mpg. I was both surprised and thrilled. My vehicle has never been easy on fuel.

The vehicles I have owned over the last 15 years seem to get the best mileage between 60 and 80 mph. Anything under 55 and the mileage gauge starts dropping a tenth or two every 20 minutes or so.

If I remember correctly, when Nixon dropped the speed limit to 55 it was stated that 50 would increase even Moore but would be intolerable. I thought 55 was.
Andy

Drove 400 miles on ice with my semi at 45 mph. Thought I would get great mileage...worse than normal speed of 65-70 mph.

Yes and no... The most efficient rpms for an engine will always at or below maximum torque. So... gearing the car to go as fast as possible with the engine close to max torque figure will generally get you there. And then factor in bearing drag, wind drag, and you seem to find that 40 to 60 mph are the sweet spots with the overdrive transmissions also based on gearing. Poor gearing can lower the sweet spot to 42 mph on a 2004 chevy impala as tested. engineers dont like to run the vehicle at its max efficiency because, the lest little hill or wind gust will slow the car down dramatically, so they tend to design it back down a bit for a fair reserve. When od came out, it immediately added around 3 mpg to most all vehicles, and extended engine life by 50%. Further aerodynamic designs, less weight, more gears to keep rpms down, electronic high voltage ignitions, computer controlled timing and mixture control, have allowed even another 6 mpg in cars. Once you exceed max torque, your trading rpm for power, and you loose mpg's exponentially. see https://www.mpgforspeed.com/
This uses wind drag only as a factor. With wind drag, if you double the hp, you will increase top speed by only 25% but increase fuel consumption by 125%... old rule of thumb. remember the fast the engine turns, the more power is lost of stopping the pistons going down, and then sending them back up. Where a rotary engine does not suffer this problem as much. Cam chains also suffer from the change of direction losses.

Every vehicle I have ever owned, the MPG sweet spot is 45-55mph. A big study in the 80's found that that is the point where wind drag becomes a bigger factor than powertrain drag. The key for each vehicle ahs been to find the lowest speed at which I was in the top gear in that speed range. 87 6.2 diesel, 54mph kept it in 4th with the converter locked up. 2014 Equinox, 48mph unless it's hilly, then 57. 1982 3/4 ton GMC, it was 48mph. Dad never got better than 14mpg with it and I could make it get 17mpg.

Biggest key is to 'drive it like there's an egg under the pedal'. And not be in a hurry.

At 70 the Equinox gets ~30MPG, at 48, it gets over 35mpg.

Is that Imperial Gallons Bruce? Either way that's pretty good mileage for a full size truck.