The Stovebolt Site Forums The Shop Area Driveline 1953 3600 - wrong speedometer gear?

My '53 Chevy 3600 with stock 4 speed transmission, stock 4.57:1 rear end and 15" wheels has never read the speed correctly on the speedometer. It is consistently 6-7 mph over the actual speed. I think the wrong speedometer gear was installed in the factory and it has a gear for 17" wheels instead of 15" wheels. What would be the best way to correct this or am I thinking wrong about it? I don't want to change out the wheels and tires just to make the speed read correctly.

If it is "consistently 6-7 mph over" it isn't a gearing problem. A gearing problem would be X mph over at 15 mph, 2X at 30 MPH, 3X at 45 MPH and 4X at 60 MPH.

Get a speeometer adapter from vendors like Jim Carter Truck Parts? Pretty easy instructions to follow:


https://oldchevytrucks.com/sp249.html

Thanks Craig, that does look pretty easy. I suspect using a GPS speedometer to get the true speed would be accurate enough.

Yes, I use an AP called speedbox. I let the wife tell me what it is saying while I'm driving and looking at the truck's speedo.

Woodcmiami

At what speed is it "consistently 6 - 7 mph over"? How far over is it at twice and half that speed?

When it is cool enough to drive I will test it at those speeds. It reads about 41 mph when I am going 35.

Changing the speedometer gearing will solve your problem if your test results are something like this:

actual MPH ......... 20 25 30 35 40 45 50 55 60
speedometer reads 23 29 35 41 47 53 59 64 70

You've already done the 35 MPH test, just do two more, maybe 25 MPH actual and 50 MPH actual.

Thanks, it really doesn't like to go 50 mph so I will try 25 and 45 mph.

First lesson of the day, speedometer needles are magnet driven, odometers are gear driven.

The magnet can be dirty or weak and make the speed wrong.

Odometers are gear driven off the cable and gears. Tire size, rear axle, and speedometer gears will change the odometer.

Odometer should be corrected first, then address the speedometer.

Drive ten miles at a steady speed on a marked highway and write down the mileage shown compared to marker signs.

Adjust gears or add adaptor to correct the odometer.

Speed doesn't matter to the odometer.

https://www.speedoservice.com/speedometer-calibration/ These guys can fix you up with parts or service.

Speedometers / odometers are calibrated to 1000 rpm cable = 60 mph.

thanks, I plan on checking the accuracy of the odometer and speedometer when the weather breaks in a couple of weeks. It is going to be 107 F today, too hot to sit in a metal truck cab!

Joe is right in saying the odometer test will tell you if the "gearing" for the speedometer is right. Do the "10 mile" test he recommends, but keeping a "steady speed" is not necessary.

Joe and Bill,
Good info. So many of us just look at the speedo and not the odometer for their accuracy. Time for me to put down my "Honey Do" list and take the truck out for a 10 mile odometer test spin.

There is a way to check it on your driveway, I will see if I can post it when I get home today. You mark out a distance on the driveway and roll the truck for 1 speedometer rotation.

Its 1000 cable rotations per mile. So you have to take in account the gear ratio and tire diameter.

Add A/C, it helps setting in the oven!

Not going the A/C route in that old truck. Everything is original mechanically and I intend it to stay that way.
I was wondering if the odometer does end up reading wrong because of the wrong gears then does that means the actual mileage would be wrong too? I have had it for 37 years and it started with 16K miles and now has 32K. I guess I could calculate the actual mileage once I figure out what the error is.

How to check speedometer with out driving ( far ). A helper to count would be helpful.

Unhook cable end at speedometer head and pull out from under dash so you can see it, wrap a piece of tape around end as a flag.

Mark out 52' - 9 1/2" on a flat surface,

Roll up to the starting mark on the ground, place tape on the truck with mark to accurately align with the mark on the ground. I used a plumb bob.

Roll forward while counting turns of the cable till you travel the full 52' -9 1/2".

If it's geared right, the cable will rotate 10 times.

Repeat a couple times to verify the results.

Joe H, You put 52'-9 1/2" then 52"- 9 1/2", can you clarify this?

52 feet 9.5 inches is 1/100th of a mile.

If Joe is right about 1000 revolutions of the speedo cable to be a mile (I'm not arguing, I just don't know) the cable should turn exactly 10 times. If 9 1/2 times then the speedo will be 5% slow. If 11 times the speedo will be 10% fast.

Bill is right, I typed it wrong and went back and changed it. I found the information back in 2001 from a now closed speedometer shop.

Got it, that makes sense.