Calibrating speedo
Thread Starter
Pro
Joined: Dec 2009
Posts: 606
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From: Waco Texas
Calibrating speedo
Hey guys, thanks in advance for any help. I am simply trying to adjust my speedo based on 3.90s and 295/30-20 inch rear wheels.
It was on factory tune with 3.42s and was about 5 mph slower than actual and now after flashing the 3.90 to the ecm I am 9 mph off? I will remeasure the tire as a tire calculator says 27 and I think I got 26? Any ideas tuners?
It was on factory tune with 3.42s and was about 5 mph slower than actual and now after flashing the 3.90 to the ecm I am 9 mph off? I will remeasure the tire as a tire calculator says 27 and I think I got 26? Any ideas tuners?
Race Director
Joined: Oct 2002
Posts: 16,477
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From: Gone
CI 6-7-8-9-10 Veteran
St. Jude Donor '03
The gear ratio has zero effect on speedometer accuracy, as the VSS senses axle speed on the output side of the rearend.
Rear tire height is the only variable there is.
27" is about right for a 295/30/20....there is no way they are only 26" tall.
Rear tire height is the only variable there is.
27" is about right for a 295/30/20....there is no way they are only 26" tall.
Thread Starter
Pro
Joined: Dec 2009
Posts: 606
Likes: 1
From: Waco Texas
Thanks for the reply man. Shoulda read for a lil longer before asking as I found it after an hour of looking and rewording search phrases. I am tuning my personal ride and used to TA/Camaro tuning and you need both the gear ratio and tire size. I (being impatient and car at home , me at work) didn't personally measure them and relied on the tire sites for specs and found 27", well after reading your post I reflashed back to 3.42s and the correct measured tire height of 26.5" gave me a gps verified correct calibration. IMO the rocket scientists at HPTuners should have that part inaccesible?
Do you you have any tuning suggestions fir a cai, free flow exhaust(no headers), 3.90 geared ls1 car to bump it hair for now? Spark advance/retard lower rpm PT? A/F fattening? Thanks again man!
