Do The Math - Tire Revolutions per Mile

[Deaf Bob]

Holy crap! There ain't nobody who needs killin' that's even worth that much....GAWD!

Accurate up to a mile, body'd drop before it is heard if at all..

[Russell]

Use the 3.31 gear and change the rotations over mile to 826. If my math is right the speedo should be right. If I am wrong it might.... and it could adversely....

I am starting to second guess my math. 826 should have worked for the gear being wrong.... Got to thinking and reread you post where you said you were going 70 and speedo read 80, in that case I would suggest 880. As before my math comes with 100% money back you get what you pay for

I would think the tuners could give you the right number, But I will be glad to help hone it in. If the above does not make speedo match gps. Post all known values tire size, true gear ratio, eec gear ratio, speedo speed and gps speed. I like numbers

[pdub]

I am starting to second guess my math. 826 should have worked for the gear being wrong.... Got to thinking and reread you post where you said you were going 70 and speedo read 80, in that case I would suggest 880. As before my math comes with 100% money back you get what you pay for

I would think the tuners could give you the right number, But I will be glad to help hone it in. If the above does not make speedo match gps. Post all known values tire size, true gear ratio, eec gear ratio, speedo speed and gps speed. I like numbers

Well, I'm pretty fond of numbers too...lemme seeeeee .... 285/40/ZR18, final drive ratio 3.55 what do I do with all of that? GPS says it's going slower that the speedo says it's going. That's all I've got. And the faster you go...well...it's all clearly over my head...whatever it is ain't right.

[Russell]

the faster you go the more its off. Because of the ratio. You put numbers on one side of the = the numbers on the other side you move them around and.... the easy way to make little changes is GPS MPH / TRPM = Speedo MPH / new TRPM --> (TRPM * Speedo MPH)/ GPS MPH = new TRPM

70/770 = 80/x ---> (770*80)/70 = x ---> x=880

[pdub]

the faster you go the more its off. Because of the ratio. You put numbers on one side of the = the numbers on the other side you move them around and.... the easy way to make little changes is GPS MPH / TRPM = Speedo MPH / new TRPM --> (TRPM * Speedo MPH)/ GPS MPH = new TRPM

70/770 = 80/x ---> (770*80)/70 = x ---> x=880

Ummm okay, so 880 is the number I should punch in for tire revolutions per mile?

[Russell]

I think so, go for a test drive report new speedo mph and gps mph and if not correct I will give you another number to try or you can you own numbers / math. in general if the speedo says you are going faster than you are you want to increase the tire rev per mile

[pdub]

I think so, go for a test drive report new speedo mph and gps mph and if not correct I will give you another number to try or you can you own numbers / math. in general if the speedo says you are going faster than you are you want to increase the tire rev per mile

Wow, that was a great shot at it Russell. That was darned close. I had to play with it two more times and it turns out 840 is the number it wants. At 840 the speedo reads dead-on-target everywhere across the spectrum. That's really cool, thanks so much man!

[Russell]

Glad you where able to figure it out!