rwilliamtaylor

I just checked my speedometer against a hand held GPS and discovered that it is off by about 5 mph at 80mph. The car reads 85mph and the GPS reads 80mph. Has anyone else experienced this?

I have 315/25/19 Toyo Proxes T1Rs which are a different size, but should not account for this much error.

jimq

every Porsche I ever had read higher then actual speed.

schred

Agree, I always wonder why Porsche does this, seems to be the case with all of them.. old and new and all models? Does anyone know why they do this as it is curious.
FS

jimq

It saves you 5 mph off your ticket you think you are getting

fly2low

It would be worth checking, but memory tells me that federal law acknowledges that there is speedometer error, and the error CANNOT be on the low side. Every car I have driven has overestimated the real speed the car was travelling.

slant911

It seems that mine reads a bit high too. However, when I ran the mile run here my speedo at the trap was reading about 170-171 and that is exactly the speeds the radar was saying I was going. Maybe it gets more accurate as you go faster???

1AS

The peculiar thing is that the speedo reads high, but the odometer is accurate. Otherwise, the warranty would lose value artificially, and create a liability. It has to be intentional. AS

MK19

Mine is high too. I thought that the dealer could calibrate it electronically........where's Firehawk on this ??

Thanks,
MK

jbossolo

BMW's are also prone to this. My 2002 M3 was at least 10MPH "over". Guess it gives you a false sense of bravado!

Oak

yeah, I thought I broke the 200mph mark but my gps clocked me at only 191.

turbolou

Interesting ! I could swear that I'm not going as fast as the speedo indicates on my 03 TT I even complained about that when my car was under warranty & was assured that it was my imagination not owning the car all that long..Now reading this post makes me wonder ?

Beej

My car is the same - 8 km/h (5mph) optimistic at 100km/h/62mph. I find this incredibly annoying! The dealer cannot re-calibrate it

I was wondering though if the error was a constant percentage of speed or an absolute error - anyone checked their's at different speeds to find out?

Cheers,

Beej

greg911

Mine is about 3 mph under the speedometer speed vs. what the the GPS reads at almost all speeds above 40 mph.

TXGold

Good question. Anyone?

Beej

Ok so I finally got around to doing some testing with my Navman handheld GPS mounted in the car. What I found is that there is both a constant and a variable (% based) error with the speedometer indicated speed in my car.

The FIXED error is approximately +3km/h (2mph), while the VARIABLE error is approximately +5% of actual (real) speed.

Ie:

* at 60km/h (37mph) true speed the speedo reads 66km/h (41mph)
* at 100kmh (62mph) true speed the speedo reads 108km/h (67mph)
* at 130km/h (80mph) true speed the speedo reads 139-140km/h (87mph)

and so on......

This means that to reach a *true* 300km/h (187.5mph) the speedo would have to be indicating 318km/h (199mph!).

This was determined by holding the car at various constant speeds on flat, straight roads for several seconds and then comparing the GPS speed to the speedo.

And my measurements were with 315/30 tyres which actually reduces the standard error by 1-2%. I have also compared the tacho at these various speeds and the tacho backs up the actual speeds as measured by the GPS vs the speedometer indications.

Now if only there was an easy way to get this fixed... I am thinking of trying 295/35/18 rear and 235/45 front tyres, as in theory the slightly larger diameter of those tyres (because of the higher profile) should cancel out the 5% variable error. I don't think such a small change in tyre profile will likely have any real/noticeable effect on handling etc.

Thoughts on this idea anyone???

Cheers!

Beej