Correct speedometer gear

[Andrew F.]

Hello, I installed a new speedometer cable and gear in my 1966 (194676S119356 ) with a 19 tooth gear. My speedometer is reading slow, at 37 MPH it is really 45 mph, using a hand held GPS. It has a AM rear end at 3.36 . My question is , what is the correct gear tooth for that transmission? Thank you for your help, Andrew Folgmann 67260

[Harry S.]

Re: Correct speedometer gear

Blue one with 20 teeth.

[Duke W.]

Re: Correct speedometer gear

It can depend on the tires. Originally '66 Corvettes had 7.75-15 bias ply tires that were rated at 775 revs/mile, and the OE speedo driven gear was probably 20T. So you have to tell us your tire details, specifically revs/mile. Look at the manufacturer's Web site or a big retail site like Tire Rack. All have specs for specific tires. If you can't find your specific tire look at other tires of the same size. Industry standards require all tires of a given size to have a vary narrow range revs per mile spec, which is actually measured on a tire dyno by manufacturers.

You said you changed both the cable and driven gear. How many teeth did the one you removed have?

Your car could also have had a axle ratio change. You CAN"T take anything for granted on a 55 year old car that you likely have little if any historical documentation.

Regarding speedometer error there are two kinds, - offset and gain error. Offset error is a constant MPH error at all speeds. Gain error is proportional to engine speed. Sometimes it can be a combination of both.

In order to fully characterize the error you need to take error readings over a wide range of speeds, say 20 MPH up to 80, then analyze the data.

Your speedo is reading 18 percent low. One tooth on the speedo driven gear will change the reading about 5 percent, so there's something rotten in Denmark.

Duke

[Richard M.]

Re: Correct speedometer gear

Yes, tire diameter and accurately measuring axle ratio is a must. I went through this many times.

Here are a few calculators which may help.

http://bgsoflex.com/speedo1.html

https://www.monstertransmission.com/Speedometer-Gear-Calculator_ep_122-1.html

A article detailing selection process....

https://www.motortrend.com/how-to/calibrate-gm-mechanical-speedometers/amp/

[Duke W.]

Re: Correct speedometer gear

Forget the online calculators. They don't take into account pneumatic tire distortion that increases revs per mile relative to the rigid body calculation from the tire inflated OD in the unloaded state. You don't even need the computer.

With the GPS, take the car up to 60 MPH true and read the tach as accurately as you can.

Axle ratio = RPM @ 60/tire revs per mile.

Of course, as I said in post #3 you'll need to do a little research on the computer to find the revs/mile spec for your installed tires.

Duke

[Dan B.]

Re: Correct speedometer gear

Another thing to consider is the transmission drive gear when all else fails to cure the problem. There were 2 possible internal gears depending on the rear end ratio that could have been installed. Are the trans and rear original to the car?
I had to pull the tranny to change one of those a while back to cure a mismatch problem.

[Richard M.]

Re: Correct speedometer gear

Dan Yes I worried about one I had here too after it was on the frame. It was a Camaro Muncie on a '67. Thankfully it was the correct internal gear.

Duke, I didn't have the luxury of complete cars when I did those. I had them up and apart on a lift during body-offs. That's the time to get on the computer. Granted, Andrew has a running car so your approach makes sense, but not always.

You stated....
"...pneumatic tire distortion that increases revs per mile relative to the rigid body calculation from the tire inflated OD in the unloaded state."

I have never heard that technicality before. I don't know if he's looking for that much accuracy. That's really accurate.

Regardless, we need to hear from him if he's listening....

Rich

[Duke W.]

Re: Correct speedometer gear

I've compared manufacturers' loaded revs/mile specs to the "rigid body" revs per mile that is easily computed from the inflated OD and have found that the former is about three percent greater than the latter.

There are several error sources in speedometer gearing. Each tooth count changes the calibration 4-5 percent so there is inherent error from the factory, usually high, and then lower that OE revs/mile tires can further degrade accuracy.

