[motor29]

I drove past a police speed indicator the other night, my speedo said 30 the display said 46 mph. I took the car on the highway the other night speedo said 90 my friends newer car speedo said 115. The higher the speed the more the speedo is off, how can I fix this ?

[Andretti]

You need to change the nylon drive gear at the transmission. Depending on the year & transmission they are color coded to reflect the ratio.

[motor29]

Thanks alot I will check into that. Who carries these gears?

[69convt]

GM as well as others.

[Cammer-6]

may need a ratio adapter too.

[PontGuy]

There is a hold down fork that holds the speedo gear housing where the cable attaches to the trans. Remove the screw, remove the fork and just pull the housing out. This is easiest with the cable still attached. Then pull the gear and count how many teeth are on it. Given that number and the ratio that the speedo is off you can calculate how many teeth you need and what you need to buy. Usually it is just a gear, but sometimes a new housing. These are readily available and cheap. A ratio adapter is usually not required unless big changes to the rear end gearing/trans have been made.

If everything is working right the ratio off should be the same regardless of speed. If you are reading 30 and the radar said 46 then the ratio is 30/46 or .65, and if you got good readings compared to your friends car then 90/115 is .78, so one or the other is not accurate. The easiest way is to use a GPS that shows speed (if you don't have one maybe you can borrow one) and cruise down the highway to get accurate numbers.

If you tell us what kind of trans it is, the number of teeth you currently have on your speedo gear, and accurate speed comparison numbers then we can tell you exactly what you need to do.

Walt

[motor29]

Thanks for all the detailed info, my trans is the original m20 4 speed (rebuilt a few years ago). I did just change the rear end went from a BOP 3.55 non-posi (gears were shot) to a 8.2 gm 10 bolt 2.73 gears from a 70 chevelle. Can you figure the speedo gear I need by this info or do I still need to pull the speedo gear from the trans. Thanks again.

[PontGuy]

That makes sense, since the change in the gear ratio is 2.73/3.55 = .77 which is almost exactly what you got comparing your speedo reading to your friend's.

Unless someone here knows what the correct speedo gear for an M20 with a 2.73 axle is you are still going to have to pull the gear and count teeth. Multiply the number of teeth that you have times .77 to get the number of teeth that you need. For example if you have 26 teeth now then you will need 20. A speedo gear only costs about $5 so if you are still off by a bit you can increase/decrease by 1 tooth or so without spending a bunch of money. I've done this just to reduce error caused by putting big tires on a car.

Its been a long while since I had a Muncie so I don't remember the range of teeth available. Also I think GM may have used different speedo drive gears in the trans for different rear end ranges, which may limit you without going into the trans or buying a ratio adapter. But my bet is that all you will need is a new speedo gear.

Walt