I just swtiched to 35's actual 315 70 17 which measure 34.69 say 34.75 for pro cal use. when I program the the new tire size in my speedometer is still 9 miles per off at 60 mph. so when my speedometer reads 60 I am actually doing 69. I have checked this 3 ways 2 with gps speedometers on my i phone and also outhouse signs with radar that always tell me to slow down. I can't figure out what to do. should I reset the speedo with a larger tire size that would match the 9 mph error. or is there something else I am missing .

 

Well, first off, you have to think about the rolling radius since the tire compresses at the ground from the weight of the Jeep. I once had those tires and the procal setting that worked for me was 33.75" (an inch less than what you have). You may even need 33.5". I can't recall, it was right between those I think.

Second, make sure you are setting the procal right. To this day I sometimes get it backwards in terms of the picture and switch direction.

 

I put a spot of paint on the tire back the jeep up until the spot hits twice then measured it comes out to 108 inches, which when divided by pi 3.1416 is 34.69 so tried the pro cal at 34.75 and at 34.5 both are off by a factor of 10 % easy.

 

Don't know what to tell you, man. I guess you can wait until 10 more people tell you that you generally go an inch less when adjusting tire size. I assure you it is not the procal, but the user.

 

Are u taking into account the gear ratio along with tire size? U may need to confirm ur gears. Dont trust the window sticker-a few people have found that to be incorrect.

 

x2 my Toyo 35x12.5-17 are set at 33.25" my KO2 34x10.5-17 at 32" and my speedo and odo are spot on.

 

Yep, just gotta make it work. Shoot, I just went to 5.38 gears, 40s, and my tone rings have a different tooth count. Shockingly, I'm only off by 5 mph, haha. I'll have to play with the procal one of these days.

 

Or don't bother measuring. Do it the hard way, just plug in numbers and check the speed until you get the speedo and gps to match.


When I first bought the procal way back when, I didn't want to wait until I got home to measure the tires. I guessed at the height, drove it, stopped and adjusted the number then checked again. Adjust, check, adjust, check, etc. Finally got them to match, about an inch and a quarter shorter than where I started...

 

I am running 315/70/17 with the procal and my speedo is spot on with my iphone speedo app. I took a 3 foot level and set it on top of the tires and measure from the bottom of the level to the ground and my tires measure 33.75". Also I am running about 29 PSI. When the dealership installed everything they air the tires to 40 PSI and the tires measured about 34.25", which was way to much PSI.