Tire size vs Speed

[nolosses4]

Does anyone know the formula to find out how fast im actually going on the highway when I put, say 31's on my jeep?

[RichP]

What formula, use your watch and the mile marker posts. put it at 60 and see if you do it in a minute... The only other thing I remember off hand is there is a chart around for the correct spedo gear.... seach might find it...

[goodburbon]

new tire diameter-old tire diameter
_____________________________ +1

old tire diameter

multiply the answer by your speedo reading and that is your speed

[Osprey413]

basically, there is a 5mph gain when you go up to a 31

[bburge]

The missing data is:

what's your tire size now?
what speed are you trying to compare?

When I went from 31's to 33's; my speedo was off by about 10%. So I was off 5mph at 50mph, but I was only off 2.5mph at 25mph. It's a ratio thing.

Got a GPS? It makes this sort of thing a lot easier. If you're just planning ahead - it's a word problem where the final boil down is the ratio of tire circumfirences(sp). Plug in the different diameters in the C=D(pi) and you get close. (Rememer, your tire isn't round on the ground. Ithas a flat spot on the bottom.)

If you try and estimate MPG with this relationship; you will be very unhappy with the results. They will be way off.

bburge

[Yucca-Man]

basically, there is a 5mph gain when you go up to a 31Not exactly; an increase in tire size makes a percentage gain in perceived speed, not a straight increase of 5 mph. Check out http://4lo.com/calc/gearspeedo.htm or any of many other calculators at www.4lo.com

[red91]

Not exactly; an increase in tire size makes a percentage gain in perceived speed, not a straight increase of 5 mph. Check out http://4lo.com/calc/gearspeedo.htm or any of many other calculators at www.4lo.com


Then it must just be my A$s 'cuase when I went to 31's from 235, it seems like my speedo got slower meaning that at 70 I feel like I'm doing 60.

hmmmm.

[Rough 01]

try searching..... Pinion speedo gear. http://www.naxja.org/forum/search.php?searchid=278745

Or go to http://go.jeep-xj.info/HowtoSpeedoGears.htm



Later,
Rough

[Eagle]

I did a bunch of research and calculations a few years ago, using the most accurate comparison I could find -- the revolutions-per-mile data from tire company web sites. You'll find that tire sizes are indeed nominal, and the actual revolutions per mile vary depending on manufacturer and tire model, even for the "same" size. So the following is approximate.

Going from 215/75R15 to 30x9.50R15 is a change of approximately 6%.

Going from 215/75R15 to 31x10.50R15 is a change of approximately 10%

[RichP]

Just to confuse more, when I went from 225's to 235's my spedo was even more accurate measured with a stop watch and by a state trooper :D [friend of mine from the guard].....

[Yucca-Man]

Rich - I noticed mine was almost dead-on when measured with 30" BFG ATs, according to a 40+ mile stretch of I-25...