SeanD

Hi,

When changing the revs per mile in a custom tune, do I use front or rear wheel for reference? I have a staggered setup, and just put in new 20" Steeda Spyders, so I need to adjust my tune to get the Speedo correct.

Thanks,

Sean

jlmounce

rear wheels

Cavero

+1. Use the manufacturer specs from the website.

gmantheman

Actually I found use the manufactures spec X 1.03 to compensate for the load to be spot on. When I used the manufactures specs, my speedo was still of by 2-3 mph.

Cavero

Long story short, that's the way Ford made the speedo. If you want the full version, check out this thread:

https://themustangsource.com/showthread.php?t=479703

SeanD

Manufacturer specs on my Nitto Invo 315/35-20 are 726. If I got to some of the sites that give you the revs/mile after entering in the tires specs, I get around 703.

I guess I will have to take it out with my GPS and double check the numbers.

jlmounce

You can't really count on that to be spot on however. The factory sets the speedometer a couple miles per hour optimistic, however the computer knows what quick you are actually going.

Always use the manufacturer specs for revs per miles which is available on the tire manufacturer's website.

gmantheman

I Nittos as well and they usually run a little bit small. I can almost guarantee if you put 747 or close to it you will be spot on. 726 * 1.03. I tried using nittos numbers but according to my GPS I was off a little. Now I am according to the GPS and those electronic speed signs, I am very close to being spot on.