Speedometer not accurate

[K4fxd]

Mine is accurate according to GPS and road side radar signs.

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[Linkster1666]

Why are you comparing GPS speed with an electric/mechanical sensor?

You understand how GPS works? You have to have at least Four sats for any speed reading, let alone an accurate one. Back in the olden days, you set a GPS waypoint, you could get back to it within 150 feet.

And having used GPS for about 30 years, you are at the mercy of many, many things. First and foremost is the software and the hardware that is deciphering the satellite data. The settings of the sats, they need updated constantly, due to a variety of factors. Thirty years ago, WHOSE Birds where you using. Space weather, a minor flare can and will mess with the birds, your hardware AND your Data. Line of sight, not as big a deal now or is it?

So you may want to do a LOT of research on Who made your hardware and what the limitations are. Also, Bob the App Writer may not know shit. (Sorry Bob)

Just an opinion.

 

[joe603]

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?

Both my 2014 Durango R/T and my 2016 Charger SCAT didn't have this issue...Again, just wanted to see if something was wrong with my car, or if it was a known issue. Thanks.

 

[Zooks527]

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.

And then your tires start to wear down from full tread diameter, the circumference decreases, and the speedometer starts reporting a higher speed than actual once again.

 

[Bob Lob Law]

Why are you comparing GPS speed with an electric/mechanical sensor?

You understand how GPS works? You have to have at least Four sats for any speed reading, let alone an accurate one. Back in the olden days, you set a GPS waypoint, you could get back to it within 150 feet.

And having used GPS for about 30 years, you are at the mercy of many, many things. First and foremost is the software and the hardware that is deciphering the satellite data. The settings of the sats, they need updated constantly, due to a variety of factors. Thirty years ago, WHOSE Birds where you using. Space weather, a minor flare can and will mess with the birds, your hardware AND your Data. Line of sight, not as big a deal now or is it?

So you may want to do a LOT of research on Who made your hardware and what the limitations are. Also, Bob the App Writer may not know shit. (Sorry Bob)

Just an opinion.

My GPS app used 13 satellites to calculate my speed and it was spot on with the radar signs(2 of them) that I intentionally drove past to check. My car was dead on, until I changed wheels and tires.

 

[Linkster1666]

My GPS app used 13 satellites to calculate my speed and it was spot on with the radar signs(2 of them) that I intentionally drove past to check. My car was dead on, until I changed wheels and tires.

I consistently find the Radar Signs 2 MPH off anyway so now we are at a 4 mph margin?

 

[WildHorse]

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

Move. In my neck of the old backwoods its 11 mph over b4 you get a ticket.

 

[Bluemustang]

If you changed tire size, that's why.

 

[KingKona]

Both my 2014 Durango R/T and my 2016 Charger SCAT didn't have this issue....

There is no issue.

Neither your Durango or Charger had 100% accurate speedometers. They were both off by a MPH or three or more.

 

[ice445]

And then your tires start to wear down from full tread diameter, the circumference decreases, and the speedometer starts reporting a higher speed than actual once again.

That's a pretty negligible change with summer tires that don't have that much tread depth to begin with lol

 

[Zooks527]

That's a pretty negligible change with summer tires that don't have that much tread depth to begin with lol

For the 275/40-19 Michelin Pilot Sport 4S rear tires that came on my car:

Initial diameter: 27.70"​

Initial tread depth: 0.266"​

Tread depth when worn: 0.0625"​

Diameter when worn: 27.29"​

% diameter change when worn: 1.5%​

Speedometer error when worn at 75 mph: reports 1.1 mph faster than actual​

If the tires were 225/45-17, the error would be 1.9%, or 1.4 mph faster than actual​

​

Which is in the middle of what the OP said he was seeing. If they were all-seasons instead of summer performance, the error number would be even greater.

 

[joe603]

I just had new front tires put on yesterday...we'll see if it improves. Again, this isn't some earth-shattering issue, just something I noticed and wanted to see if it was a S550 thing or my car...like the BBQ tick.

 

[Skye]

Conceptually, it's diffcult for me to see things being exact amongst one another.

Police: using sounding radar or laser

GPS: coded radio signals and stratum one timing

Car: derived from mechanical, electrical, possibly both means, maybe multiple input sources

Phone: probably GPS, via a programmed app or OS

+

Environment variations: tire wear, interfering weather, radio or physical conditions, location, etc.

Different systems and interfaces, each with their own measurement methods, and what I think could be most important: different 1) rates of measurement and 2) tolerances. Further still, what are any of the systems described above certified to? Level of accuracy? How long ago?

At the end of the day, IMO, it's what are the authorities measuring the car at. Anything over, you have to prove they're wrong. Have a different reading? Courts would expect the driver to take it up with whomever created and maintains the other system and then come back and prove the police/trooper was inaccurate.

Scenarios I describe aren't often feasible, if possible at all. Drivers should allow for a bit and give themselves something of a speed budget, so they are always under the speed of interest. If the car is consistently (key phrase) reporting a certain amount over the cars actual, verified speed, that amount, that gap, is the budget and they should not exceed it.

Example: car reports 65 MPH / 104 KMH but actual, authority-measured speed is 63 MPH / 101 KMH. The budget is 2. The driver needs to drive between 65-67 MPH, as reported by the car, preferably 65. The driver could drive at a car-reported 67, but they run the risk of all the variances ultimately showing otherwise to the police (higher, triggering ticket).

 

[ORRadtech]

Personally I'd rather the speedo read 2 over than 2 under.

Also, I know that it's jurisdiction/ LEO dependant but I've never been ticketed for anything under 10 over. Hope that doesn't jinx me...

 

[K4fxd]

Also, I know that it's jurisdiction/ LEO dependant but I've never been ticketed for anything under 10 over. Hope that doesn't jinx me...

Don't drive in Indiana, got popped for 5 over coming into some hick town.

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