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Speedometer not accurate

joe603

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Hey guys, just wondering if anyone else's speedometer doesn't match up with Android auto displayed speed? Mine shows the car is about 1-2mph faster than it actually is. I thought it was just the delay from my GPS speed, but I noticed today when going through a local police warning sign with your speed displayed matched my GPS.
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raptor17GT

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a speedo cannot under read but has a limited over read allowance. I'm sure on mine even the Cruise control speed set is showing a couple mph lower than the actual speedo. Tis not a problem
 

Bob Lob Law

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Mine is off by 1-2 as well. That started when I took off the stock 20s and put on 19-10/11 from LMR as the overall height is shorter.
 

Skye

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Ya, I was just about to ask: have you changed the tires? Do they differ in size from factory originals?
 

WildHorse

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1-2 mph.. Lol who cares. Could be one of a billion reasons why its off.
 

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ice445

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Speedo is calibrated for the stock rim/tire combo. As long as you can get the revs per mile right (and something that can reprogram it), you can get the speedo dead nuts on. I got the revs per mile for my setup (285/35/19 indy 500) from tirerack, programmed it to the car, and my speedo perfectly matches my GPS.
 
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joe603

joe603

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Nope, OEM rims/tires. 2mph is the difference between getting a warning and a ticket...
 

2021 Mach 1

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Any Ford I've owned, the car speedo always said I was 1-2 mph faster than what GPS said.
 

PhillyMike

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Every car I have ever owned is slightly slower vs. speedo when I go by one of those roadside measurement devices.
 

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joe603

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but your speedo is 1-2mph higher yes? So you use that as your guide and you'll never be over ?
I get that...and I have been doing that. I was just wondering if it was my car, or if it was a normal Mustang quirk.
 

PoCoBob

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Every vehicle built the speedometer reads faster than what the car is going. Corporate lawyers instituted that decades ago so people can't sue the manufacture over speeding tickets. Plus as your tires wear down the distance per revolution is shorter so the speedometer doesn't take that into account.

Many years ago one of the import car companies got nailed for their speedometers being out by 10%. They were accused of deliberately doing that so the warranties would expire 10% sooner than what was advertised. If they offered a 60,000 mile warranty the car only had to go 54,000 miles before the odometer reached 60,000.
 

raptor17GT

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I get that...and I have been doing that. I was just wondering if it was my car, or if it was a normal Mustang quirk.
every car is like that, no matter make or model. Some are even further out, i think 10percent overread is the max they can be.
 

KingKona

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I get that...and I have been doing that. I was just wondering if it was my car, or if it was a normal Mustang quirk.
It's not a quirk, nor specific to a Mustang. It's completely normal with every vehicle.

No vehicle has a perfectly accurate speedometer.

What do you think happens when tires wear?
 

5doorsoffury

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if you own a performance car in a area where 1-2 mph is the difference in punishment you should move or get a diff car cause that's a crappy reason ...

Now to answer the question you go into forscan or a tuning device and you adjust it to match the GPS speed...

GPS Speed can be 1-2 mph off so yea you can go chasing it all you want but one takes a real world reading based on tire speed the other is triangulated by a very weak signal sent from miles above your head that's calculated based on differences in arrival times with a signal that travels at the speed of light
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