Another Speedo Gear Question

[johnsonsinutah]

Sorry, I searched but couldn't find an answer to my question, so here it is. I have a 98 GT convertible and just changed to 4.10 gears, I had a 23 tooth speedo gear installed at the same time. Driving home I had my wife follow me and give me her speed and it appears that at 80 MPH on my speedometer I was going only 70 MPH actual speed. Does this seem about right in terms of the variance? I assume the variance is proportional to some degree so is there a formula that I can use to convert it into percentages? Additionally, and for another day, can I use a speedcal on my car?

Thanks..as always..

 

[Pearl02]

How many teeth was on the original speedo gear? What ring gear was in the car before? Pearl02.

 

[jomull55]

what color is the speedo gear? auto or manual trans? i think i have one for 4.10 laying at the house i can count the teeth on it when i go home

 

[COramprat]

If you have the orange gear that is for the 3.27 stocker gears. The white one is for 4.10's. If you want to really take care of the issue go ahead and get a Speedcal. I had the 4.10 gear and it tore up after a year.

 

[281CI96Coupe]

+1 For a speedcal. I have it on my car.

 

[johnsonsinutah]

Sorry, I have an automatic, all of the guides I've been able to find indicate that there isn't a speedo gear that will correctly compensate for 4.10's, the 23 tooth gear I had put in is supposed to be the closest. Using one of those gear calculators, it came out showing I needed a 25+ tooth gear...which they don't make. I'm ok with the speedometer off, I just need to figure out what meant by the "closest". Does that mean 5% off, 10% off, 15% off??? I'm currently in the 10% off range at 70 MPH (true speed). Also, I"m trying to determine if the 10% off is consistent throughout the speedometer range, by that I mean when my speedometer shows 40 MPH, does that mean I'm really doing 36 MPH. I just need some kind of guideline to mentally compensate. I don't want to be driving 25 in a 35 and not know it!!

 

[2000GTdriver]

Looks like speedo error is actually about 12.5% and that should be consistent throughout your speed range. Of course that depends on the other cars speed being right. A good test is to drive on a highway with mile marker signs and compare your odometer distance reading to the actual mile markers. That gives you a correction factor that does not depend on the acuracy of another cars speedometer.

 

[johnsonsinutah]

Thanks for the input, I found a formula somewhere and I think my speedometer variance is 15%, the speedo gear I used should correct for the change from 3.27 gears to 3.55, leaving the unadjusted variance the difference between 3.55 and the 4.10's I installed. I think I'll have to breakdown and get a Speedcal, I do enough math at work!