hey, i just got a gps/nav unit and thought id try to tune my speedo a little closer than it is, here is what i came up with my current settings:

94 z28 auto 3.50 rear gear, p275/40zr17 tires(stock for camaro ss)

speedometer scalar=29
speedometer scaalar, fraction=1.057

35-45 mph speedo is 1 mph fast
50 mph speedo is correct
55-75 mph speedo is 2 mph slow

it is close and i can live with it but id like to have it right on, and if you enter my data into tunercat speed calibrator it comes up with scalar=28, fraction=.985 and the speedo is over 6mph off

is there a more exact formula than what tc uses for its calibrator

 

The gps is probably not accurate enough for what your doing. I would check your speed against the mileage markers on the highway. 60 mph= 1 minute between markers. Our aviation gps's are right on when it comes to position but the groundspeed is usually off unless your moving really fast like 450 Knots.

 

maybe it isnt,,,, but i have 2- 2005 gm vehicles and on the same road the gps matches the speedo's exactly at the same speeds im testing the camaro and on the same road

 

I will have the same gears and tire combo so let me know what the final calibration works out to. I may want to use your calibration after I install my 9" with 3.50 gears. Thanks.

 

right now i have:

speedometer scalar=29
speedometer scalar fraction=1.057

and it is real close only 2 mph slow at 60-75

we got another snow fall last night so testing is off till it warms back up, i will let you know what i end up with

anyone else running this combo 3.50 gear and a p275-40-17 tire??

 

Every hand held I have used is pretty darn accurate on the ground and in the air. I calibrated my speedometer using a hand held, just change the tire sizes until you get the right speed.

 

I have a question on setting the Gear to Tire calibration on Tunner Cats.
I have a 93 TA with the OBD1 and a chip.
I just put a 9" with 4.10's and I need to rest my computer for the change.

I have a 275/40/17 tire with my 4.10's, can any one help?