The Stovebolt Site Forums The Shop Area Engine Shop Slow Speedo

I am running a NWC T-5 in my '63 C-10 and the speedo is about 7-8 mph slow best I can tell. I clocked the truck against a GPS unit and with 60 showing on the speedo the GPS was showing 68. Is there a way to calculate which driven gear I need or do I have to just start buying gears and trying them out?

If the speedo is slow get a gear with 1 or 2 fewer teeth. Compare the odometer tenths with the mile marker on the interstate. If you get the odometer calibrated pretty close the speed will probably be right.

So the only way to determine the correct driven gear is to get a handful with various tooth counts and try them until I find one that's close?

If you have a speedo shop in the area they can adjust your speedo. I don't know how they do it but mine was adjusted 10 MPH.

No not really, but yes, sort of. You know you need to speed up the driven gear by about 12-13 percent. So you need a driven gear with 12-13 percent less teeth (1 or 2).

If you don't want to take your tranny apart, you can get a ratio adapter to correct your speedo. It mounts on the outside of the tranny on the speedo cable input. I see that there's a bunch on Ebay.

Everybody, and I mean EVERYBODY that builds old cars/trucks needs to keep this site on hand. http://www.700r4.com Just measure the heigth of your rear tire and know your rear end ratio and go to "calculate speedometer" on this site and it gives the exact gears you need--the drive and driven gears. The drive gear on the tailshaft requires taking off the tailhousing but that not a hard job, and it works every time.

Coilover, what you mean to say is that EVERYBODY that uses 700r4s on this site should keep that on hand. That calculator doesn't work for every transmission's speedometer gears.

Thanks for the input guys. I'll look at the ratio adapters.

YES IT DOES WORK WITH EVERYTHING, your speedometer gear is on your transmission OUTPUT shaft. If it wasn't the speedometer would read wrong on every gear but high or overdrive. It monitors driveshaft RPM, not engine RPM. The engine rpm will rise and fall with the trans in different gears but the only thing that counts for speed is driveshaft rpm. Only two things will cause a varience, differant gearing or tire size change. Your driveshaft will turn faster or slower depending on the rear gear ratio so if you change it the speedo will read wrong. Same for rear tire HEIGTH, width means nothing. In other words the ONLY thing that counts is driveshaft RPM, the speedometer doesn't care if the driveshaft is being turned by a three on the tree or a double overdrive 6 speed Tremec.

The math is correct, but the actual gears do not interchange. It only works with everything that those gears are available for the transmission you are using. You can not find all the gears suggested by that program for every transmission.

For instance my SM465 would need a 15 tooth drive with a 41 tooth driven with 6.13 rear gears and 33 inch tires, but unfortunately there is no 15 tooth drive gear or 45 tooth driven available for the SM465. However a 6 tooth drive with an 18 tooth driven are available.

If I had a 3.08 rear with a 29 inch tire, I would need a 17 and 36, unfortunately on my transmission I would have to settle for 8 and 17 speedometer gears.

Does that make sense?

Texmo,

Check your odometer against mile markers before doing anything else. If the odometer reads close to to correct, you need to have your speedometer calibrated rather than getting an adapter or new speedometer gears for your transmission.