The Stovebolt Site Forums The Shop Area General Truck Talk Odometer Accurate, Speedo Not So Much

I recently purchased a cleaned and lubricated speedometer for my '48 GMC 1Ton Dually flatbed using 7.00x18 tires. A drive around my looping street clocked at 1.3 miles, matched by the reading in a different vehicle. The indicated speed is way off, reading about 12.5 mph when the correct speed, observed by following a helper in the other car, was about 30. An indicated 19 on my gauge was an observed 40 mph. Both the odometer and the speedometer functioned smoothly.
Since the rear end and transmission are the same as the '48 Chevy, HO72 and 4 speed, I went to the 1929-1954 Chevrolet Master Parts Catalogue for some information. It shows the 1 Tons using a particular speedometer, with some of the 17" wheel equipped trucks using an adapter for that speedo. The same # speedo is also used in many of the 1-1/2 trucks as well.
My questions are, how can the indicated speed be so far off while the indicated odometer seems to be accurate? Is there an adjustment I can make to get the speed reading accurate? I'm a newbie to the world of speedometers but my understanding of how they function is that there is a spring that restricts the magnetically induced rotation of the needle. Could the spring be the problem? Is there something else that could slow the rotation of the needle?
As always, thanks for the education! Jim

The first thing I thought is the needle has been removed at some point. When it was replaced it's know not resting at true zero, it's resting at -12 mph making the speed look low.

Mike B

Who cleaned and lubricated it? If you are measuring mileage correctly then you have the right speedometer. Odometer is a direct drive affair, while the speedometer is not. It has a magnet which turns inside of a cup. There is air space between the two parts. There is also a coiled hair spring involved which you cannot alter in any way while disassembling/assembling (which is quite a challenge without 3 hands, 2 of which needing to right-handed).
My guess is that it may not be properly cleaned or needs lubrication.

Thank you Mike and Carl for the replies. I've heard back from the seller who has provided me with pictures of the back of the mechanism which allows for easy adjustment of the spring without having to take everything apart. I'll attempt my adjustment in a couple of days and let you know how it comes out.

Research in the Master Parts Catalogue is always fun! For the 1948 model year there are 7 different part numbers for the speedo as used in the 1/2. 3/4, 1, & 1-1/2 Ton models, with 7 different 'speedometer gear teeth' used. The gears range from 4-12 to 6-20, which I guess are like a ring/pinion relationship, though not presented as 4:12 or 6:20. If in fact there is such a range in the 'speedometer gear teeth', it's no wonder that many gauges aren't very accurate if they came from a different model truck than they're being installed in. By the way some particular models also use adapters attached to the back of the gauge. Lots of possible combinations for gauges that all look the same!

All speedos were set for 1000 rev. per mile. The gearing is adjusted to allow for gear ratio & tire size by the gears in the transmission. If the odometer is correct the speedo should be also. If not the needle is not set or adjusted right. Let us know how your adjustment goes. I use the mile markers on the freeway to check the odometer accuracy.

Jim, can you share the instructions as to how to adjust the spring?

When I was doing boat repair work at a nearby marina, I would have a local speedometer repair shop intentionally mis-calibrate a tachometer to give a customer with unrealistic expectations about his Graymarine flathead six the impression it was turning a couple of hundred RPM faster than its actual speed. The comment I usually got was "Man, I can really FEEL how much faster the boat is running now!"

Happy customer, cash in the pocket!
Jerry

Carl, PM sent. When I adjust mine I'll take pictures and make them available for everyone.

OK, here's the update. I was able to adjust the indicated speed on the gauge but not enough to make it accurate. When I purchased the unit I thought it was an original gauge, cleaned and lubricated. Instead, It's more like a hybrid, an original face and housing with what I think is a reproduction mechanism. This mechanism is adjustable which may be different from the original which I have been told in PMs is not. I'm awaiting contact from the seller to verify whether the mechanism is repro or original.
My guess is that the repro units need to be adjustable instead of using different gears, as on the factory gauges, so that they work with a variety of combinations of transmissions, rear ends and tires. Obviously they don't have enough of a range for all applications. Looking back at the information I read in the Master Parts Catalogue I need to find an original gauge from a 1 Ton or a 1-1/2 Ton truck. Guess I'll be on the hunt once again!
If anyone wants a picture of the adjustable mechanism please PM me with your email and I'll send a picture.