brookewebb

So my shiny new tomtom says that I am doing 60mph, but my speedo reads more like 66mph.

Which one should I believe?!

Obviously this is a 10% difference in reading which becomes a little concerning when you take the number of speed cameras around into account. Might also explain the unscientific but resoundingly impressive 0-60 times I always count out to myself as I accelerate away. 1, 2, 3, 4, 4 and a quarter, 4 and a half, 4 and three quarters, 5 wow!

DaveK

I believe that GPS speeds are more reliable than speedos.

I do get some discrepancy between my Tom Tom (PDA) vs my speed camera detector, but not nearly as much as with the speedo.

I figure my speedo is about 10mph out at 70mph, so reckon I'm safe doing an indicated 80mph. However - on Sunday morning, there was a nasty mobile camera on the M3 as I came around a slight bend doing 90mph. I hit the brakes and slowed to an indicated 80 (don't ask why I didn't slow all the way to 70 - I don't know myself!) - so I guess I'll find out soon if that is a safe speed........

Indycam

Do you guys got measured mile markers on your roads over on that side of the pond ?

last toy

I believe the speedo is calibrated for 16 inch wheels at the factory. So if you have changed your wheel size, that will account for a descrepacy but should not be 10%. next time you see a cop with his laser, stop and ask him what he clocked you at unless he stops you first.

Johnny G Pipe

Speedos almost always over-read by 5-10% IIRC, in most cars. When playing with speed cameras, always listen to the GPS!
The factory Vmax is 162..but many of us have 'seen' the needle pass 170.

Occasionally plod helpfully place 'your speed' active signs by the road - to help us calibrate our speedos, I presume. How nice of them.

Caveman

My road angel shows 7 or 8 mph difference to both my Porsche and BMW. Friends have said they get similar discrepancies.

brookewebb

I've got 17 inchers but as you say - shouldn't make such a large difference. As guess as long as I take the speedo reading I'll be more than safe....

911addict

The car speedo is less than the true SAT speed, 10% is quite possible.

As far as 0-60 time, if your car is in sweet condition and you get the gear change and tyre grip just right you could bring the time down toward 5 secs. Is yours fully standard?

And Porsche official figures always conservative.

TR6

Speedometers are notoriously inaccurate in most cars. I would trust a GPS more than my speedo. By the way, does anyone make a GPS unit that specifically measures 0 to 60, quarter mile times, etc? That would be cool.

robmug

GPS is the accurate one, as it is ACTUALLY measuring distance over time; the speedo is a mechanical device which is designed to over-read (for a number of reasons)

jimq

my car reads 60 both digital an analog and the GPS will read 58. not far apart at all

brookewebb

But then again, GPS is speed over ground, and doesn't take into account the incline or decline of a road, and would, to a triangulation of satellites appear to be making less progress than the true mileage.

jimq

not much grade change here in florida

C4Russ

I don't see this as accurate. GPS measures movement for all 3 axis, including elevation change. Aircraft regulary use GPS for departure and arrival data, including airspeed. Aircraft can and do have greater inclination then just about any road a car would travel and they have no issues with incorrect speed readings.

brookewebb

Not sure I agree but not up to speed with avionics sufficiently to argue the point.