Calibrating the Speedometer

My speedometer was right out to lunch. It read about 30 km/hour fast (I'm in Canada) at 100 km/hour which for those of you using imperial is about 75 mph when I'm actually going 60 mph. You calibrate your speedometer by changing one or both of the speedometer drive and driven gears located in the transmission tailshaft housing. I came across two extremely helpful websites that aided me in figuring out

I used this NovaResources.com's Recalibrating GM Speedometers to figure out what driven/drive gear ratio I should be using. The drive gear is located in the tailshaft housing and the housing must be removed to change the gear. The driven gear is mounted where the speedometer cable enters the tailshaft housing. A single small bolt holds it in place.

I needed to know my tire diameter based on my tire size of 255 - 60R15 and used this tire diameter calculator to figure that out. It worked out to 27 inches.

I know my rear end ratio is 3.08 and using the Nova Resources formula (rear axle x 20.2 / tire diameter), I calculated that I need a driven/drive gear ratio of 2.30. I looked up on their page and found that a 10 drive gear and a 23 driven gear will work. I chose the 10 drive gear because that's what happens to be in my T350 tranny right now.

10 tooth drive gear Blue for a T350 automatic transmission

Note: Each tranny model will have its own unique drive and driven gears - more on this later.

To find out the GM part number for the 23 tooth driven gear, I went to the Chevelle Engineer's Speedo Gear Selection page. He lists the speedometer gears for the most common muscle car transmissions I found that a Black 23 tooth driven gear was available - I went to the local GM dealership and picked one up for $10.50. Installed it and now my speedometer was only 10% out! What the heck?

23 tooth driven gear Black for a T350 automatic transmission

I found out that most speedometers are designed for 1000 rev/mile but my speedometer is in km and Canadian which always screws things up. Some speedometers are designed for 900 rev /mile and guess what - mine was. Some speedometers will have the 900 or 1000 calibration setting printed on the front or back of the gauge. Mine didn't. So back to the drawing board!

I found out that there was a catch! There are two diameters of drive gears - my 10 tooth drive gear (1.84" dia) would not work with a Black 23 tooth driven gear (1.76" dia)! Eventually, one or both of the gears would get chewed up as one is meant to work with 1.76" diameter drive gears and the other 1.84" drive gears. There isn't a 1.84" 23 tooth style driven gear available so I have to replace both.

For a T350 tranny, adding one tooth to the driven gear reduces the speedometer display by 5%, I am out 10% so I need 2 more teeth on the driven gear or 25 teeth. The driven/drive ratio is 25/10 or 2.50. Unfortunately, there are no driven gears above 21 available for my 10 tooth drive gear so back to the Recalibrating GM Speedometers page to find a gear combination that would work.

I found several combinations that would work within 2.5% error and one that worked out exactly 20 tooth driven gear (blue)/8 tooth drive gear (orange). The GM part numbers looked good and a visit to Mr. Speedometer got the 20 tooth blue driven gear. They didn't have the drive gear and a visit to the local GM parts dealer resulted in finding out that most of the T350 parts are no longer available

20 tooth driven gear Blue for a T350 automatic transmission

So now it was to plan B (or was that plan C now?). I phoned up Mr. Speedometer and asked if they had my next choice of gears which was 21 tooth driven gear/9 tooth drive gear for a ratio of 2.44 which is 2.5% off. Yes they had, so on the road again it was! Now I was getting pretty paranoid so when I checked out the Green 9 tooth driven gear, I counted the gears. Guess what! It was wrong - it had only 8 teeth! It was the ORANGE drive gear but moulded in GREEN plastic! So I picked it up.

8 tooth ORANGE drive gear for a T350 automatic transmission (moulded in the wrong color!)

Now, I was ready to change to the new speedo gears. First I removed the speedometer cable and the driven gear sleeve with the driven gear from the transmission.

Speedo cable unbolted and driven gear sleeve removed

Old driven gear sleeve

The old driven gear sleeve leaks where the driven gear connects to the speedo cable. The seal is worn and I couldn't find a source for one. I had to purchase an aftermarket replacement for $35.

New driven gear sleeve

To remove the tailshaft, you have to remove the driveshaft. Unbolted the driveshaft universals at the diff and pulled it out. Forgot to mention - expect to get about a liter of tranny fluid dripping everywhere. Next was removing the tailshaft - 4 bolts and it was off, pretty simple.

Tailshaft housing removed - you can see the end of the speedo cable also, sorry but the drive gear is already removed in this picture

The drive gear (not shown above) is held in place by a clip which is located by a tiny slot in the tranny output shaft. The old drive gear slid off easily, once I figured out you had to release the clip by pushing down on the edge. The new drive gear was a tight fit and had to be gently tapped with a hammer and dowel to fit. It was too tight to push on by hand.

Drive Gear Clip

Drive gear clip in place on the output shaft

Drive gear in place on the output shaft

Re-installed the tailshaft housing, slid in the driveshaft yoke and connected up the driveshaft to the diff. Lastly was to install the new driven gear and sleeve.

Driven gear in place on the sleeve

Driven gear and sleeve installed in tranny. Notice the slot. There is a bar held in place by a small bolt - this holds the sleeve securely in place.

Took the beast out for a test drive and the speedo is now dead nuts on and there are no leaks. TaDa!

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Copyright July, 2011 Eugene Blanchard