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2nd Gen Non-Engine/Transmission How do I calculate the right tire size so my odometer is correct?

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Engine/Transmission (1998.5 - 2002) 4X4 Vacuum lines .

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[I posted this in a 2nd-Gen forum, and thought it would make a nice entry in the FAQ. Please double-check my math, and delete this editorial note. ]



This is for folks who have a means of programming their tire size in order to correct their odometer. The more mathematically astute can apply the ratios to correct turns-per-mile and other correction methods.



Your best best is to drive 100 miles on the highway, noting the mile markers at the start and end, and your odometer at the start and end (including tenths). The resulting ratio will let you compute the exact tire size to program. This works best on long-ish road trips.



For instance, suppose you have programmed in 33" tires. Drive 100 miles on the highway, as measured by the mile markers.



If your odometer shows you've travelled 105. 8 miles, the ratio is 105. 8/100, or 1. 058. The corrected tire diameter to program in is then 33*1. 058, or 34. 9". Program this in, then drive back, measuring again. Your odometer should be almost dead-nuts on.



If your odometer shows you've travelled 96. 3 miles, the ratio is 96. 3/100, or 0. 963. The corrected tire diameter to program in is then 33*0. 963, or 31. 8". Program this in, then drive back, measuring again. Your odometer should be almost dead-nuts on.



I did this with my TruSpeed; my odometer has never been off by more than 1-2% since. I occasionally adjust it to account for the loss of tread as the tires wear. Yes, I can be anal about things once in a while, and it helps to keep the math fresh.



Once you have your odometer recording correctly, your fuel economy calculations will more closely reflect reality.
 
Another option, which is more cost effective is to go to:

http://www.onlineconversion.com/tire_size_conversion.htm

This will give you the diameter. Multiply this by Pi, or 3. 1415, which will ultimately give you the radius of the tire in inches. Divide this by 12, which will give you the radius in feet. Divide 5280 by this figure and gives you revolutions per mile. You can do this with any tire, which will give you an accurate enough computation and save you the cost of going out and driving your vehicle.

In this case, a 285/75/16 would give a radius of 8. 587 feet, to round up. This would mean approximately 614. 88 revolutions per mile. I don't know how the dealership programs in this new value, but I think that it's done based upon Revs/mile.
 
another option to calculate

here is another way to calculate odometer error using percentage. lets use a stock tire as a 245/75/R16.

245 is the width of the tread in mm 75 is the percentage of the sidewall height of the width, and of course 16 is the rim size. so 245 * 0. 75 = 183. 75/25. 4 (mm in 1 inch) =7. 23 inches sidewall height * 2 for the top part of the tire and the bottom = 14. 468 + 16 (rim) = 30. 468 inches total tire height. multiply that by Pi (3. 14159) gives you total inches covered in one revolution of the tire = 95. 718.



so if you do this calculation for your new tire say a 285/75/16 you would get 32. 83 inches.



now you can calculate the percentage difference between them which in our example equals 7. 2%. so your new tire size is 7. 2% bigger than stock. once you find this percentage constant, you can make a chart for the MPH and odomoter, so if you are not recalibrated you can at lease have a reference of how fast you are going. i made one and keep it in my truck. i figured out every 5mph, of indicated vs. actual. so now i know how fast i am actually going if i set my cruise on 60 indicated, i am actually going 64. 8mph.



hope that is clear as mud;)
 
Your math and formula look correct for calculating the circumference of the tire. However, the tire is not exactly round when it is rolling. It's a little flat at the bottom. So, compare your calculated circumference with the actual rolling circumference. To get the rolling circumference take a piece of chalk and put a mark at the bottom of the tire and a mark on the road at the same point. Now rotate the tire one revolution and put another mark on the road, now measure the distance between the two marks and compare it to your calculated circumference.
 
Just look at your gps, it gives speed and i had the local cop "shoot" me with his radar and the gps was dead nuts on. My speedo was reading 4 mph faster.
 
I have not adjusted my truck's odometer, but I have programmed my attitude monitor to read corrected MPH. I really didn't have to do any complicated math or physical labor. I simply used the internet to reference the specifications of the OEM tires vs what I'm running now. The manufacturers listed circumference and that's what I used. I programmed in the circumference of both the OEM size and brand as well as that of the size and brand that I'm running now. I have been running like that for a couple years and it seems to read pretty darn close to both GPS units and local sherrif's radar carts. It's plenty close enough for me. Heck, I used to run a 4 speed Chevy with 44" tires and 5. 13 gears up and down the highway with only a post-it pad with a hand written RPM vs gear vs MPH chart on it.
 
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