VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1937-1945 Transmission Speedometer Gears

I have a stock '40 3 speed trans. mated to a 3:55 rear differential. I would like to correct for speedometer error due to departure from stock 4:11 rear diff. I've been unable to find trans. driven gears from vendors such as Filling Station, Chev's of 40s, Summit racing, TCI, etc. Would a driven gear from a '53 power glide trans, originally mated to a 3:55 be a potential solution? Or should I be looking to source a driven gear from an earlier GM 3 speed trans? Would welcome advice on sourcing driven gear.

When I changed the gears in the rear end of a '58 Chevy panel truck (many years ago) from 4:11 to 3:38, a local automotive electrical shop (who also worked on speedos) made me a small gear box that went between the speedo cable output of the tranny to the speedo cable itself. The input to the box just screwed onto the tranny and the output had a screw-on connection for the cable to mate to.

The gears in little box (by little, I mean roughly 1" x 2") corrected the rpms of the speedo cable so the speedometer read correctly.

Hope this helps, Dean

Any speedo shop should be able to help with the item Dean mentioned. I forget what they're called.

You can also order these ratio adapters that mount at the speedo cable end online if you look around. You just provide them your actual speed with a GPS vs your speedometer reading at the same point in time. They can custom make them or have various standard ratios once they know what percentage your speedo is off.

The Power Glide gears will not work in a standard transmission. I would go for an adaptor.

Thanks to all for your input, very much appreciated. I will now pursue a ratio adaptor to correct speedometer reading. My vehicle is under repair at the moment and once it is back on the road I'll clock my actual vehicle speed against speedometer reading to confirm variance. But in the meantime, I'll start searching for folks that make the adapters. Bob

For what its worth, speedometers are calibrated internally to the odometer. The best way to set up speedometer gearing is to the odometer.

If you have any plans of changing the tire size to something different than it is right now, do that first. Try driving 10 miles or more, watching the mile markers. Note how much the odometer gains or loses, and divide that down to a percentage. Do it more than once to make sure you don't get fooled by a mile marker in the wrong place. Use that to decide how much change you need.

If the speedometer is still wrong, that is a speedometer problem, not the gearing.

The company that makes the small gear unit the screws onto the speedo cable attachment on the tranny is: Instrument Sales and Service in Portland, OR., 800-666-5469 or 800-333-7976. Your speedo cable then screws onto the output of the gear unit. I bought one years ago and it was very accurate. They will have you drive 5 miles, like on a road with mile markers and then read what your odometer says for the 5 miles, that’s how they figure the gear ratio.