Speedometer Error
Thread Starter
Drifting
Joined: Oct 2005
Posts: 1,549
Likes: 1
From: Roanoke Virginia
Speedometer Error
In HP Tuners I can change the speedo for different tires. I went from the Runflats (26.7" dia) to Mich PS2 (25.8" dia). However I noticed the stock tires set for the car was 26.1". What's up? That does not match the stock runflats that came with it!
Drifting
Joined: Sep 2004
Posts: 1,551
Likes: 4
From: Livingston Texas
I'm not sure why there is a discrepancy in the tire size on the stock file either. I have LS2 Edit and it shows tire revolutions per mile to be 770.87 on the stock file. The stock runflats should be 775 rev per mile. This is less than a 1% error so it is well within the +-4% accuracy.
Advanced
Joined: Jan 2007
Posts: 92
Likes: 0
GM takes into account the "loaded radius" on each tire not just the manufacturers specs. So a loaded tire will squat slightly with the cars weight on it.
You can do a roll out on your stock tires and see if it comes up to 26.1" . I bet it does.
You can do a roll out on your stock tires and see if it comes up to 26.1" . I bet it does.
Thread Starter
Drifting
Joined: Oct 2005
Posts: 1,549
Likes: 1
From: Roanoke Virginia
Then in conclusion I took Tirerack.com's info and will reprogram it this way: 800/783 x 770.88 = 787.62 Which is runflats/ps2 x existing factor = resultant tire revs. Sounds about right, and it compensates for their number being a little off, which Tirerack could be off for all I know.
Advanced
Joined: Jan 2007
Posts: 92
Likes: 0
Or just measure the roll out to be exact.
63360 (Inches per mile) / Tire Turns per Mile = Roll out in inches
Roll out in Inches / Pi = diameter
63360
------------- = Tire turns per Mile
Diameter * Pi
Or
63360
------ = Tire Turns per mile
Roll out
Then to use the auto calc just do a difference between the new and old tires.
New
---- = Trans Shift Point Multiplier.
Old
63360 (Inches per mile) / Tire Turns per Mile = Roll out in inches
Roll out in Inches / Pi = diameter
63360
------------- = Tire turns per Mile
Diameter * Pi
Or
63360
------ = Tire Turns per mile
Roll out
Then to use the auto calc just do a difference between the new and old tires.
New
---- = Trans Shift Point Multiplier.
Old
