Speedometer gear change (up teeth or down)?

Ok guys I have a sbc 350 with a powerglide and 3:73 gears and mt speedometer is funning high by 8-10 mph . Do I get a gear with more teeth or less teeth to correct this. And how many teeth do I go up or down to be correct or at least a lot closer?

 

8 to 10 mph at what speed?

If it's at 60 mph, then that's about 9/60= .15 so it's 15% off. You need to slow it down, so you need more teeth on the driven gear. If it has, say 22 teeth now, you'd need 22 x .15 = 3.3 more teeth on the gear, so you'd get a 25 tooth gear. If they make one.

otherwise you get to find a drive gear (on the output shaft) with fewer teeth. Or get an adapter box made.

Or if it's always off by that many mph, and the odometer works right, just take the needle off and move it 8-10 mph to the left.

 

At 55mph ( following my wife at 55) it reads 63-65mph. Sorry forgot that

 

You need a few more teeth on it, 15% would be a good place to start. You gotta pull the gear and see what you have first though....

also I like to get the odometer to read correctly first, then adjust the speedometer itself if it is still off. If the gears in the trans are right, the odometer will be right.

GPS is really handy for this

 

Thanks squirrel I'll pull the gear to see what I've got

 

A good link.
http://www.transmissioncenter.net/speedometer_calibration_______va.htm#350

 

Each tooth will change it about 2 1/2%. Less teeth to gain, more teeth to loose on reading.

 

Ok I have a 20 tooth gear and my rear tire size is 215-70-15 , how many teeth should I go up?

 

Try a 22 tooth gear

http://www.jegs.com/i/TCI/890/880004/10002/-1?parentProductId=762271

 

I did the math by your formula and came up with a 23 tooth gear did I do this right?

 

Yeah, you did. but I looked up the gears and the 23 tooth is bigger, and requires a different drive gear to mesh with it properly.

 

Ok I'll order it tonight thanks squirrel .

 

If you can't get it right with the trans gears, years ago SW made boxes to change spedo readings. They were used mostly on GM 4x4 pickups. If you can find one of these, that might be the way to go

 

You need more teeth on the gear if your speedo is too fast. That slows the gear down. I do not know for sure but I just went from a 30 tooth to a 26 tooth on my Olds and the difference was about 8 mph. in the opposite direction you are going ( my speedo was slow) So... From my experience. Up the tooth count Approx. 2 per 1 mph.

 

I have run into this as well--off too far for gear to help-I call Com'l Speedo in Sacramento and tell them what the speedo reads at actual 60 mph-they then make a gear box and mail it to me--works when gears won't quite get you there-also many listed on ebay

 

Well I put a 22 tooth gear in today and it's just about perfect thanks to all.