Speedometer is off by 5mph

[ColoradoBoss]

Took a road trip this weekend and noticed my speedo is 5mph slower. I have changed tires to a 35/12.5/17 but I did do a tire size correction with the Cobb AP. I'm really scratching my head on this one since the speedo should only be off by 1% or so. Don't know if any one has any suggestions or not? I have altered the AFE bias for a more accurate MPG reading. I'm not sure if that would affect the speedo, I got the information about changing the AFE bias from the f150 forums.

 

[FordTechOne]

Not every tire of the same spec will have the same circumference. Does Cobb allow for tire circumference to be used instead of tire size?

 

[ColoradoBoss]

I don't believe so, but I will check when I get home form work.

 

[ghost410]

Is it consistently off by 5mph at all speeds or at a certain speed? If you could calculate the exact percentage off it is, should be easy to correct in the Cobb. If it’s a consistent 5mph at all speeds, there’s other issues

 

[ColoradoBoss]

It appears to be consistently off at all speeds. only other thing that i can think of is the AFE bias, so i'm thinking of putting that back to stock and the cobb tire size back to stock and check my speed again.

 

[ghost410]

If it was the tire size correction in the Cobb causing the issue it would be a set percentage do The mph it was off would increase with speed. So I’d rule that out

 

[snt505]

Not every tire of the same spec will have the same circumference. Does Cobb allow for tire circumference to be used instead of tire size?

I don't believe so, but I will check when I get home form work.

Cobb does allow you to put whatever height you want in there. I think stock the number is like 33.4" (less than even the ideal size of 34.35"). I have had Yokohama 37s that are 36.8 or so without a load and now BFG 37s that are like 36.3 unloaded. I have just adjusted that number to compensate, but always go a little lower than the unloaded actual height.

 

[Steve-PNW]

Cobb does allow you to put whatever height you want in there. I think stock the number is like 33.4" (less than even the ideal size of 34.35"). I have had Yokohama 37s that are 36.8 or so without a load and now BFG 37s that are like 36.3 unloaded. I have just adjusted that number to compensate, but always go a little lower than the unloaded actual height.

can confirm