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
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 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
8 tooth ORANGE drive gear for a T350 automatic transmission (moulded in the wrong color!)
Speedo cable unbolted and driven gear sleeve removed
Old driven gear sleeve
New driven gear sleeve
Tailshaft housing removed - you can see the end of the speedo cable also, sorry but the drive gear is already removed in this picture
Drive Gear Clip
Drive gear clip in place on the output shaft
Drive gear in place on the output shaft
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.
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