: speedometer calibration

03-20-05, 03:07 AM
Well, I found out tonight that the speedo in my Caddy is off by about 20% No wonder I've always felt like I'm flying... :eek:

Does anyone have the info to figure out how many teeth are on the speedo drive gear for a '70 TH400, and how to figure out how many teeth should be on the driven gear?

03-20-05, 10:32 AM
do you have stock rear end gear and tire size??
those can throw off the speedo too. and unless some of the teeth broke off i can't imagine the teeth would just go out of spec that far. what have you done to the car?

03-20-05, 10:52 AM
Well, I haven't changed anything in the driveline. The car came with 235/75-15 tires on it, which I believe is pretty much the stock size, right?
I'm thinking either some teeth have worn off the driven gear, it was replaced with the wrong gear at some point, or the speedometer head has a screwup.

We tested this timing the car between two highway milemarkers. With the speedo steady at 60, my fiance timed us 50 seconds for one mile which is 72mph. I've also noticed that I tend to pass ALOT more people in this car than when I am driving other cars that I own. It kinda seemed like I was going faster, but it wasn't like I was having to mash the gas all the time, or the ride felt like I was going fast... LOL

03-21-05, 01:33 AM
dont feel bad, my speedo is so bad that it is showing in KPH what i am doing in MPH.

03-21-05, 11:18 AM
wow, that's a long way off! Unfortunately, they only put MPH on the speedo back in '70.

Hopefully I'll be able to get to checking the rearend ratio and speedo gear teeth later today, and see if everything matches there.