Calibrating the Speedometer

My speedo consistently reads 5mph faster than the GPS. Is there any way to get this corrected? (18" wheels if that makes a difference)

I have the Origin B2, and it reads 4mph under the speedo. I contacted Origin and they told me that they set them up this way! Their argument is that you won't get done for doing 4mph over. My brother has the same in his C4, and his reads exactly the same as mine.

Car speedos invariably are innaccurate to some degree, usually reading a little faster than your actual speed. Interesting that Origin admit to selling an innaccurate instrument[ ]

Thanks guys.

Rather than give you an accurate guage and allow you to have responsibility, in keeping with the general dumbing down I expect the future will hold a more simple system. The speedo will be colour coded, with a green zone, an orange zone, and a red zone. The green zone will be for in towns. The orange zone will be for out of town (if there is any given the amount of building there is), where there are orange signs. This gets round the fact that a significant number of people do not understand the national speedlimit signs and the general concept. Then there will be the red zone, which will cut the engine management, pull you over the to side, use the car phone to call the police giving your location from the sat nav. You will then be given electric shock therapy and have all you worldly posessions confiscated. The speeds will be dynamically changeable by satellite system, allowing the speeds to be changed by local councils, busy bodies, the time of day, the weather conditions and whether a bit more revenue is needed this month. Tampering with the system will block the engine management for 6 months forcing you to ride your state provided bicycle (which due to Government inefficiencies costs twice as much as if you just bought it yourself).

Classis [ ]

Excellent

What's weird is that it is constant. A brand new tyre has a circumference 40-45 mm larger than one with a minimum legal 1.6mm of tread (assuming a new tyre has about 8-9mm of tread). This equates to about 2% of the overall circumference of an 18" wheel fitted with a 265/35, which is about 2 metres. So that's 2 mph at 100 then. If your tyres are at the wrong pressure, they will bulge at high speed (they do anyway, but this is largely what the speed rating stuff relates to, I believe) further increasing the circumference, and so reducing the indicated speed. Porsche knows this which is why their designers position the speedo completely out of view, instead preferring you to pay attention to your tacho, which is far more important anyway! In my experience, if you think you may be driving too fast, you generally are. I was chatting to a serving Met. traffic division officer recently, and he was expounding a theory employed by his colleagues that everyone gets done for speeding in town at 45 mph because that is the slowest you can comfortably drive in top gear in an average car these days! Something to do with the way we were taught to drive - 'Get the car into top gear as soon as possible'. In 1970 a Vauxhall Viva was doing about 25 in top (that is 4th gear). Doesn't explain automatics though. Also, your GPS cannot be that accurate, can it, as it can only postion you to within 15 feet?

The GPS reacts instantly to small changes in speed (as quickly as the digital speedo anyway) so I'm inclined to trust it. Equally, I used it in my BMW and Elise regularly. The BMW used to over-read by no more than 2mph and the Lotus was (surprisingly) spot on. GPS is a lot more accurate than people think and if it's combined with other systems, can be relied upon to provide unparalled accuracy. I use GPS at work on a daily basis and it's accuracy is uncanny - almost spooky, and is likely to improve!! So the only conclusion I can come to is that the car speedo is not good. Some interesting thoughts though....