Speedo Calibration

Hi all, not sure if this topic has been covered before. But I've noticed that my displayed speed appears to be out by about 8% at 100kph. Ie it will show 108 while the GPS is displaying 100. Infact it'd be fair to say that even at 50kmh it appears to be out by about 7kph. It's a Turbo and was delivered with a new set of pzeros. I've checked the tyre sizes and they're spot on. I know a bit of descrepency might be normal, but this seems a little bit much to me and I'm just wondering if there is anyway the speedo can be recalibrated. Kiwi

Mine's the same - maybe worse. My speedo reads 33-34mph when the GPS shows 30mph. I think they always err on the 'safe' side. You would also find a discrpancy between the total distance recorded in the DME and that recorded by the Odometer. I believe the DME reading is more accurate, and I reckon most 996's display a mileage 5-10% higher than they have actually covered.

I'm not convinced that SatNav provides a reliable measure of speed. I have found that simply changing direction can influence the device's estimation of your speed, even when your speed hasn't actually changed. Sometimes, the nav's quite close to the speedo, other times it reads up to 10% low. I'd be interested to hear other people's experiences. :?: Cheers, Roy

Your speedo should work from the rpm of the wheels so is pretty much an instant reading. The GPS/SatNav wil calculate your speed based on time between signals and distance travelled so is really only an average. If you set your OBC to mph, the longer you drive about the longer it takes to change the average speed displayed but your speedo will change rapidly with throttle changes. Similar principle but the GPS is much quicker at updating the info. Try driving around in a fast circle and see what your SatNav gives as your speed. TB

It's not just the Sat Nav, Here in New Zealand the powers that be seem to have fallen in love with these little speed signs and they've sprung up all over the place. They display your speed as you go by and if your a few K's over it flashes 'Slow Now'. Infact due to their abundance I normally pass 1 or 2 on every trip out I do and they always point out this large descrepency. Okay, so I'm getting the feeling that there is little I can do and it's just erring on the side of caution. But, can I then ask does this erring also flow through to mileage recorded. As I'm just wondering then if essentially the recorded miles are being overstated by about 8% also? Kiwi

the legal requirement is that the indicated speed must not be more than 110% per cent of the true speed plus 4 km/h and the indicated speed must never be less than the true speed. Therefore, a speedometer displaying 79 mph when true speed is 70 mph is only just legal.

Hi,my car also shows + 4 mph at any speed when checked againt a Snooper radar detector.It registers the same speed before and after a pair of new rear tyres were fitted.Its better to ere on the side of caution on this one so I allways glance at the detector when I spot plod or a camera. :wink:

As I said, that's what I reckon happens.

Most car manufacturers aim for about 4% fast as it allows them to stay within the law taking into account tyre wear and manufacturing tolerances. However, there is mounting evidence that BMW (and possibly others) are setting theirs at a much higher speed (e.g. 8-10% fast)... Not that I'm cynical or anything, but this has significant marketing advantages: Shows a faster speed than is really the case, making acceleration times on things like pocket dyno look good and top speeds significantly over the 155 mph restriction agreed with MB, VW and Audi) Suggests fuel consumption is significantly lower than is really being achieved - including showing better average mpg figures on the on-board computer. I would imagine that official emissions tests do not use the car's instruments, but if they did, that would have a massive impact on CO2 related taxation!

......every bit of pork I have overstates by at least 3mph vs two sat navs or my snooper.