The Stovebolt Site Forums The Shop Area Engine Shop Speedometer Adjustment

Hope someone can tell me if my speedometer can be adjusted. It's in a 53 half ton. The needle changes and is not jumpy as speed increases or decreases but it is reading way to slow (15 mph when I know I am going 45-50). Is there an adjustment screw or spring? The cable is turning fine.

Thanks- Pete

Pete,
Is there a squeeking/chirping noise? I had this problem and had to get a replacement speedometer, because the plastic on the backside had worn out. (and I was too cheap to have it rebuilt). You can check to see if its your speedo or the gear in the tranny by disconnecting the cable at the tranny and hooking the end of the cable into a power drill. Careful! The speedo is only meant to go one way! If the speedometer increases normally, then the prolem should be the gear in the Tranny (about $5 at any tranny shop).

Nope.....there's no adjuster screws! But don't be afraid to take it apart and gently clean out the grunge. There's nothing scary in there! You'll then be ready for when you need to replace the dial decal! Cheers!

Hopefully others will chime in. When I installed a new speedometer gear in my tranny, I noticed that I was a constantly 10 mph faster than what my speedo was saying. So I took my truck in to a local speedo shop and they put my truck up on a special rack with rollers and started up the ole truck to see what it's speedo was reading vs what the rollers were reading. They agreed with my findings and made an adapter that bolts between the speedo cable and the speedo gear. This adapter can be either a reducer or increaser depending on what you need.

Nevertheless now my speedo is reading true speed.

If its a gearing problem, it will be off the same percentage at all speeds. Also the odometer will be off the same percentage. This is a easy problem to correct with a gear adaptor. Any speedometer shop can help you with that. Check the links page or do web search for speedometer repair.

Like the above post said, these are easy to work on and clean. Take it out and go inside to a well lighted work table. Slowly pull it apart and clean it with your choice of cleaners, I like brake cleaner. When reassembling use a lite machine oil spareingly where needed. The odometer numbers and gears are a little tricky but not to bad.

If the cleanning doesn't help, it will need to be sent out for recalibrating. If the magnet is weak, the odometer will be close or right on if the gearing is correct and the speedometer will be off . The faster you go the farther off it gets. Unlike gearing which will show a steady percent off 2%, 3%, etc.. this will be nonlinear. Does that make since? The faster you go the higher the percentage it reads off.

Mine has a weak magnet, so at 65 it was showing 56. To correct it, I just left the glass out so I could place the needle where I wanted. I hit the highway and checked it against the mile markers with a stop watch for two minutes and two miles. Ten miles in ten minutes will let the differences show more, but here in town it hard to drive right at 60 for ten miles with out holding up traffic or getting run over. The odometer was right on so I knew the magnet was weak. Since it read 9 slow, I pulled the needle and moved it up 9 mph. Now at slow speeds it's off but at 65 it reads right on. I can judge speeds under 35 or 40 pretty good, its the faster speeds I don't want to get caught at . Joe

Got the same deal going on with my 36 GMC. Speed shown is slower than actual. Does it function the same as later models?

If your speedometer is running half speed check to make sure your cable isn't slipping. That is usually the symptom. Make sure the ends aren't loose.

Yes they are the same as the later model units. Each are gear driven from the output shaft of the transmission to a cable. The cable spins a small magnet in the speedometer head. The needle is hooked to a very light spring which holds tention. As the magnet spins, the needle trys to keep up only the spring holds it back. In these three items is the calibration. Each have to be correct for the gauge to read correctly. 90% of all speedometers operate on the same ratio. The cable should turn 10 revolutions in 52 feet 9 1/2 inches or 100 revs per mile. 100 cable rpms = 60 mph. Joe

Thanks for all the suggestions. It sounds like the speedometer mechanism (magnet?)is going to be my problem as the odometer is correctly registering the miles. I have already cleaned the components but may take it apart and try again. It's going to have to wait awhile. I don't break too many speed limits with it anyway!
Pete