: Speedometer variance

10-23-11, 10:43 PM
I have noticed most car speedometers read a few mphs faster then what the vehicle is actually doing. I have noticed this on quite a few different cars with a few different GPSs. I found out this past weekend that my car reads a few mphs slower then what the car is actually doing. So, the speedometer was reading 78, I thought I was actually doing 75, and the State Trooper clocked me at 81 in a 70. So I went home a grabbed the GPS and it read 53 when the speedometer was showing 55.

The car appears to be 100% stock, I just installed the CAGS eliminator this weekend so I don't really think that someone changed the tune just for the tire size

So my question is, has anyone seen this before?

10-24-11, 12:12 AM
With stock sized conti dw 245/45/18's using the car's built in gps system, my 07 speedometer at highway speeds reads 2mph slower than the gps. With the stock goodyears at highway speeds it would run 1mph slow. I believe that 265/40 or 255/40 would get the speedometer dead accurate or reading slightly high respectively. Oddly this is not the first vehicle I have owned that the speedometer read lower than actual, 05 Nissan Xterra offroad 4x4 with 6speed did the same thing with its factory tires. I doubt many vehicles with factory tires would read low except for low production specialty packages and models.

10-24-11, 02:25 AM
If you have access to EFILive or HPTuners you may fix that through pulses per mi/km

I am runnin 305/30R19 in the rear and no issues with speeding cameras or cops