speedo / GPS speed difference?

[ace3]

i thought iremembered something about this ...

my GPS reads ~4mph faster than the speedo does. in the two other cars i've driven with this GPS, the speed is exact, or maybe 1mph difference.

wasnt' sure if anyone else had had this issue.

[andykeck]

That's very common, particularly with cars from the EU. There's some sort of law that speedos can never read low even considering different-sized aftermarket wheels and tires. I posted a link to a Road and Track article on the subject a while back. The good news is that in every case I personally am aware of, even though the speedo reads high, the odo is dead on.

[Graydon]

Here's an article from Car & Driver about why European speedos read high:

http://www.caranddriver.com/article....&page_number=1

But what you are saying is that your GPS is reading higher than your speedo... it should be the other way around.

[andykeck]

Originally posted by Graydon
Here's an article from Car & Driver about why European speedos read high:

http://www.caranddriver.com/article....&page_number=1

But what you are saying is that your GPS is reading higher than your speedo... it should be the other way around.

Yeah, that's the article I was talking about. I guess I forgot where I saw it.

I completely missed the fact that Ace3 has the opposite problem. That's just weird.

[etypeiii]

Yes, this is weird. My GPS reports about ~3 under the indicated speed on the Elise speedo.



DLY

[ChrisB]

Originally posted by andykeck
That's very common, particularly with cars from the EU. There's some sort of law that speedos can never read low even considering different-sized aftermarket wheels and tires. I posted a link to a Road and Track article on the subject a while back. The good news is that in every case I personally am aware of, even though the speedo reads high, the odo is dead on.

You are correct about the European law, but Ace said his GPS, which he believes to be accurate, read 4 mph faster than the speedo. In other words, the speedo on his car reads slow...just what the law was intended to avoid.

[Racer X]

If someone were to go an indicated 80 MPH in my car, on a track , my GPS would indicate 76.5MPH. I have not yet checked my odometer against my GPS (handheld, don't get ruffled that I added a bunch of heavy luxury items to my car).

So I low speedo reading would seem to be rare versus other people's experiences.

Greg

[ace3]

er ... yeah ... i'm a moron.

the GPS is ~4mph slower than the speedo.

[Graydon]

Originally posted by ChrisB
You are correct about the European law, but Ace said his GPS, which he believes to be accurate, read 4 mph faster than the speedo. In other words, the speedo on his car reads slow...just what the law was intended to avoid.

Umm, yeah... we both already pointed that out.

Originally posted by ace3
er ... yeah ... i'm a moron.

the GPS is ~4mph slower than the speedo.

Your elise is perfectly normal then.

[acw]

4% is great. In comparison, Porsche speedo are about the same. Ferrari speedo are ridiculously optimistic (10-15%)

AW

[Hollywood]

80 on my speedo=75 on gps
86 on my speedo =80 on gps

[Graydon]

Originally posted by Hollywood
80 on my speedo=75 on gps
86 on my speedo =80 on gps

That's because speedos are off by a percentage, not a fixed mph.

[eletourneau]

I compared the speedo of my E500 to my portable Nav system, and they were both in sync.

Maybe Mercedes has a USA tuned speedo.

[Craigy]

On my S1 elise, 70mph on the stack speedo is 70mph GPS.

I wouldn't mind trying out a run over a measured mile or something to make sure, but I think it's accurate.

Craigy

[Craigy]

Originally posted by Craigy
On my S1 elise, 70mph on the stack speedo is 70mph GPS.

I wouldn't mind trying out a run over a measured mile or something to make sure, but I think it's accurate.

Craigy

I forgot to mention that this is because there's no real space for larger diameter wheels on the S1. (15" front, 16" rear)

Craigy

[Ancient Wedgie]

Originally posted by Hollywood
80 on my speedo=75 on gps
86 on my speedo =80 on gps

This is the same result I clocked on my speedometer this weekend.

I clocked 48 seconds for me to travel the marked mile at 80 mph on my Elise's speedo.

It should have taken 45 seconds.

The difference means that my speed was really 80x45/48 = 75 mph.

If the odo were equally incorrect, it should indicate 1x48/45 = 1.0666miles per mile traveled.
I read my trip odo after two roadside markers = 2 miles.
It read roughly 2.1 miles (the best it can do in the short distance and with an odo that only reads in tenths of a mile).

Thus it seems both my odo and speedometer are equally optimistic.

This means that my 36000 mile warranty is really only good for 36000x45/48 = 33750 miles.

How does one get this fixed?
Or are we expected to go along with the fraud?
Is there a precedure to get our warranty periods adjusted?
In my case, until 36000x48/45 = 38400 miles on my odometer is reached?

[Lumn8r]

Originally posted by andykeck
That's very common, particularly with cars from the EU. There's some sort of law that speedos can never read low even considering different-sized aftermarket wheels and tires. I posted a link to a Road and Track article on the subject a while back. The good news is that in every case I personally am aware of, even though the speedo reads high, the odo is dead on.

On my Neon R/T, both the Speedo and the Odo read high. The Odo is off by about 3%. So although it reads 104000, I have only driven a little over 100000. That's okay though since I plan to keep it until it dies. It is a great car and the depreciation has reached the point that I'd have to pay someone to take it off my hands anyway.

[xxxotic]

Originally posted by Hollywood
80 on my speedo=75 on gps
86 on my speedo =80 on gps

I got the same thing.

I noticed that in FL all highway mile markers are not exactly right because of the road construction and widening and moving of roads. Starting at mile marker 100 on i-95, my other car is off by 3/4 of a mile at marker 200, but then it decrease to about 1/2 a mile at marker 300, so you might not want to use the to see if the odometer is correct.

I havent done GPS distance to odometer distance test on the Elise yet. I thought only the speedo was off, not the odometer from what people posted previously.

I wonder if the LSS tires have slightly different diamaters than non-lss tires and this could also throw the equation off a little bit based upon what tires you have,since gears inside of the speedometers probably are the same and are not based on if the car has lss or not.

[Ancient Wedgie]

Originally posted by xxxotic
I wonder if the LSS tires have slightly different diamaters than non-lss tires and this could also throw the equation off a little bit based upon what tires you have,since gears inside of the speedometers probably are the same and are not based on if the car has lss or not.

I have the standard tire, not the LSS.

EDIT: No way would I have picked up my baby and then driven 1000 miles in that monsoon last weekend were she shod with AO48s!!! I may be nuts , but I'm not insane.

[Ground Loop]

Originally posted by xxxotic
I wonder if the LSS tires have slightly different diamaters than non-lss tires and this could also throw the equation off a little bit based upon what tires you have,since gears inside of the speedometers probably are the same and are not based on if the car has lss or not.

I don't know how the speed sensor is set up (maybe shared with ABS?), but from the sensor to the ECU to the dash -- it's all digital, all software.

It's not inconceivable that they would have a selection somewhere to program in wheel circumference, but I'd be surprised.