Calculating Milage

I've figured this out before, but forgot how I did it. I have 31's with 3:55 gears. I'm assuming the factory tires were 28s. How many miles do I subtract per 10 mile increments. 100 miles if that is easier.
Thanks

dono Bennett - i think that the miles get more and more off as speed is increased - (at least that is what people say about their speedometer)

boise49ers said:

I've figured this out before, but forgot how I did it. I have 31's with 3:55 gears. I'm assuming the factory tires were 28s. How many miles do I subtract per 10 mile increments. 100 miles if that is easier.
Thanks

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Distance traveled on an old wheel (assuming 28) is 28PI. Distance traveled on the new wheel is 31PI. Now if you divide the distance traveled for one revolution you will travel about 10 % further. So whatever miles you've got on the cluster, multiply it by 1.1 and call it a day.

something to consider...I've checked my XJ on 31's (stock speedo gear) against GPS and it is only about 5% off (speedo slower than actual speed)....which makes me think that it would have read about 5% fast (speedo higher than actual speed) when stock since the 31's are ~10% larger diameter. YMMV :dunno:

TekkaMaki said:

something to consider...I've checked my XJ on 31's (stock speedo gear) against GPS and it is only about 5% off (speedo slower than actual speed)....which makes me think that it would have read about 5% fast (speedo higher than actual speed) when stock since the 31's are ~10% larger diameter. YMMV :dunno:

Click to expand...

You're right: the stock speedo is not always 100% right, plus there are variations in tire sizes (both stock and non stock based on brand and tread wear). If you don't want to get pulled over for speeding it's better to go over on the calculations

Another good test is to reset your trip odometer on a long stretch of highway and keep tabs on mile marker posts. I've found that to be slightly more reliable way to verify speed vs using the gps as there are slight delays between you seeing your speed on the speedo and the GPS (as in GPS is always a hair behind) which sometimes causes even more confusion.

a new speedo gear is less than $10 at the dealer and about 5 minutes to install

BrettM said:

a new speedo gear is less than $10 at the dealer and about 5 minutes to install

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Yeah, there's that one site that spells it all out for you to get the right new gear, and IIRC it has install pics (like two, I could probably even do it)...

But I bookmarked the sucker at work and I'm OFF today! Go go presidents.... I'll drop it in here tomorrow unless someone beats me to it.

Kejtar Thanks. That was the how I did it before.