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14.9 vs 15.5 Tires

B

Bryce Frazier

Well-known Member
Drove my Super M home 22 miles today. Took exactly one hour, and the guy following me said I was running perfectly at 21 mph, up hill, down hill, etc.

It has 14.9's on it right now, but they are trashed. I have 15.5's to put on it, but was wondering what they would do to the road speed? 15.5's were invented to be the same height profile as the 13.6's right? So, am I correct that the 14.9's might actually be a TINY bit taller than the 15.5's?

Bryce
 
(quoted from post at 19:19:52 02/15/16) I believe the 14.9 x 38 tractor tires are about 3 inches larger in diameter as compared to the 15.5 x 38 tires.
Click to expand...

Close to that from what I have measured.
 
They will slow you dowm a bit.But not enough to notice.You're farming,not raceing! lol. BTW,I have 13.6x38 on both my M/SMs. Plenty fast.
 
Bryce,

Time to put your math skills to work.

circumference will be pi times the diameter, so the difference in speed will be the ratio of the sizes.

Measure the height of the 15.5, divide by height of 14.9, multiply by your measures speed, result will be your new speed.
 
Bryce a 14.9x38 is 64 inches tall, A 15.5x38 is 61.6 inches tall. So your 15.5 tires will be 2.4 inches shorter or 1.2 inches on the radius.

Here is a tire size chart for you to save.
a215644.jpg
 
We found it depends on the brand. The Firestone's we sold were shorter than any other brand 15.5 x38's we sold. I think they were far better than the other brands. I also sold, Goodyear, Titan, Kelly Springfield made, and a lot of forign made ones. Kelly and Goodyear long bar-short bar were taller than the others, but the long bar short bars wore terrible on the road. As I was getting out of the business 10 years ago, Goodyear came out with a long bar,long bar 15.5x38 that was taller than the long bar short bar version, unfortunately I cannot remember the name of them now. I don't see where you listed the name of the 15.5's. What I am trying to say is--You may have to run a tape around them and do your own calculations.
 
Ya want road speed then try 16.9x38 and if that is not fast enough then go to the 18.4x38 . But trust me the 16.9's will be fast enough for a chin strap for your hat.
 
Don't sacrifice lugging power for speed. Also get too tall and you will be bumping that head on bridges as tall as you are. LOL
Richard in NW SC
 
Awesome! Thanks JD! I saved it on my computer, will probably print it out and put it in my shop binder as well....
 
Ya, I figured that they would all be a little different.

Right now I have 1 Firestone 15.5, and one Titan I think? I don't remember. They are identical tread design, 23 Degree, Long Bar, Long Bars! ;)
 
If the 14.9 measure 29.4 inches from axle center to ground and speedometer is correct. RPM is jacked up on the engine, would be turning close to 1770 RPM at 21 MPH if I figured correct.
Tire brands mater, had one 14.9 brand shorter than another brands 15.5 before.
 
Bryce, Many years ago I backed my 39 JD A in a barn that had a shed on the front. Was a very close fit.
Went back to get it one day and one of the posts on the shed had sunk a little for some reason.
Had to braze the air intake back on. The straight pipe on the exhaust was just short enough to clear.
Richard
 
Ach! Ya! So dat's why I have to set the brakes vhen I park on a level surface! I always vondered about dat. (;>))
 
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