As long as I've got an instrument to look at I want it to be as accurate as possible. I've gone so far as to take speedometers to a properly equipped shop to have them recalibrated to bring them within one percent accuracy. The GM spec is 60 MPH at 1000 revs cable speed, but a speedometer shop can change this by altering the magnetic flux of the head to get it dead on if you can tell them what percent alteration is needed.

Duke

[Richard M.]

Re: Correct speedometer gear

Duke, I'm learning. Thank You....

Could you explain "rigid body" versus "inflated OD".

Which of these is the factor used in tire manufacturer's specifications for tire diameter?

Do modern car electronic speedometers take these factor differences into account while driving low vs high speed? I.E. at high speed does the OD increase due to centrifugal force? Is this what you define as "inflated OD"?

Rich

[Duke W.]

Re: Correct speedometer gear

Inflated OD is the OD of the tire inflated - probably to max cold pressure, but with no load.

Rigid body would be something like a steel railroad wheel. Steel deflects under load, but the amount is so slight a steel railroad wheel can be considered to be a rigid body, so the revs/mile is simply computed from pi and the OD. Pneumatic tires defect under load which effectively reduces their rolling radius, which is why actual revs/mi on the road are about three percent greater than using the inflated, no load OD or radius to calculate revs/mile. Tire manufacturers actually measure revs/mile with the tire loaded and rolling on a big drum.

Another tire tech spec is static loaded radius. I believe it's measured at max load and cold pressure as placarded on the tire. Of course, it's always less than inflated OD and using SLR and pi to compute revs/miles will probably yield essentially the same number measured by the above test.

I'm not that familiar with modern electronic speedometers, but I'm sure they can be calibrated more close to true than the old mechanical type like on our vintage Corvettes. I recall there are DOT standards on accuracy. They can read a little high (not sure of the percent) but cannot read low with the OE tires.

Bias ply tires expand with speed, but steel belted radials, especially H speed rated or above that have nylon cap belts expand much less and have little impact on speedometer accuracy, but I can't quantify it.

For a typical tire in the same OD range as OE C2 tires with 10-12/32nds inch new tread depth, revs/mile will increase about two percent when the tire wears down to the wear bars at 2/32nds inch remaining tread depth.

Duke

[Joe L.]

Re: Correct speedometer gear

Another thing to consider is the transmission drive gear when all else fails to cure the problem. There were 2 possible internal gears depending on the rear end ratio that could have been installed. Are the trans and rear original to the car?
I had to pull the tranny to change one of those a while back to cure a mismatch problem.

Dan------


If a 19 tooth driven gear was removed (and it was functioning properly) that would indicate that the "large" drive gear was also installed. That gear is only compatible with 18-21 and 22 (silver) tooth driven gears.

[Ken A.]

Re: Correct speedometer gear

The GM spec for a corvette is 60 MPH=3000 RPM

[Duke W.]

Re: Correct speedometer gear

A '66 with a 3.36 axle and 7.75-15 OE tires (775 revs/mile) is turning 2604 revs at 60 in top gear. The manual transmission speedo drive gear is 8T and the driven gear is 20T. Do the arithmetic for the speedo cable revs at a true 60 and let us know what you get.

Duke

[Joseph L.]

Re: Correct speedometer gear

See the SoCal Chapter February 2021 Bonding Strip newsletter (BSVol41N1) for information on speedometer gears. The answer is there.

Joe

[Dan B.]

Re: Correct speedometer gear

Dan------


If a 19 tooth driven gear was removed (and it was functioning properly) that would indicate that the "large" drive gear was also installed. That gear is only compatible with 18-21 and 22 (silver) tooth driven gears.

I worked on one a while back that had a 3.08 rear (replacement) and an 18 tooth driven gear that "worked" OK with the wrong drive gear, but the speedo still read slow. Pulled the tranny and changed the drive gear and that fixed it dead on accurate. About the same speed differential OP reports, which is why I asked if the tranny and rear are original to the car.