What causes speedometer to be off?

When I drove my 1952 Mercury and checked the speed with a speedometer app on my smartphone it seemed to be about 8-10 mph off… When it says 50 it’s really about 40 mph. Any ideas what would cause this? I know the tires are the correct size - not sure if that would make it read inaccurately.


Well? First off speedometers can have an inaccuracy of up to 5% in the US.

Second, you’re comparing an old mechanical system to indicate speed which is probably worn out to a higher tech GPS.


Incorrect differential ratio can definitely cause this. So can a worn out speedometer. Either is entirely possible in a 60 year old car.

The speedometer on a 1952 car is a mechanical device that uses a rotating magnet and a spring. Possibly the spring is worn out and no longer has the correct tension.

Also since tyres have been changed likely several times, they may not be original size.  That can also cause error. 

I would also add that while that app is likely to be reasonably accurate, it may not be totally accurate.   How consistent is the readout?

A smartphone might be using GPS for location or it might be triangulating off cell phone towers, with lesser accuracy. The best way to check your speedometer is to get on a stretch of highway, keep your speed constant, and measure the time between a two mile markers (have a helper do this so you aren’t distracted). This will also check your odometer. If your odometer and speedometer are both reading high, then it is the tires, unless a prior owner changed the differential gearing or something in the transmission. If only the speedometer is off, then the mechanical parts inside it are just worn out.

Any speedometer app that used cell tower triangulation to determine speed would be rated so low it’d be hard to find it in the marketplace.

The Apple iPhone uses 3 methods to determine location; a location database of wi-fi hot spots, cell phone towers, and GPS. I don’t know if an app developer can specify “GPS only”, as Apple support docs say that when the phone can’t find a satellite it will fall back to wifi and cell towers.

Interestingly, there were comments on another automotive forum that suggested that smartphone apps will be less accurate than the speedo over the short term (less than a couple of miles) for this very reason.

I’d be curious to know the truth either way. In the meantime, checking the timing over a couple of measured miles seems relatively foolproof.

Well, I haven’t played with the iPhone, but the speedo apps on the Android platform use GPS. I can’t imagine that Apple’s developers would be stupid enough to measure speed off of wifi routers or cell towers.

I’d have to suggest that whether it’s using trangulation off of towers or satellites, the cell phone is probabbly more accurate than the 1952 speedo. The only way to find out is to use a stop watch and milemarkers on the highway, or to compare those milemarkers to the '52 odometer.