The Stovebolt Site Forums The Shop Area Engine Shop Speedometer 80 or 90 MPH

What is the correct speedometer for a 50 3100.
80 or 90MPH

One parts supplier is listing 1947-49 at 80 and 1950-54 at 90.

Another supplier shows 1951 was the last year for 80 and 1952 begins with 90.

What is correct?

Tom,

I have read that the first model year with a 90 mph speedometer is 1952. Best to get answers from owners of 1952 trucks (and hope they have not been modified in the last 54 years).

Tim

[ Added later: as you most likely already know, Jim Carter & Classic Parts (Chevy Duty) show 1952; but, Chev's of the 40's shows 1951). ]

Oilrag,

TClederman is correct. 52 was the cutover to the 90 mph speedo.

Thanks Tim & Eric for the info.

Other than the face plate, the 80 & 90 MPM speedometer are identical. The 80 has high beam on the top, 90 has it on the bottom.

The guts, gears and everything else looks to be the same.

The speedometer needle is spring loaded and is driven by a magnetic field. There is an adjustment for the spring. Is the adjustment the only difference between an 80 & 90? There was about 1/4" difference in the setting between the 80 MPH & 90 MPH.

I would like to test the speedometer and calibrate it before I put it in.

Using RPM's in high gear (3rd) how fast, MPH is the speedo going.
Does 2000 RPM = 55 MPH or 1500 RPM = 40 MPH ???

Wheel/tire size is not a factor because the truck is off the ground.

Unfortunatly, the tire size DOES matter. The overall diameter of the tire will affect the accuracy of the speedometer, as well as the final gearing. I'd also like to say that if you are going to test the speedo for operation, do it with a drill attached to the cable. Running the truck off the ground is a disaster waiting to happen. You can easily over speed the rotating parts, as there is no load on them. If you have a drill that has rpm settings, that will work just as well. You will want to re-check the accuracy by driving it with the tires that you plan to run, that's the best way to get an accurate reading. There is also a good calculator for this in the Links section, Randy's Ring and Pinion. I tried it myself, and it's very easy to use.

I understand that changing the wheel size will affect the accuracy of the speedometer. That's not the problem. The speedometer will read the same at XXX RPM's no matter the tire size.

I'm not sure how I would calibrate it with a drill. Maybe that's the way to do it. Can you expand on that.



Bill, I must be having a "senior" moment because I don't understand how driving it will tell me if it is accurate. I'm just an apprentice, I'm not that good.

Also, I checked out the link with the calulator and it shows over 2500 RPM at just 30 MPH with 16" wheels and a 4.11 rear. With a 3.55 rear almost 4500 RPM at 60 MPH with 16" wheels. That's too many RPM's for me. Couldn't use the other calculator since I don't know the trans ratio.

On the left coast we have mile markers on the highways. Simply drive the truck on the freeway and look for a marker. At 60 mph, it should take you 1 minute to get to the next marker. Bring a stopwatch. you can also check the accuracy of your odometer. I think a variation of + or - 5% should be acceptable. If you have any doubts, take it to a speedo service center.

I use the calculater on www.4lo.com. Me thinks you used the rim size, and not the tire diameter...Most 16 inch wheels will have a tire diameter of about 30 inches.

Unless you have overdrive... transmission ratio will be 1 in your top gear. if you don't have 4wd, transfer case ratio will be 1

2761 rpms at 60 mph with 4.11 rear and 30 inch tires. Just under 2400 rpms with the 3.55

You are right. Me did use the rim size. I hope I do a better job on the speedometer than I did using the calculator. Thanks for the rpms.

Another way to guage your speed is with one of those hand held GPS units. I've got a magellan gold and it shows MPH on one of the screens.

I would recommend bringing a friend along to measure the time between mile markers (so you can keep your eyes on the road and your mind on driving ) and go at least two miles or preferably five, while maintaining a constant speed. This should give you a good measure of the spedometers accuracy.

or get a cop to check you out..

I took mine to a speedo shop and they calabrated it.Didn't take the truck.The magnets have to be changed I.E.stronger or weeker with a set RPM input.So I don't think math / tire size works here???However tire size effect overall speed in the end.If that concerns you have speed o shop calabrate thuck as a whole.

The speedometer is calibrated to 1000 rpm at 60 mph, as is just about every speedo in the world.

My '50 has the original 80 mph speedometer.

You really want to calibrate the odometer to read 1 mile per mile before you try to calibrate the speedometer.