How is speed determined
Le Mans Master
Joined: Aug 2005
Posts: 7,392
Likes: 218
From: Merritt Island, FL
The pickup point is in the tailshaft of the transmission. There's a VSS (vehicle speed sensor), located on the LH side, a few inches ahead of the driveshaft. The ECM counts the number of pulses received over time, and calculates the speed from that.
Melting Slicks
Joined: Dec 2008
Posts: 2,772
Likes: 64
From: Mystic CT
Of course the wrong size reat tire affect accuracy. The sensor only kows how many times the tailshaft has turned and the ratio is calculated to be correct with the correct rear gears and tire circumference. If you change either of those you will have some error. If you put in more rear gear ex. go from 2.59 to 3.07 the speedo will read high if you go to a taller tire it will read low.
WS (wheel speed) = tail shaft speed / axle ratio
(WS x 60 x tire circumference in ft) / 5280 = mph
WS (wheel speed) = tail shaft speed / axle ratio
(WS x 60 x tire circumference in ft) / 5280 = mph
Race Director
Joined: Apr 2002
Posts: 10,036
Likes: 346
From: Anaheim CA
You can buy a box that can compensate for different tire sizes and rear end ratios. I don't know the manufacturer offhand, but I'm sure there are people here who know. A friend of mine put one on his car when he went from a 3.07 rear end to a 3.54.
Le Mans Master
Joined: Oct 2004
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From: South-central Missouri
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Team Owner
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if you put a stock set of gears in the back you can get a new sensor from GM. they fit more then just a Corvette
Le Mans Master
Joined: Aug 2000
Posts: 8,838
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From: Australia
Dakota
The two VSS units that were fitted from the factory relate to the size of the speedo driven gear used to give the correct readout
