billy mild

http://www.abbott-tach.com/era.htm



According to that site it seems it is for altering the speed? The previous owner mentioned that the car had 3.73 gears in the car. Would that affect the speedo to need one of these boxes?

mike100

for 3.73 there is a factory plastic gear to alter the speed of the VSS (veh speed sensor), but i guess that is a more expensive way to do it. To be fair, you can usually get more dialed in with those pulse signal boxes for exact speed calibration for gears or tire size differences.

you lucked out again on the mods. free stuff that you really want.

billy mild

Is there anyway to confirm I have 3.73’s without taking the diff apart?

mike100

jack up the car and put it in neutral. take a paint pen or otherwise mark the driveshaft on the rear yoke going into the diff. mark the tire too- turn the tire exactly one turn and verify if the d/s yoke turns 3 and 3/4 turns. if it turns 3-1/2 times, you have the stock 3.45's.

this doesn't work unless the diff has limited slip, but vettes all get that so it should be an easy check even without a helper.

billy mild

Has anyone made a gearing calculator that can figure out speed based upon the rear end ratio?

A lot of people do this for motorcycles.

billy mild

It appears the car is around 2000 RPM at 70 MPH. Is that stock gearing?

WVZR-1

What year car and what gear? If your car is a '90 I'd suggest that you get a scanner connected to the ALDL and confirm the accuracy of the tach. What size tires? 315/35R17?

2000 RPM w/3.73 is 4th (40.5), 5th (54) and 6th (81)

w/3.45 it's 4th (43.8), 5th (58.4) and 6th (87.6)

Those are done using a dimension I measured from a 315/17 tire and NOT the advertised dimensions which are generally in error but close.

mike100

has to be tested in 4th gear as that is 1:1 ratio and makes the math easier. Did anybody mention how inaccurate C4 tachometers can be? There is a resistance that gets hot and increases on the gauge circuit- it can be corrected, but even the good ones are 200 rpm off. The difference between stock and 3.73's would be about 200 rpm at 60.

My earlier suggestion leaves no doubt, but you do have to jack up the rear of the car.

WVZR-1

I always use 4th 1:1 when I do any speedometer calculations etc. but with the ratios known of 5th & 6th the calculation isn't difficult. I like 4th for throttle control purposes. Most people that I know can drive more consistent speeds/RPM's when in 4th or lower gears. Using 5th & 6th gets certainly tougher. When we had to do measured miles for calculations it was certainly tougher, GPS makes it much easier, and our math was all done with paper and pencil.