My speedometer is 3 mph fast at 70. I know this is within factory specs but I want it to be more accurate. I have stock 235/55/R17 on my car. Tire rack shows that to be 768 revolutions per mile. I set my Xcal to 770 but the speedo was even more off. I don't know what my tuner set the revs per mile to but it is closer than what I can get it to. 800 revs per mile was close. Funny thing though, no matter what the speedo read, my odometer was still reading correctly according to the mile markers. So my question is, what are you guys setting your revs per mile at with the stock 17s? Or has anyone even messed with it?

 

According to an Excel spreadsheet someone on MF gave me, it's 742 rev/mile. How are you measuring the speedometer?

 

+1
How are you determining that your speedo is off?
Measure around your tire with a tape measurer.
Divide 63360 by the measured inches around the tire.
Thats your revs per mile.

 

You need to being checking your speedometer against a GPS. We can't tell from your post how you verified it. My Stang reads 1.5-2 MPH slow. My SUV is more like 1 - 1.5 MPH. It depends on what speed I check it at.

 

Verified on a police radar sign that is permanently mounted next to the highway. 60 to 70 on my speedo is really 57 to 67. I haven't tested it at a slower or higher speed. I don't own a GPS but should invest in one.

I would imagine 742 would make the speedo read way too fast as 770 was making it read faster than 800.