Hmmm, I was always told it was because the red electrons knew to go to the rear, and the white ones being extra nosey wanted to see where you were going so went to the front. Meanwhile those pesky orange electrons could never decide if they were going to work or not.

OK I'll get my coat. Taxi!


Seriously though, over the years I have driven a lot of vehicles with calibrated speed indicating devices, and have got used to having an accurate readout. Thus one of the first things I do with my own vehicles when new to me is to do a calibration curve on the speedo. For years I always used the measured miles that the road policing units used, and this can take some time to get enough readings over a sufficient range of speeds to get a decent curve.

Now I use a gps device with a few additional readings on times runs on a measured mile to check the gps accuracy. It's a much quicker process and effectively has proved to me that using a gps with a decent signal and tracking a reasonable number of satellites on a straight flat stretch of road is perfectly adequate.

In 30 + years there is only one make of vehicle that I found was completely bang on throughout the range as delivered from the manufacturer and that was from Citroen, both BX and Xantia models. No idea on more recent models.

My own FL2 is, as other have suggested, generally about 1-2mph fast throughout the practical range in UK, ie a gnats over 1mph fast at 30, and a gnats under 2mph fast at 70.

EYorkshire wrote:

Here's an interesting 'doppler effect' question

Q: What is the simplest way to observe the optical Doppler effect?
A: Go out at and look at cars. The lights of the ones approaching you are white, while the lights of the ones moving away from you are red.

What would we do without Google eh?

To get back on topic, my MY2013 shows a consistent 3-4mph fast. (I trust my sat nav) The speedo and the lights are poor on this model.

I've always found that the discrepancy is a percentage and is consistent so if it's 5 over at 50 it will be 10 over at 100. On some of our freeways we have speed check readouts on electronic boards. We also have temporary speed check readouts at road work sites. These are good places to check your speedo and gps for accuracy. We also have odometer check areas on freeways, usually a 5km stretch with a sign at every kilometre. Do you not have them in the UK? My gps is pretty close to the check readouts. I have larger tyres on, 245/65/17 and they make the speedo pretty accurate. I know that 245/70/17 will actually make the speedo read slow so can be a real trap.

RE the distance markers in UK.

No we don't have anything simple like just described.

We have two things, Every 100m (109 yards 1.084ft) there is a small post at the side of the emergency lane. These are all numbered, but the font is rather small. In theory a passenger could do a count of these but not very practical.

The other thing is an official measured mile, which is marked by the County Surveyor and traceable back to official standards at the National Physical Laboratory.

These are more or less all over the country but you have to know where they are, but they are what the police use to calibrate speedos and do a daily check on Video speed time/distance measuring systems.

I don't know if they are the same all over the nation but the ones I use have at start and end points a black and white road edge boundary marker with red reflectors on top, UK folks will know what I am talking about, There will also be a short white line painted at the base of the sign a foot or so perpendicular to the road.

Well just filled up again and did the usual check with the miles travelled and gallons used (sorry for the imperial) and have clocked over 40mpg. Driving style, type of journeys, etc. has not changed and I have been getting 36-38mpg but on the odd occasion has dropped to 34mpg. I am quite aware of my driving style and always try to minimise fuel consumption so I am wondering if the odometer or fuel pump accuracy is variable (or both come to that matter!)? Anyone else noticed a 16% difference in mpg figures?

Kickstart wrote:

Q: What is the simplest way to observe the optical Doppler effect?
A: Go out at and look at cars. The lights of the ones approaching you are white, while the lights of the ones moving away from you are red.