tire size / speedometer gear question

[Blue88Comanche]

so for the longest time ive been using the proper speedomenter gear (according to the charts) for a 31" tire with a 4.10 diff ratio.  but my speedometer is still off compared to several GPS apps.  According to General Tire my Grabber AT2 tires are 30.8" tall, but when i measure them on the jeep the front is 29" and the rear 29.5" I also don't know if using full time 4wd affects the speedometer.

 

So my question is after checking my tire pressure again when i get home.  should i use a gear that matches the manufacturer tire size or actual installed tire height?

[FrankTheDog]

To be even more accurate, measure from the ground to the center of the axle and multiply it by 2.

That will give you your rolling diameter which will be less than your overall height.

[Marine1Texas]

Being in 4 wheel drive has no effect on it. GPS apps on 99% of all phones can measure off 2 to 5 mph. I have a GPS odometer that I used before I got my new gearing for 4.10 on 31's. The GPS always reads 1 to 2 mph faster then the speedo. 1 inch will not throw off your speed that much, only if your going above 65 you would see any difference. I use 37 tooth on mine. The previous one was a 39 I noticed only above 45 the speedo was off by 5 about's. My tires measure at 30.8 so the 37 tooth is very close. The front tires have more weight on them so it will be a tad bit spread out on the bottom near the ground loosing some height.

 

How long will it be till you change tires? it would suck to change tires in  say a year or less and the new ones are closer to 31 then the ones you have. that my 2 cents is all!

[Blue88Comanche]

Being in 4 wheel drive has no effect on it. GPS apps on 99% of all phones can measure off 2 to 5 mph. I have a GPS odometer that I used before I got my new gearing for 4.10 on 31's. The GPS always reads 1 to 2 mph faster then the speedo. 1 inch will not throw off your speed that much, only if your going above 65 you would see any difference. I use 37 tooth on mine. The previous one was a 39 I noticed only above 45 the speedo was off by 5 about's. My tires measure at 30.8 so the 37 tooth is very close. The front tires have more weight on them so it will be a tad bit spread out on the bottom near the ground loosing some height.

 

How long will it be till you change tires? it would suck to change tires in say a year or less and the new ones are closer to 31 then the ones you have. that my 2 cents is all!

I have no idea when I'll be changing tires. I'd like to cause I want some that are taller, but they still have a good amount of tread left. But reading 50 on the dash puts me at 52-53 actual MPH, while that's not such a big issue for driving, but 4-6% going to play a bigger role on the odometer accuracy over time, that's what I'm more concerned about.

 

 

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[Hippie66]

Speedometer only has to be within 5% on a brand new car.

[Hippie66]

Scratch that... 10%. Must be within 5 mph at 50.

[Eagle]

 

I have no idea when I'll be changing tires. I'd like to cause I want some that are taller, but they still have a good amount of tread left. But reading 50 on the dash puts me at 52-53 actual MPH, while that's not such a big issue for driving, but 4-6% going to play a bigger role on the odometer accuracy over time, that's what I'm more concerned about.

 

 

 

Find some way to check the accuracy of your odometer before you do anything. It's not uncommon for manufacturers to set speedometers to read a couple of MPH faster than the actual speed, even if the odometer is correct. I have a 2000 Cherokee and I also have my late wife's 2000 Cherokee. Both are bone stock, and the speedometer on both reads 2 to 3 MPH fast when going past th0ose police radar trailers that display your speed as you approach.

 

FWIW, I've looked at a lot of tire specs over the years, and all the 31-inch tires I've looked at are actually between 30.4 and 30.6 inches in diameter. If you want taller than that, you'll have to get 32s -- which will probably be around 31.5 inches.

[Blue88Comanche]

 

I have no idea when I'll be changing tires. I'd like to cause I want some that are taller, but they still have a good amount of tread left. But reading 50 on the dash puts me at 52-53 actual MPH, while that's not such a big issue for driving, but 4-6% going to play a bigger role on the odometer accuracy over time, that's what I'm more concerned about.

 

 

 

Find some way to check the accuracy of your odometer before you do anything. It's not uncommon for manufacturers to set speedometers to read a couple of MPH faster than the actual speed, even if the odometer is correct. I have a 2000 Cherokee and I also have my late wife's 2000 Cherokee. Both are bone stock, and the speedometer on both reads 2 to 3 MPH fast when going past th0ose police radar trailers that display your speed as you approach.

 

FWIW, I've looked at a lot of tire specs over the years, and all the 31-inch tires I've looked at are actually between 30.4 and 30.6 inches in diameter. If you want taller than that, you'll have to get 32s -- which will probably be around 31.5 inches.

Using GPS I can start to see a deviation in the odometer after about 1-2 miles.

 

But when sizing the tire to the speedometer gear, do I use the side wall diameter or the rolling diameter of the tire.

 

 

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[1989 Eliminator]

To be even more accurate, measure from the ground to the center of the axle and multiply it by 2.

That will give you your rolling diameter which will be less than your overall height.

 

This^

Tires deform due to the weight they hold. To more accurately find out your vehicle speed you need to measure your dynamic rolling radius.

 

Then find out your current RPM, the gear you are currently in (if you are in OD then use that...), and the differential gear ratio.

MULTIPLY... (RPM * 2 * 3.14 * 60 * [measured tire RADIUS in inches]) to get your inches per hour.

Then DIVIDE that answer by the transmission gear ratio, the differential gear ratio and 63360 to convert inches to miles.

 

(RPM * 2 * 3.14 * 60 * [measured tire RADIUS in inches]) / (Transmission Gear Ratio * Differential Gear Ratio * 63360) = MPH

 

The final answer is your actual vehicle speed in miles per hour. I found that my GPS was pretty accurate to my actual vehicle speed since I also have bigger tires on my truck.

[Eagle]

 

Using GPS I can start to see a deviation in the odometer after about 1-2 miles.

 

 

 

Maybe, but I doubt you're really seeing an accurate check on your vehicle's odometer. Your vehicle's odometer records how far the vehicle actually travels -- a GPS only knows where the vehicle is relative to a set of satellites. Say you had a three-lane highway all to yourself for 100 miles. If you get in the center lane and drive, you'll go 100 miles. If you start in the right lane and drive the full distance weaving back and forth from the right lane to the left, you'll tavel more than 100 miles -- but a GPS will only know where you started and where you ended, so it'll show the same distance traveled. A GPS is okay (but not great) for checking a speedometer, but I don't think it's at all useful for verifying an odometer.

[Minuit]

I'm running 235/75/15s with 3.55s and a 34 tooth green speedo gear. Those speed trailers the cops put up every now and then always seem like they read a little lower (significantly, as in about 20%) than my indicated speed. On the highway, the odometer is dead on with mile markers at all speeds.

 

I agree with Eagle - use something more precise than a GPS to see. Highway mile markers across about 10 miles would be my suggestion - that would also give you a rough idea of the percentage that your speedometer is off by. Nothing is absolutely perfect and you can't necessarily trust highway mile markers to be exactly a mile apart, but basing your decision off of just a GPS will probably just make you chase your tail.