speedo calibration

dcdoug · #1 (**permalink**) 11-07-2009, 01:14 PM

I have a Tremec 600 and a 3:31 rear. When I got the car it had a 17 tooth driven speedo gear and the speedo consistently read about 6mph lower than actual speed. So I thought I needed to go with less teeth to make it read faster and installed a 16 tooth driven speedo gear and it was slow by more like 10mph. So I thought I had gone the wrong direction and installed a 19 tooth gear and it seems to still be slow by about 10mph. I am using a GPS for actual speed and it might not be perfect, but it does seem to be fairly consistently 10mph slow. How is the above even possible?

Is there something else going on here that I don't understand?

Gunner · #2 (**permalink**) 11-07-2009, 01:50 PM

Quote:

Originally Posted by dcdoug

Is there something else going on here that I don't understand?

Well, the answer to that is an obvious yes.

All things being equal, you should have seen about a 6 percent gain in displayed speed with the first change, and a 10% drop in displayed speed with the second. But you knew that.

Do you have a reversed speedo? Is the reverser unit possibly causing some interference?

Do you have a steady speedo or a bouncy one? The sticking that causes bounce might be contributing to a misread.

That's all I've got on tap. BTW, most GPSes are accurate to better than a percent on freeway speeds.

MaSnaka · #3 (**permalink**) 11-07-2009, 05:42 PM

DC,
Just a wild thought here. Are you sure the gears are correct being left hand or right hand. When my top loader was out being rebuilt someone before me had put a speedo gear with the threads in the wrong direction. Being nylon they just cut their own path and spun the speedo fairly close to right, so I never questioned it. That might explain the inconsistant change with yours. Like I said...a wild thought.

John

dcdoug · #4 (**permalink**) 11-07-2009, 09:09 PM

I do have a reversed speedo and my next thought was to look at the reversing gear on the back of the speedo. I think I need to go back out and find a longer straightaway and take more exact readings on both the 19 tooth gear and the 16 tooth gear. There has to be more different. It was a bit ad hoc the way I did it, but I really didn't think I would get such a strange result. I don't really get much speedo bounce, so there isn't much interference and the 17 and 16 inch gears I changed out didn't seem to have any strange wear marks, but I will also look at them more closely. It's a place to start anyway. I will also make very sure the cable was fully inserted into the 16 tooth gear and there wasn't any slippage, which might also account for the result.

Gunner · #5 (**permalink**) 11-07-2009, 11:05 PM

Re-lube the cable well while you're at it. Never hurts, might help.

dcdoug · #6 (**permalink**) 11-08-2009, 12:31 PM

OK, I put the 16 tooth gear back on and made sure that the cable was properly seated into the driven gear. I think that must have been the problem or maybe something else was interfering, but anyway now it reads as accurately as I think I am going to get it. Problem solved.

Gunner · #7 (**permalink**) 11-08-2009, 08:05 PM

The reflection of gamma-rays from the surface of the moon must be correctly aligned.