I just noticed either the Speedo or Tach is off in my 2007 GT. At first I thought they had put 3.31 gears in it!
However it has a 3.55 Sticker on the rear axle and a CD Code on the door so I doubt that.
If the rear axle was mislabeled the tach and speedo would match but the car would actually be going faster than it registered
on the speedo if the PCM was programmed for 3.55 gears.

At 70 mph (Car Speedo) in 4th I should be at 3181 rpm and but I am showing just under 3000 rpm (Car Tach).
2966 rpm would be correct with 3.31 gears.

1. Tach reading 200+ rpm low?
2. Speedometer off by 5 mph?
3. 3.31 Calibration in the PCM with 3.55 gears?
4. 3.31 Calibration in the PCM and 3.31 gears despite window sticker, door, and rear axle all showing 3.55?

I was prepared for the speedo on these cars to read 2-3 mph high but 5 mph seems excessive.

 

Since you don't have a handheld tuning device, drop the car off at the dealer and have them check your speedometer... Worse case scenario they reflash your PCM for the 3.55's and your problem is solved.

-Jeff

 

+1 Jeff

 

They can reflash the program as long as it's a stock gearing and tire size. It can also be set or 18's when you have 17's or the other way around.