Speedo & new gears

Does anyone know if it's possible to figure out how far off my speedo is now that I have 3.73s in place of the original 3.45s? Also, is it safe to assume that my odometer is off my the same margin?

I tried using some speed calculators on the net (specifically from thirdgen.org) to get an idea of how fast I'm going, but it only appears to be accurate for whatever gear in your tranny has a 1:1 ratio (which happens to be 4th in my T5). Is it possible to find out how fast you're going using similar calculators, but using different gears (Ex: how fast would I be going in 5th @ about 3000rpm, assuming stock tire height).

I always figured my speedo was telling me I was going about 5mph faster than I really was, but the speedo isnt very accurate anyway (If you hold a constant speed, the needle kinda waves back & forth a little).

Any help would be appreciated. I have a feeling that when I get an answer to this, I'm gonna feel stupid. :bow:

The easy option (for now) is to use the gear calculator for 4th gear to find the speed you want, then, shift to 5th and see where the tach falls. Otherwise, the answer will depend on if you have a electronic speedometer or a cable speedometer. You can get calibration boxes to adjust the electronic versions. The cable type may need new speedo gears to correct it. Being an 87, and the wavering speedo needle makes me think it is a cable type.

What are you using for your drive and driven gears in the tranny and what size tires?

i just swapped gears to, when cahnging from 2.73 to 373. your going slower then your speedo registers i assume. so is thier a subtraction equation you can do in your head while driving like maybe about 20mph over or so, let me know guys im having the same problem and dont want to change the gears at the moment, im working on headers

It will be off a percentage. At 0 mph, it is right, the faster you go, the farther off it will be.

my 83 z28 has the exact same problem, i'm not sure what was originally in the car, but when i got it it had 3.73 limited slip and the speedo is way off, it also kinda bobbles around and it seems about 20 mph too fast at about 55 mph(reading about 75-80). any way i could fix this easily?(as in cheap and not too difficult) thanks
Dustin

Originally posted by WhiteLightnin83
my 83 z28 has the exact same problem, i'm not sure what was originally in the car, but when i got it it had 3.73 limited slip and the speedo is way off, it also kinda bobbles around and it seems about 20 mph too fast at about 55 mph(reading about 75-80). any way i could fix this easily?(as in cheap and not too difficult) thanks
Dustin

Ok guys come on.:rolleyes: Just go to a tranny repair shop and theyll hook you up with the parts you need. Itll be cheap and easy. Things like this dont need to be worried about.

Dennis

seems about right lightning, ill just subtract 20 off i guess

dieseldennis is correct, your average transmission shop can fix you up or you can get the gears (if still available, I used to buy them at the dealer) and change them yourself.

THey are still available from GM, I just did mine a few weeks ago...if your gonna get a new drive and driven gear, get them from GM, you get a whole new carrier already assembled.

about $38 total.

If you buy gears for a 700R4, buy the clip that holds it to the shaft. Everyone breaks the first one they remove.

You'd probably have to change both drive and driven gears for a 3.73.

All gm speedo`s work at 1000 cable rpms per mile. There are a few ways to determine what is needed. Easiest is to have a gps with you while you drive and figure how fast or how slow it is. To decrease the reading you increase the driven gear tooth count and vice versa for increasing your reading. Every tooth change is worth 2% so 4 teeth would give you a 8% change. 3.73 can be done without reduction boxes, you`ll need to change your drive gear on the tail shaft..big PIA but it`s nice to have a speedo that`s at least close. Turbo 400 driven gears are the same as t-700, all the gears are still avaliable at gm.

here`s something that you can plug your #`s into to get going
http://www.thirdgen.org/newdesign/tech/calculations.shtml
I`ve got all the part #`s if you need