My Talex gps speed camera alert system reads 4 mph slower than the speedometer in my TDI 2000 Octavia......Which is correct?

The GPS will be more accurate given that your position is triangulated.

The GPS is correct. The speedo is not:thumbdwn:

I too have a Talex. At 70mph on the speedo the Talex says 65mph

Tom Tom mobile shows that my speedo over-reads by about 5%. I believe 5-10% over is pretty normal for all cars.

Mine's the same,

about 10% out.

Only problem is, I know it's 10% out, so on motorways I now drive at 10% over.

Tend to be too close to the camera limits now

I have a dymo strip attached to the dash showing corrected indicated speeds for when I am not using GPS. These are 32, 43, 53, 64 and 74.

This a common fault on most cars. That is why the police are 'usually' acceptant of 10% variant. They usually work on a 10% +3 mph before showing you how well the strobes on the car works.

all car speedo's have to read fst by law (closest to accurate I had was the digital speedo in the C2 GT, it always read exactly 1mph above what the gps showed!) my vRS too reads about 10% fast...

Just go with whichever one reads highest..

Neither GPS nor speedometer will be 100% accurate, but the GPS will be the more accurate

Interestingly, I always assumed that speedo's were calibrated to read 10% fast (a big enough margin for error to ensure you were not exceeding the speed limit), but discovered on a recent drive out with a chap with GPS that my speedo was actually pretty much spot on! D'oh!

Chris

This a common fault on most cars. That is why the police are 'usually' acceptant of 10% variant. They usually work on a 10% +3 mph before showing you how well the strobes on the car works.

All speedo's must be accurate or overread (ie reading 30mph on the dial when the actual speed is 27mph) so that you are knowingly speeding if you exceed it. No speedo's are calibrated to underread. The 10%+3mph is the ACPO guidelines and is slightly different.

Chris

When I was doing my internship at the Suzuki plant, I walked across to the speedo calibration bay. They aligned the speedo to read 44km/hr at an actual speed of 40km/hr. Apparently it is a must that the speedo should be at least 10% more than the actual speed. They said it's a safety thing. Although the speedo error should never be negative.

But on my Honda the instruction manual said that the speedo error factor was 1:1. and it was true. Autocar's tested speed for that car was 181.4 km/h and at max the speedo always used to show 181 km/hr when it was new.

Once the car got old my GPS showed 175km/h while the speedometer showed 185.But i had also uprated my tyres, didn't correct for that.

Turn your computer onto the average speed setting, hold a steady 70mph, hold down the button on the underside of your wiper stalk to reset, after a few seconds you'll relise that your car knows exactly how fast it's going...