MPH/tire size/RPM/mpg

I tried searching first...
My wife was doing 65mph in the Oldsmobuick. I blew by her at 60 (my XJ's speedo) and beat her home by over 5 minutes on a 55 mile trip. I stopped for coffee, too. She seems to think I was doing 75, at least. In just 3 or 4 miles, her headlights were never in my mirror again.

AX-15 trans:
At 60mph (on my speedo), my rpm's are 1,800 in 5th.
At 60mph (on my speedo), my rpm's are 2,050 in 4th. (4th is 1:1, right?)

Gears are stock 3.07's.

Tires are 31" BFG MT, full tread remaining.

I'm just trying to figure out a pretty close approximation of my actual speed when my speedo says 60. I know the faster I go the worse it is.
I can't find a formula to calculate road speed, any help?
Superlift Tru Speed isn't in the budget now...
Thank you, Holiest Brethren of the Church of Jeep Tech,
Mike

I think it comes down to something like this:
stock tire is 28" in height (give or take 1/2"). You have 31" of height now. So with a 29" tire you travel: 29PI at each revolution. With the 31" tires you travel 31PI. So that means that you go 10.7% further for each revolution and if your speedo says you're doing 55mph, you're actually doing almost 61 and at 60 you're doing almost 67mph.

Now to fix your speedo so that it works right, search this forum for "speedo gear" and figure out which one you need. They will run you less then $20 from the dealer (with an oring) and after you put the correct one in you'll be pretty close to accurate.

old tire dia./new tire dia X old gear(teeth) = new gear

stock 215's are about 27"

27/31 = .87

.87 X 31teeth = 27 teeth

Drive past 10 mile markers on a straight section of freeway and compare to your odometer reading for ballpark accuracy.

What were your stock tires?

The difference (error) when going from 215/75R15 tires to 31x10.50R15s is approximately 10%, plus or minus 1% depending on the particular tire used. So if you were indicating 60, you were actually traveling around 66 MPH.

http://www.gotjeep.net/speedo.html
Jeep on!
--Pete

ComancheClub.com

Thank you, fellas.
225/70 or 75 were stockers on this (door sticker). I had figured about 10% error, that's why I was alarmed at the difference between my wife's speedo and mine.
I checked my odo from yesterday.
Ride home from work is 34 miles. Then I went from home to meet wife yesterday (55 miles). From there back to here is 55 miles. Total, 144 miles.
My odo today after those trips yesterday showed 120.
Difference is 24 miles.
120/144 = 0.833 or 8.3%
55mph x 1.083error = 59.565 corrected mph
60mph x 1.083error = 64.98corrected mph
65mph x 1.083error = 70.395 corrected mph

Gotta go check the link from Pete M, too.
(Holy Toledo Speedo, Batman. I checked the link. SIMPLE and beautiful.)

mrfreakinwhite said:

Ride home from work is 34 miles. Then I went from home to meet wife yesterday (55 miles). From there back to here is 55 miles. Total, 144 miles.
My odo today after those trips yesterday showed 120.
Difference is 24 miles.
120/144 = 0.833 or 8.3%
55mph x 1.083error = 59.565 corrected mph
60mph x 1.083error = 64.98corrected mph
65mph x 1.083error = 70.395 corrected mph

Click to expand...

Your math is way off. 120/144 is 0.833, but 0.833 is 83%, not 8.3%

You calculate percentage by dividing the DIFFERENCE by the original amount. We're talking about your odometer here, so the original value is the 120 miles indicated. The difference between that and the actual distance is 24 miles. The error is 24/120, which is 0.2 or 20%.

Even if you use the actual amount, the number would be 24/144=0.16, or 16%.

So your discrepancy is a lot more than what your figures indicated.

Eagle said:

Your math is way off. 120/144 is 0.833, but 0.833 is 83%, not 8.3%

You calculate percentage by dividing the DIFFERENCE by the original amount. We're talking about your odometer here, so the original value is the 120 miles indicated. The difference between that and the actual distance is 24 miles. The error is 24/120, which is 0.2 or 20%.

Even if you use the actual amount, the number would be 24/144=0.16, or 16%.

So your discrepancy is a lot more than what your figures indicated.

Click to expand...

Aye... Way off. EAGLE, you probably just saved me from a ticket for speeding!! I sure am glad you're on the ball.
So @ 20% error, I have:
55mph x 1.2error = 66mph actual
60mph x 1.2error = 72mph actual
65mph x 1.2error = 78mph actual

For the heck of it, @ 16%, I would have:
55mph x 1.16error = 63.8
60mph x 1.16error = 69.6
65mph x 1.16error = 75.4

This explains why everyone drives so slow all of a sudden. And of course, this is using my own odometer which is now known to be inaccurate as a basis for the measurements. Close enough to what I was experiencing to call it right.