94 LT1 Trans am, inaccurate speedo reading
04-25-2017, 11:37 PM
#1
Staging Lane
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94 LT1 Trans am, inaccurate speedo reading Hi my 94 trans am (headers, flowmaster muffler, msd system) is reading like 15-30 mph faster than the actual speed of the car (tested with gps and with another car) stock rims, stock tyre size, stock gears, auto trans 2.73 rear axle (already changed the cluster and got the same issue) please help
04-26-2017, 07:56 AM
#2
TECH Junkie
You spelled tire as 'tyre' (Canada, Australia, etc..) which makes me wonder if you are in the US. It almost sounds like it is reading kilometers vs mph. 80 Kilometers per hour is about 50 mph which would explain the speed discrepancy you are experiencing. If I'm totally off base I apologize and will let someone else chime in with the mechanicals. Just want to eliminate something right off the bat. Good luck with the fix.
04-26-2017, 08:04 PM
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If it was a 93, I would say you had the wrong chip in the computer. I had a 93 M6 that was off because it had an auto computer chip in it when I got it. Soon as I swapped the chip to a M6 chip it was spot on. Has anyone done any tuning on it that could have possible messed it up?
04-26-2017, 08:16 PM
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You spelled tire as 'tyre' (Canada, Australia, etc..) which makes me wonder if you are in the US. It almost sounds like it is reading kilometers vs mph. 80 Kilometers per hour is about 50 mph which would explain the speed discrepancy you are experiencing. If I'm totally off base I apologize and will let someone else chime in with the mechanicals. Just want to eliminate something right off the bat. Good luck with the fix.
04-26-2017, 08:18 PM
#5
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If it was a 93, I would say you had the wrong chip in the computer. I had a 93 M6 that was off because it had an auto computer chip in it when I got it. Soon as I swapped the chip to a M6 chip it was spot on. Has anyone done any tuning on it that could have possible messed it up?
04-26-2017, 10:08 PM
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04-27-2017, 01:51 PM
#8
If you bought a new "stock" PCM, then it may not be the same gearing as yours. These cars came with different gearing, like my bro's car has 3.23 gearing but mine had 2.73. When I bought my car, the previous owner put in 3.73 gears but never Flashed the PCM for the gear change so my MPH was 20 off [70MPH actual=90MPH on the speedo]. See if you can find out what the gear ratio is from the person you bought the PCM from and if it is different than your car just have someone change your .BIN on your new PCM with your cars gearing. Chevrolet dealerships can do it by just putting your cars specific .BIN from your cars VIN number on the PCM also. Hope this helps!
04-27-2017, 05:49 PM
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If you bought a new "stock" PCM, then it may not be the same gearing as yours. These cars came with different gearing, like my bro's car has 3.23 gearing but mine had 2.73. When I bought my car, the previous owner put in 3.73 gears but never Flashed the PCM for the gear change so my MPH was 20 off [70MPH actual=90MPH on the speedo]. See if you can find out what the gear ratio is from the person you bought the PCM from and if it is different than your car just have someone change your .BIN on your new PCM with your cars gearing. Chevrolet dealerships can do it by just putting your cars specific .BIN from your cars VIN number on the PCM also. Hope this helps!
i pulled the ecm from a wrecked '94 z28 at a junkyard. let me see what i can do and i let you know