I have a friends tune I have to fix and all I have time for is to stop by and DL the tune and leave so no experiments for me. The speedo is fast from actual MPH. Dont I increase the tire size to slow it down and decrease the tire size to speed it up?

 

If you use the Speedo Calculator in EFIlive/HPT, you'll input any changes (tire size, gears, etc) and it'll calculate the appropriate VSS pulses/mile for you. Put it in the right cell and you'll be golden.

But to directly answer your question, smaller than stock tires will cause you to be going slower than your speedo is reporting. The smaller tire circumference will cause you to travel less distance per revolution.

Larger than stock tires will cause you to be going faster than your speedo is reporting due to the larger tire circumference moving you along at an increased distance per revolution.