Thread: Dash Speedo / HUD accuracy

Originally Posted by Sabrewings

The HUD and the gauge needle receive info from the PCM which is measuring pulses from the VSS. The discrepancy comes from the needle being off cal or the gauge markings not being perfectly accurate. The HUD will always be right since it displays it digitally.

Now, if you don't make adjustments, using a different rolling diameter tire will throw it off since it doesn't know that you changed anything.

how much would you say an inch throws it off? basically a 1/2 inch all the way around? how do you calibrate you speedo?

Originally Posted by gotboost3

how much would you say an inch throws it off? basically a 1/2 inch all the way around? how do you calibrate you speedo?

Go here and put in the original stock tire size and your new tire size. It tells you all the differences:

Tire size calculator

For calibrating the speedo, you just take info from that page and someone with a tuner adjusts the "pulses per mile" value in your PCM.