speedo is off

[Buzzzz]

Had a meeting with the new england chapter (hi guys) this weekend and mentioned that my speedometer is reading faster than I'm actually going. When it says 80, I'm actually traveling 65 (I've checked this by driving along side a second car). Everyone there seemed to think that the previous owner must have swaped gears. Do you all agree? Would there be any other reason the speedo would be off? And if so, does this give me an HP gain over a bone stock car? (Just curious). It has a lot of snap to it, that's for sure.

 

[Snake Bitten]

Check you RPM's verses a bone stock car traveling at the same speed, if you have higher RPM's at the same speed, you have lower gears...

 

[Buzzzz]

I don't have any cars in my neighborhood to compare with. Does anyone have approximate RPMS for bone stock cars? I could compare mine with that.

 

[1TONY1]

Jack up your car and check driveshaft revolutions verses one tire revolution to see what your gear ratio is.

 

[JonB]

Thats a pretty big error. Does not fit 3.55, but is very close to a 3.73. I suspect your car originally came from Fitzgerald? Those guys LOVED to stick 3.73's in Gen 1s.

Other bad news: Your odometer is off by the same amount.

It records 80 miles for every 65 you drive

It records 8000 for every 6500.

If you can compute your error EXACTLY with another vehicle beside you, I can provide a correction box, pre-calibrated for figures you provide. I would prefer your readings at 3 bracketed speeds versus a KNOWN-CORRECT SPEEDO.

When I show 30, the control car shows _______
When I show 60, the control car shows _______
When I show 90, the control car shows _______

25, 50, 75 OK too...........

 

[Buzzzz]

Hi John, no it didn't come from fitzgeralds. I'll take it out and get those exact numbers with a control car and give you a call.

 

[1TONY1]

Why not use a gps to get perfect mph instead of another car that MIGHT be correct.

 

[joe117]

Here's one of many websites that you can use to do the math for you in determining what rear end you have. They list the ratios for the Vette 6 speed and I think they are the same as Viper. I know the 4th is 1:1 on both. Just make an accurate note of your speed in 4th at some accurately noted rpm. The rear tire data must be figured too. I think you can do that on this page too. http://www.angelfire.com/fl/procrastination/rear.html

 

[Ron Hickey]

By my (rough) calculations, a speedometer reading of 80 mph would be 71 on 3.45's, 69 mph on 3.55's, 66 mph on 3.73's, and 61 mph on 4.01's. This makes the assumption that the speedometer was 100% accurate with stock tires and 3.07 rear gears.