Tire size parameter limit too small..

[Country Boy]

I have a truck Ive tuned and the speedo is still 5 MPH faster than actual GPS speed. It has 40" tires and 4.56 gears. I already have the tire size set to 39.75 and I tried to go over 40.00 and it gives "Invalid Tire Size". Can we get a raised limit for tire sizes? Also. which way do I change the gear size to slow the speedo down? I dont have the time to experiment, but drop in the tune and go.

Thanks.

[Bill@HPTuners]

No, your best bet would be to try adjusting the PPM constant to dial in the speedo.

[Country Boy]

I would adjust them down, right? Any idea by how much? I dont have the time to experiment since he cant leave. Hes a paramedic and only in this side of the state when hes at the station. I need to slow the speedo down 5 mph. Too much for a guess and its going to jack up the shifting.

[mecanicman]

It works opposite the way you are thinking. To many pulses per mile will make the speedo read slow, to few will make it read high. The math is simple, take your speedo indicated mph and divide by actual mph. Multiply that value by your pulses per mile and your done. So 65/60=1.083 Multiply this by your ppm which is essentially a 8% increase. As long as your GPS and speedo reading are bang your correction will be too.

[Country Boy]

It works opposite the way you are thinking. To many pulses per mile will make the speedo read slow, to few will make it read high. The math is simple, take your speedo indicated mph and divide by actual mph. Multiply that value by your pulses per mile and your done. So 65/60=1.083 Multiply this by your ppm which is essentially a 8% increase. As long as your GPS and speedo reading are bang your correction will be too.

Swesome! Thanks a lot.