Blacksmoke

What exactly does the tune do when the revs are different on the tires?


Does it just correct the mpg the computer says you are getting and the speed it says you are going or what?

I don't see how correcting it could help your performance any and could only see the mph being like 2mph or so difference as far as the correct speed you are going correct?

I went from the stock 18's on a 2006 Mustang GT 18x8 and stock tires (whatever those were)
to 18x10's all the way around and 275/40/18 tires.

I do know that I am getting only 14.5 average mpg in the city according to the computer. (which seems low to me)
I am not hot rodding it and keeping the RPM's above 3000 (usually at 4000)
Wonder if this is just an incorrect reading from the wheel/tire change?

Am I hurting the actual performance of the car or the mpg by not having a tune with these bigger wheels/tires...?
Need a little advice here...

Thanks

Argonaut

The car's computer is set to use a certain diameter tire. AFAIK it uses this diameter to calculate: MPG, Speed and Odometer. When you change the overall diameter of the tire by going to a bigger or smaller tire than what the car is set for...all those calculations will be off. A tuner, such as the SCT, will allow you to input the new tire diameter and thus correct the situation. As you suspect none of this has anything to do with engine performance, its just the numbers being reported are now wrong because of your tire change.

Having said that - the 275/40/18 has a diameter of ~26.66". You indicate your stock wheels were 18s and I am not familiar with the stock size that came on those rims. I do know however that my 2007 came stock with 235/55/17 with a diameter of ~27.17". Thus, in my case, switching from stock to the 274/40/18s would result in the speedometer reading ~2% too fast with corresponding wrong reporting of the MPG.

socalwrench

Don't forget that ABS system also uses the vehicle's speed to adjust braking force and action.

You need to adjust for any tire height or gear ratio changes.