how reliable is GPS speed?

**Charlie** · #11 (**permalink**) 30-07-2004, 02:53 AM

My GPS speed is usually about 5mph lower then my speedo using TomTom3 on an iPAQ 4150 with a Leadtek BT receiver. I can't decide which is telling the truth...if either. It could depend on a lot of things.

pmarley · #12 (**permalink**) 07-08-2004, 03:05 PM

As speed is Distance Travelled compared to time taken the only way is to drive a measured mile (ie. sign post saying junction 1 mile) for one mile at exactly 60mph - you should see which is correct.

My GPS is correct - My car is wrong. I had a Ford Ka as a loan whilst my car was being serviced - The Ka's speed was more reliable than my Golf.

The Golf has a digital display for Aircon etc. Pressing three buttons together brings up diagnostics and option 16 is Speed in Kilometres (being German !) I set my GPS to KPH and TTN3 to kph and it is exactly correct to 1kph.

Appears that the digital to Analogue conversion (ie. the Speedo) is wrong

Paul

pmarley · #13 (**permalink**) 07-08-2004, 03:11 PM

One More thing......

I was done for speeding in my old car in Lanarkshire.

I denied speeding and the car was put on a rolling road it was 10% out at all speeds upto 70mph which is also what my GPS works around. I still had to appear in front of the Sheriff and explain why I was speeding. 3 points and a £30 fine ! I had been caught at 100.18mph though....