Koz

Here we go again. My speedo is off by 20% so it seems that I need to change the speedo gears. I have a 19 tooth gear in there now. How many gears do I need to makup the 20%. There was a post a while back talking about percentages per tooth.. but of course I can't find it.

Thanks in advance.! :cheers:

gerry72

It's 5mph per tooth. Once you know how many teeth you need you just look up the part number and head to your Chevy dealer's parts counter.

Fevre

You have to pull the speedo adaptor out of the trans to see what color gear you have then you go from there.

Koz

Thanks for the input but I'm color blind.. I thought it went by teeth.. I have a 19 tooth gear now.. Since it was determined that I have a 20% variance.. 20% of 30mph would be +6mph.. 20% of 60mph would be +12mph.. How many tooth gear would you use? The local dealer said to try a 21 tooth gear and see how that works.. Said they weren't too expensive. I rather get a concensious here before buying anything..

gerry72

You're dealing with a gear ratio. The percentage calculation won't work here. I'll write it again: EACH TOOTH REPRESENTS 5MPH. This is whether you are travelling at 10 mph or 100 mph. If your speedometer is registering 5mph faster than your car is travelling, you need to slow it down by adding one tooth or in your case going from 19 to 20 driven teeth. For the in-between ranges, the recommendation is that you go to the next gear that will mean your car is slightly slower than the speedometer reading.

It's hard to say which way you need to go as far as teeth. Is your speeometer faster or slower than your actual speed?

Koz

Speedopmeter is faster than actual speed by apx. 20%. I thought I read that if I'm off by 5mph across the board then I could adjust the needle on the speedo but if it's off by percentage then you need new gears.

I used one of the stationary radar detectors that flash your speed when you drive by. I was doing apx. 36 on speedo when it read 30 but on the highway I have to do apx. 65mph to keep up in 55mph traffic. That's where I got the 20% variance from.. :confused:

Fevre

I'm with Koz, the relationship would be proportional not linear. For simplicity say the gear in the trans has one tooth and the drive gear has 10 so it takes 10 revolutions of the trans gear to make the drive gear rotate once, add 1 tooth to the drive gear and you have slowed it by 10% since now it takes 11 revolutions of the trans gear to make the drive gear rotate once. If it took 60 revolutions of the trans to register 60 mph with 10 teeth, adding the one tooth would cause it to take 66 revolutions. At 30 mph it would take 33 revolutions with the added tooth keeping the 10% relationship.

gerry72

If your speedometer is registering about but no more than 10 mph fast at around 60mph you need a 21 tooth gear. You can do the math for the ratios but what you're trying to achieve is a cable rotation of 1000rpm for exactly 60mph indicated and actual velocity.

Fevre

Well you didn't state to use 60 mph as the base. Also I am not sure about the vette trans' but for T5's there are diff trans gears so that might throw off it off if in fact there are diff trans gears avail.