Gear Ratio change...need Help Tuning!

[BlackBird98]

Hi,
I just bought a 1998 Pontiac Trans Am with the A4 4L60E Automatic transmission and the LS1 engine. Well, my speedometer is off by a considerable amount. I took the differential cover off today and discovered someone has put 2.73 gears in my car (it is supposed to have 3.23 gears). My friend has an HP Tuner but doesn't know how to use it. Can someone walk me through how to change the gear ratio using the HP Tuner. Thanks!

[slow67]

1st:
Check your roll-out in inches.
Mark the tire & the ground (at nomal tire temps after driving).

Rotate one or two revolutions (divide that value in inches by 2 if you do 2 revs ). Then mark the ground where the tire mark makes it's 1st or 2nd revoltion. It should be ~88.18" If it's different, time to rescale.

Here's the math on how to do the speedo rescale:

Tire revolutions/mile = Inches per mile / Tire Circumference Inches
Tire Revolutions/mi = 63360 / 88.18 --- based on (28.07" tire * Pi)

Trans revolutions/mile = Tire revolutions/mile * Gear ratio
Your values:
(63360" per mile / 88.18" tire circumference) * 4.56 gear = 3276.33

Take that value and multiply by the Rev-Trans value (40 in *most* cases) to get the VSS Pulses/mile = 131053

Put your values in the appropriate places and your speedo's dead on. Check with a GPS if you have doubts.

[BlackBird98]

In place of the 4.56 gear on your equation, I would put 2.73 right? And how do I rescale if I need to?

[slow67]

Check your tire height too......

[BlackBird98]

Ok I will check the tire height. What about the other two questions I had though?

[brianfromhawaii]

who says it sposed to come with a 3.23? i have seen it both ways... 2.73 is very common in auto z28s and base v8 tas.