[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.
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.