I have 285/70R17 Goodyear Duratracs and they measured 32 from ground to top of tire. I put in the right number for the aev procal put the RPM's still seem a little high. What has anyone else put it in for these tires? Thanks.

 

you need to check your actual speed using a GPS and compare to your speedo. If you don't have a GPS there are several speedometer apps for smart phones.

 

Have you checked speedo with gps. If speedo is reading right rpms should be correct

 

Ha great minds think alike and apparently right at same time

 

Sorry guys, rpms are not adjustable...they are read directly from the engine and do NOT depend on tire size. Now, if you are talking about mph, then I see what you are talking about. The very best way to determine the rolling
diameter of the tire is to put a chalk mark on the face of the tire, drive it until you have two chalk marks on the driveway or such, then measure this length and divide by pi (3.421) that willl give you the rolling diameter of the tire which determines speed calculation....another way is to measure from the center of the wheel to the flat surface the wheel is sitting on and doubling the measurement.... Lesson learned, a 35 in tire is NOT 35 inches in diameter...

and, yes, I use a GPS to set mine right on.

 

I figured he meant his rpm was wrong for indicated speed
Pi is not 3.4...

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X2. And to say tire size has nothing to do with rpm is true and false. Final drive ratio doesn't change there for a 32 inch tire will cause the engine to spin faster at lets say 70 mph than a 35 will. I know with my speedo calibrated and 32 to a 35 I spin about 280 rpm less

 

If your RPMS are too high make sure the OD on the auto trans is on. If its a stick and your in 6 th and its still too high then you need to re-gear to a lower ratio... If you want to go fast and have the RPMS low your going to have to go Up ie... 456 gears to 373 will help jeep the RPMS low at highway speeds. But if you do that your going to kill your low speed performance like stop and go as well as using it to negotiate obstacles.... Like ditches rocks, logs, and hills your going to be at a serious disadvantage.

It's a give and take your going to have to give up something weather it be low speed performance or high speed you can't have it both ways.... Sorry Jeeps are not like Burger King you can't have it your way.

 

I just had 35" Nitto trail grapplers installed from 33" GY MTR with Kevlars and I am still having the same issue as the 33's. I can get my speedo match my GPS no mater which way I go. I drive using my GPS for speedo. Lowered and raised the tire size a 1/4" either way and its always off. And I also use my Procal to adjust. 2011 JK Sport 6 speed manual.

 

Pi = 3.142, not 3.421