I don't think you can even make that be wrong. If I put 2:73's or 5:56's in the car the speedometer and odometer are going to show you a mile a minute at 60 miles per hour. That was the point I was trying to make. The gages only know output shaft speed, it calculates how fast the car is going based on a ratio in the PCM. You can make that ratio anything you want and 60 MPH will always show 1 mile per minute. Now any external speed measuring device like a GPS could help with how fast the car is really going.

Ok; maybe this will help, My 96 has 2:23 based on the speedo and tack I am going 75 MPH @ 2500. Based on the time it takes me to travel the 30 mile to work the 75 MPH is pretty close to right. If I mounted 15" low profile tires that where 20% smaller in circumference then when my RPMs where are 2500 and Speedo would still read 75 Miles per hour but I would be going only 60 MPH. The odometer would only show that the distance to work was now 36 miles instead of 30. The computer thinks I am going 75 MPH and since I am going 20% slower it takes 20% longer.

 

Which is why I suggested having someone pace you to double check speed accuracy.

 

it should confirm the speedo is in itself working correct. once you do that then you go find a mile marker and see if it matches mile for mile on the odometer. forgot that part.

 

I have 3.23's and an auto trans with stock tire dimensions and at 50mph i turn about 1800-1900, if i remember correctly, and about 2100-2200 at 70 mph.

 

I have 3.73s in my 02 Z A4. 75mph is 2600 rpms for me. Still getting 23mpg highway too.