Speedometer high by 12.676% ?

[Silver01NC]

My car is supposed to have a Terminator IRS with 3.55 gears instead of the '01 3.27, so I expected my speedometer to be off. The difference between 3.27 and 3.55 is only 8.563%, so I'm thinking "80mph" on my speedometer would actually be 73.7mph. Most of my driving is 10 miles each way 45-55 on local roads so i really didn't think it was a big deal. I will say that 1st gear always seemed a little short but the pull in 2nd was nice.

I had a chance to test the speedometer last week, and my "80mph" is actually "71mph" as measured by the digital speedometer on a brand new CTS. That's a 12.676% difference, and a lot closer to a 3.72 gear ratio over 3.27 (+13.67%).

Does this sound like 3.72 gears to you guys? Does this mean my mileage is increasing 12.67% faster than actual, too, and depreciating the car? Does anyone know if the "3.27", "3.55", and "3.72" ratios are accurate, or if "3.27" is really 3.2675? It bothers me that the 3.72/3.27 ratio does not match 80/71mph closer.

Can the dealer reset the gear ratio on the computer? or do i need a handheld tuner to do that, or a Speed Calibrator card installed?

thanks -

pix -

 

[Uncle Meat]

A speedcal would be the cheapest & easiest way to correct your issue.

First you need to determine what your rear-end gear ratio really is.

The only 100% accurate technique is to count the # of teeth on the ring and pinion gears, and divide them. I'm sure you don't want to go to that extreme so try this instead...

1. Put rear on jack stands (safety first!)
2. Wrap a long cable tie around the driveshaft, but don't cut the 'tail'. Point it straight up at the 12:00 position.
3. Put a mark on a tire, like at 6:00 position.
4. Put the trans in Neutral.
5. Turn the wheel exactly 1 revolution, noting how many times the driveshaft turns by watching the cable tie. The key is to be as exact as possible.
6. Note the position of the cable tie tail.

Now, if the cable tie made 3 revolutions, and is at about the 1:00 position, then it's likely a 3.08 gear. 3.5 revolutions, then a 3.55. 4 revolutions, 4.10, etc.

Not 100%, but should get you close. The bummer is that it'll be difficult to tell a 3.23 from a 3.31, etc.

U.M.

 

[Silver01NC]

thanks UM

i can do that at the next oil change.