Speedometer reading off

[scooterdup]

Speedometer in 59 Apache reads about 15mph low...confirmed with gps app.
Any ideas on where to start figuring out issue?

I may be pulling gauge cluster out one day to do some debugging...gas gauge doesn't work either.

[idbeast]

Factory Speedo or aftermarket electronic??? If factory, change the Speedo gears, you can look it up online for which one or one's you need.

[scooterdup]

Factory...thanks for reply

[mr48chev]

Meaning that you changed the rear axle and possibly the transmission?

Speedometers are usually set to run X mph at y rpm of the cable. You don't modify the speedometer to change the speed reading you change the driven and or drive gear in the trans.

You can either change the speedometer gears in the trans or get a calibration box (for lack of a better description) that goes on the trans where the cable would go and the cable attaches to it. Inside there are gears that you change out to re-calibrate the speedometer. They use them a lot on semi trucks and they are use a lot on 4x4 trucks for guys who swap tire sizes. This is an example and being IH is probably twice the price of some for other rigs. But it is an example https://www.ihpartsamerica.com/store...yABEgJx4PD_BwE

There is an excellent of how the speedometer correction box works here and what the insides look like. https://stevemckelvie.wordpress.com/...orrection-box/

It would be less expensive to swap the gears in the trans but if it is off 15 mph at 60 that is close to 25%. 25 % might be a lot to make up. That would probably mean that you went from a 4.11 to a 3.08 or close to it rear gear setup.

[Kevin Mac]

http://www.tciauto.com/tc/speedometer-gear-calculator/

Here is a link to a spedo calculator. I went through this with my truck and bought several gears to get it right. Finally had to change the driven gear, which was not a big deal. There are videos on Youtube on how to change them. I didn't use the calculator until the first couple I bought were not quite enough to get it right. I have a 350 turbo in mine. One word of caution is that not all makes use the same color for the number of teeth unless they are OEM. Also I have a AD stock reman spedo.
What transmission you running?

Mac

[1project2many]

Is the speedometer off by roughly the same speed at all speeds? Or is it off by an increasing amount as the truck goes faster? The second problem can often be fixed with gears. The first requires speedometer work.

[mr48chev]

Originally Posted by 1project2many

Is the speedometer off by roughly the same speed at all speeds? Or is it off by an increasing amount as the truck goes faster? The second problem can often be fixed with gears. The first requires speedometer work.

Had to think about that one for a minute but he is correct in that if the amount it is of mph wise will grow as the speed increases. It will always be a certain percentage off but if you view it as say 25 % of 40 mph is 10 while 25% of 60 is 15. That might give you a couple of spots to check what percentage it is off too.

If the amount it is off never changes as speed increases it is in the head.

[1project2many]

Most folks will grab GPS data these days, but we used to do two checks to see if the speedo head was out of adjustment or if the speedo cable was turning at the wrong speed. This required driving several miles on the highway with the speedometer reading 60 and using a stopwatch to measure the time to complete the trip along with the odometer including tenths place to measure distance.

If the odometer recorded distance accurately but the stopwatch recorded more or less time than one minute per mile the problem was in the speedo head.

If the odometer and speedometer were off but by different amounts, both the cable speed and speedo head may need correction.

If the odometer and speedometer appeared to be off by the same amount, the problem is usually cable speed.

I used to make correction charts for folks that I knew wouldn't bother fixing the speedo. I'd show the actual speed on the left and the speedometer speed on the right so they wouldn't get tickets. One guy put masking tape with numbers written on them on the plastic lens over the speedometer after I did that.

[mr48chev]

The state highway about 7 miles from my house is marked off with mile posts every mile for five miles and tagged speedometer check zone. It's been there since the 60's when we would time our selves to see how fast we could make it though the five miles. One of my buddies were running along in that area doing right about 100 one behind the other when I hear a little Beep beep, beep Beep behind me, about that time a Jag XKE pulls out around us and disappears from sight in front of us. Game over we slowed down to about 75.
GPS is so easy now and so accurate plus you can get the speedometer app on your phone for free that I can't see messing with doing it the old fashioned way. Last time I tried to drive that five miles at exactly 60 I had a farm truck pull out in front of me out of a field and blow my average anyhow.