Michigan Mike

Well primarily your formula has no provision for tire size. 1 revolution of a 32" tire travels 100.528 inches. 1 revolution of a 35" tire travels 109.95 inches. 1 revolution of a 42" tire travels 131.84 inches. So the difference in distance traveled per 1 tire revolution (regardless of the gear ratio, I mean 1 RPM at the axle IS 1 RPM at the axle), from a 42" tire to a 32" tire is @ 31 inches. The computer then, thinking it's a 32" tire, calculates the road speed. But the road speed is higher because the Jeep has really traveled 31 inches further down the road for each tire revolution. Odometer is the same...more distance traveled than indicated.

Mark Doiron

Oh, I get it. But, my formula wasn't meant to calculate the change in speedo reading with a change in tire size and gear size. It was meant to calculate the change in the odometer reading with just a change in gear size, assuming that all else in the drive train/tire size are unchanged.

I figured that my simplified formula was sufficient considering that's the topic of a couple posts where people are saying that regearing, without a tire change, affected their speedos. I was attempting to verify that the result of their regear was consistent with a simple mathematical model of the change. It seems to be to me, but I'm willing to accept that my formula may be incorrect. Like I said earlier: Long time since college physics. Maybe I'll have my son look at it--he's an engineering physics major at Oklahoma University and eats complex differentials and linear algebra for a late-night snack.

And now you know why I have no personal experience with regearing my JK: My son and my money go to OU. LOL.

Michigan Mike

My son and my money go to OU

Money well spent

pagoda

So now that my miniscule brain is weeping from the physics and advanced math lessons....I'd like a small thing clarified.

On my JK with 35" tires and no change in gears; the roughly 3 percent change in MPH and distance traveled means that the mph and odometer are off to the plus or minus of what is reading on the dial?

i.e. mph is actually reading 55 mph, am I going faster or slower in actuality
and if my odometer says 10 miles traveled is roughly 10 percent more or less than the 10 miles read?

Thanks for explaining it as well in the previous posts.

bigbluejk

If you calculate it like mike said. Well lets make it more simple and just say 10% (that is an ez number). So your speedometer says 55mph. that means in one hour your odometer will add 55 miles to the clock. if your off by 10% then you add 5.5 miles to that (55*10%=5.5). So at that speed you have really gone 60.5 miles but only logged 55 on your odometer. If you ran like this forever then your odometer would have 10% less miles on it that you really went. make sense? It is all pretty confusing when you try to take actual tire sizes and calc differences of them.

Michigan Mike

You're going faster than your speedo says and you're traveling further than you odometer says.
For an estimate, around 10% further and faster.

Edit: Yeah what he said!

pagoda

Yes that makes great sense....not being the math whiz I was just looking for some method to make approximate calculations until regearing and procal can be done....thanks alot...

bigbluejk

Oh... BTW... after the discussion about this I have slowed down. I was thinking my speed was read from the transfer case, which with my 5,13s and 35s and current programming would show a faster speed than I was really going, but it is indeed read from the wheels. So where I was going 70 and thinking I was going like 65, I was really going 77. doh. lol.

bigbluejk

Oh yeah... it is all just an estimate. Really from the factory it is just an estimate as well if yo think about it. As the temperature changes so does the pressure in most peoples tires. When dealing with a tire like a 32" that holds that much air, I bet actual tire circumference could change a good bit. then your speedometer/odometer would both fluctuate wouldn't they? I dunno, just popped in my head after you reminded us all that all of these numbers are estimates.

tbone1077

OK, this thread got me curious. I took my jeep out with my GPS and checked the differences. I have a 3:73 ratio and went from 29" factory wheels to 33's.

speedometer 30 40 50 60 70
GPS 34 45 56 66 77 exactly a 10% increase at +4"

I calculated this to about 2.5 % per added tire inch, so if you went from 32's to 35's, it would be a 7.5% increase. (at 70 on speedometer, actually doing 75).