Inaccurate speedometer

[Nadine66]

Any one have experience in their speedometer being inaccurate? I have a '66 Newport with original engine and transmission. I downloaded a speedometer app and if my speedometer reads 30 mph the app shows I'm going a few miles an hour slower. The faster I go the larger the gap. If the app says I'm going 55 the speedometer says 65. My buddy has a '67 Mercury Comet and he says his does the same. Were all speedometers this inaccurate back then?

 

[MrMoparCHP]

Change in tires size is likely the problem, you can change the pinion in the transmission. There are different way to calculate the change.


Alan

 

[Big_John]

Any one have experience in their speedometer being inaccurate? I have a '66 Newport with original engine and transmission. I downloaded a speedometer app and if my speedometer reads 30 mph the app shows I'm going a few miles an hour slower. The faster I go the larger the gap. If the app says I'm going 55 the speedometer says 65. My buddy has a '67 Mercury Comet and he says his does the same. Were all speedometers this inaccurate back then?

Several reasons. First is speedometers were always built with a lean towards reading faster. Then you have tire changes were we've gone through different sizing standards, combined with changing from bias to bias belted to radials. Somewhere the tire size gets rounded off and may be a fraction of an inch smaller.

Then the speedometer itself wears and usually they read slower as it ages.

So, if it's enough to be worth the effort, you'll have to look at what size the speedo pinion is in the trans now and change it accordingly. The problem with that is they are really all compromises to gearing and tire size, so you can easily make it worse.

There are also speedometer correction adaptors that slow down or speed up the cable to correct the speedo. Not cheap with new cables needed, but they do work.

Or you can just live with it... Figure out what it really is at 30MPH etc. and drive accordingly.