YOUR RESEARCHSat-nav "speedometer"
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In general, is the speed as displayed on a sat-nav system likely to be more or less accurate than the one on the car? I'm thinking more accurate as it is not dependant on wheel sizes / pressure etc. I'm constantly exactly 5mph faster on the speedometer than the sat-nav (although there is a toiny lag while the sat-nav recalculates).
Satnav is definitely more accurate.
My values are very very close with my Kenwood 7200. Does your car have the tyres set at the correct pressures and the factory wheel sizes?
I've had a number of BMW/Audis that have been 5mph fast on the analogue speedometer. However nearly all were accurate on their own trip computer. This was handy as you had a built in margin for speed cameras. Regulations say that a speedometer can overread (I think up to 10%) but not underread. Motorways have markings at 100m intervals so its easy (when quiet) to check both your sat nav and speedo by going at a constant speed and counting 16 or 32 markers (1 mile or 2 miles to 0.5% accuracy). Before portable sat navs I used to do this for every new car so I new what the margins were. These days I have a portable sat nav which is accurate on flat straights. I've not checked our 996 yet My S2000 is only 2 mph fast with its digital speedo.
I've got 18 inch alloys which were factory fitted but I run the pressures a couple of psi lower than 'correct' as I like my fillings to stay in place!!
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