Speedometer Off – Revisited

Q: Speedometer Off – Revisited

I have a 1968 Bird and the speedometer is off 6 miles an hour fast. I have 14′ rims and 215/70/14 tires. I’m missing something here. Could someone tell me why its off? I have the 160 speed-o. Is that whats off, or is it something else?

A: I suspect the speedometer gear drive in the transmission is not correct for the current combo (tire size). Just because your running 14″ tires doesn’t mean that they are the original profile. You can try to fix it yourself by changing the drive gear or there should be a shop in your area that can do it for you and certify the speedo. Another option would be to go with reproduced originals instead of correcting the speedo.

A: Same problem here but fixed with a different colored drive gear in the trans, I cant remember which color slowed my speedo but they are bout eight bucks apeice and can be found at most gm dealers. It is the little plastic gear that is on the end of the speedo cable that conects to the trans.

A: Unless your trying to keep the car 100% original, it makes more since to get the tires you want and then correct the speedo. If you want to go to a comparable tire size as original, then check out: http://www.wrljet.com/tires.html

A: I agree with whoever said to pick the tires you like, then calibrate the speedo. But if you want original size tires here’s some advice.

Originals would have been E7014 or F7014, probably F7014. Here are some sizes that are similar:

A: If you have 215/70/14 tires, they should be pretty close to your originals and shouldn’t make you off by more than 1% or so. I would guess then that your speedo is off by 3.23/3.08 or 5% making it read about 74 when you’re driving 70. I’m going to include here an answer I posted on the classical pontiac site for a guy that swapped to 3.73 gears in case it helps.

I just did this for a 2.56 to 3.23 swap in a TH350. There are three ways to do this.

For the first one, find out exactly how far off it is… try using the odometer to check how many miles you read when traveling 10 miles according to mile markers. You should get a ratio like 2.25:1 (based on your 90 MPH at 40 MPH guess). Next, pull the speedo gear and housing out of the trans. Count the number of teeth on the gear (driven gear). Also, count the number of teeth on the gear it mates to on the output shaft of the trans (drive gear). Take the number of teeth on the driven gear and divide by the number on the drive gear. Lets say you get 35/19=1.84, now multiply by how far off your speedo is (like 2.25) and you get 4.14 (in this example). This is what the final driven gear to drive gear ratio needs to be. You may also need to change the driven gear housing since they only house ranges of gears like 36-39, 40-43, etc.

The second way to do this is to use one of these formulas:

Drive Gear = (.0495835 * Tire Dia * Driven Gear) / Gear Ratio.

OR

Driven Gear = (20.168 * Gear Ratio * Drive Gear) / Tire Dia.

And here’s a third way, call GM part supplier. They’ll want to know Gear Ratio, Tire Size, type of Trans and then he can give you GM part numbers for the gears and housing.

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