84redta

iTrader: (1)

Can anyone help me with my Speedo, seems to be 9-10% over what it is actual via speedometer app on my phone or GPS. 30 is 33, 40 is 44, 50 is 55-56 60 is 67ish. Attatched is my calibration screen from my laptop. Burned two pcms in 2 years due to fluke situations, really like to get it right once, don't want to guess n test if I can help it. Thanks for the help!

84redta

iTrader: (1)

Forgot to mention, 4.10 gears in the back (4th gen 10 bolt), lt1/t56 from a 94 i believe, and 285/40-18 rear tires on vette y2ks, not that half those details matter that I know of but trying to include all that I can.

84redta

iTrader: (1)

Ignore the first tire size, but from this screen shot,

is the discrepancy in the computers calibration?

madhatter

Yes it is. The 285/40/18 is most definitely a 27" tire.

84redta

iTrader: (1)

Bump,

Finally found some threads talking about this and said to use rev/mile as it's most accurate, can anyone tell me what all the parameters should show to confirm it's right first time please. Again, t56, 4.10, 285/40/18 tires

84redta

iTrader: (1)

That's what I thought was off after I saw it and looked up measurement with tire size. If anyone can give me a break down of all the specs from tunerpro or other software on the right of the first pic so I can be sure to get it perfect that be great, thanks!

englundjd

when I changed my gears I used tunerpro to change it and i just divided all 3 numbers by the stock gear ratio then multiplied them by the new gear ratio. I feel that tire changes would be the same.

84redta

iTrader: (1)

Yea I have cats, and it seems from first image that tire size calculation in the program wasn't working perfectly.

madhatter

So select tire diameter instead of tire size and input 27". You should be fine after that.