Chevrolet Speedometer

S. Dafoe · #1 02-09-03, 14:46

I have a late model Chev (round) speedometer that works very well but the odometer has been removed at some point int ime. The trip odometer works great and the face of the speedo itself is in marvelous condition. I have a spare speedo (purchased at a flea market) with perfect internals but has a faded face.

Has anyone done work on these speedos before? I thought the easiest "fix" would be to swap the faces. This would involve removing the speedo needle. Has anyone done this before OR what would you suggest is the best approach to getting a fully functional piece with what I have to work with?

Thank you for your continued help with my trivial questions.

Lynn Eades · #2 04-09-03, 08:23

Mr?..S...S...Secret ? Dafoe, I dont know the answers to your questions, But, I do think they are very good questions, Because to me this is the stuff that a forum of this sort is about (me thinks
anyway) Im sure you will get your answers, and those of us who want to, will learn something. Thanks for putting it in front of us.
Whats the answer Bob? is this your specialty?

gordon · #3 04-09-03, 09:25

The easy way is to out and buy a new reproduction jeep round speedo, which will interchange nicely. The US cost was about $60 last time I looked.

Stripping and rebuilding the two speedos is no real problem, even if the front galss breaks you can change it as it's just a flat sheet. The problem is you would have to fit the pointer and then road test it against a vehicle sitting at, say 30mph to see how far off it was, and whether you wanted to move the pointer and have another go at checking.

I don't know any reliable checking and calibrating procedure for them except comparison with another vehicle which has a reasonably accurate speedo, but since many of the small speedo drive gears on the transmission were commonly changed to get the right setting for axle ratios and tyre sizes a check at 30 isn't a bad idea. Less than 30 who cares if it's accurate, and if it is right at 30 it'll be close enough till one of these venerable trucks reaches the top end of what we laughingly call its' speed range.

Gordon

Bob Moseley (RIP) · #4 04-09-03, 11:57

Lynn - thanks for the thought but at this stage I am only concentrating on the early stuff prior to the advent of the sealed round gauges. One day I'll explore the later stuff.

S - Gordon's answer is a good solution unless you can find another speedo.

Bob

S. Dafoe · #5 04-09-03, 19:31

With respect to the calibration if one were to remove the speedo needle and face only why would the calibration change? Theoretically the needle should be at zero mph (or less). If the needle is returned to the same position and none of the internals are changed I would think the calibration would remain unchanged. If I were to remove the internals then I could see more of a problem.

My original concern was more or less with the needle itself and if it goes back in place easily (although you now have me thinking). Perhaps Bob can comment on the needle removal and installation itself.

Thanks guys.

Stew

gordon · #6 04-09-03, 19:41

If you think about it Stew, the speedo guts need to do something before the needle moves off the bottom stop (and this applies whether or not there is a stop pin at the zero point) This could be a lot or a little, certainly enough to change the speedo reading by 20mph or so.

The correct way to calibrate this sort of instrument is to spin it at the speed representing half the full scale deflection (say 30mph) and put the pointer shaft on while it's turning at that speed, then change the speed to 25 and 75% of full scale deflection (15 and 45mph say) and check that it's accurate at those speeds.

Since we don't know the speed it's supposed to be turning at, and we dont have calibrated speed sources to hand anyway... taking a stab at the mid range and checking that it's reasonably accurate should be good enough for our purposes. The legal situation in the UK is that any vehicle that isn't pedestrian controlled has to have a speedometer accurate to +/- 10%

Lynn Eades · #7 06-09-03, 00:16

To put it another way to Gordon, There is a hair spring attached to the needle spindle that is trying to return the needle to zero, (to the stop pin) How much tension is on that spring is the problem. One way to try to second guess where to put the needle back on the spindle , is to (before removing the needle) gently lift the end of the needle over the stop pin and see where the needle wants to point. on re assembly, put it back in the same place. this should be reasonably close.
(Bacon and eggs are getting cold. Bye.