Hi Guys,

Just got a new phone and was testing out a performance app that uses gps as a speedometer. It was free and I was bored. Anyway... got in the car and noticed that my speedometer was way off.

At 60 it was 4mph off... showing 56.
At 70 it was 6 mph off... showing 64
At 80 it was 7 mph off...
at 90 it was 8 off
and at 100 it was almost 10 off.

I have a 2005 Z w/ the Stock 17 rims with 245 45 up front and 255 40 in the rear.

She's got 126k miles on her.

I'm trying to look up/ search speedo calibration and i'm thinking that something is just malfunctioning as opposed to a simple tire recalculation (since the tires are pretty stock)

Any thoughts?

Thanks in advance

Mike

 

how do you know the app is not the thing that is off? they are not always perfect - some are better than others

there is no speedometer calibration that I am aware of, it takes the speed from the abs sensors

 

the app is prolly the thing that is wrong...

 

I thought the same thing....

To test, I was driving last night next to my wife and my speedo was a constant 4-7 mph higher (depending on the speed) from from hers. She has a stock rim/tire Jeep Liberty.

just to let you know...we were on hands free phones.

It's quite bizarre.

Could my abs sensors be off? I mean, my fuel sensors are off, my oil sensors are off....

 

Was the car faster or slower than the GPS?

Or, when the car was showing ______, the GPS read ______.

Have you compared the mph readout on the trip computer to the speedometer?

 

I dont see how the sensors would be off, my understanding of how they work is that they are either within spec or they aren't, and not in spec triggers a code

I have no idea how a Jeep indexes its speed to know if your wifes car is accurate, or maybe yours is ok and hers is not

If its a huge concern, best bet is the dealership

 

Thinking your rear tire size is wrong. The stock size is 235/50X17. According to the Miata.net tire size calculator, you your speed would read 57.2 when you are actually going 60 mph. That is pretty close to your figure. If this speedometer error concerns you, going to the correct size tires should resolve your problem. Your front size is also wrong.......they should be 225/50X17.

 

Thanks for the help and advice guys. I am planning on getting new rears fairly soon. I'll see if that doesn't help out.

 

Those aren't the OEM sized tires so that is part of the problem. Most phone based GPS apps are inaccurate because of the low sampling rate of the GPS within the phone.

 

Did you read the OP at all?!?!?! hello wake up...that was clearly listed

 

Correct.
You altered OEM tire size. Actual speed 100.0 MPH. Speedometer will read 104.8 MPH.

 

Did you?

OP says his new phone has app that uses gps as a speedometer.

Next, he was in his car (using the phone app as a speedometer), and claims his speedometer was way off. "My" could refer equally well to the phone app speedometer or to the car's speedometer.

From then on he only says "it" was off, with no clear indication which it he's talking about.


Normally, I'd take a GPS speed reading as being more reliable than a vehicle speedometer, but a cheap phone app is likely to have a low refresh rate and pick up only a few satellites. Couple that with the fact that he probably didn't stay at any particular speed for very long, and the phone app speedometer reading gets VERY unreliable.

 

As others have mentioned, it is your tire size. Keep to original specs or get used to having a speedo that reads a little off.