I'm struck by the number of people who have travelled in my car who say how fast it's going. I tend to drive roughly the same speed whatever i'm driving so can't really understand it, it must be a psychological thing maybe the proximity to the ground or the noise of the engine but my old Rover was close to the ground and no-one ever said that was fast! Maybe the speedo is out.

Have you tried comparing your speedometer speed with the speed that is shown on a sat nav?

I'll try that, it's bit of a problem getting into the cigarette lighter to plug it in as the cover doesn't open fully.

How about GPS on your phone? You can download a speedo app, or look at the bottom left of the screen when running Waze

IME the speedo an be out by up to 10%, ie: a real 70mph is an indicated 77mph, at least that was the case on my '97 Porto 20vt, but I have oversized tyres fitted which pretty much eliminates the error. On my '00 Plus however, the speedo is much more accurate.

My everyday car (Jaguar XF) has a similar error, a little less than 10%, eg: I set the cruise control at 53 or 54mph to maintain a steady GPS-indicated 50mph through the M4 roadwords, and 74-75mph to get 70 on the M3.

Oh yes, GPS on a smartphone should work well too.

Bit of a funny story. I once had a cheap smartphone with fked GPS. Sometimes it used to take up to 5 minutes to get a lock onto a GPS signal even when stood under a clear sky with a full battery and all the relevant settings set to most accurate.

I downloaded a navigation app on it once and while sat as a passenger in a car that was doing a nice and steady 70mph on the motorway, it said the car was travelling in the middle of the adjacent field............at a speed of nearly 300mph! I wish I'd taken a screenshot of the phone screen but I was too busy laughing.

I reckon if Bugatti had used that phone in that Chiron which reached 304mph, it would have probably said the car was going over 1000mph or something!

From a recent trip to my employer's office in Milan...

Imagine if this was someone's drive back home from work and they showed this screenshot to their partner who didn't trust them? The partner would probably say something like:

"If you travel 0.8 miles at a speed of 504mph, the journey should only take you 6 seconds.........WHO THE FK IS SHE/HE?!?"

I was checking the route from my hotel to the office which was 0.8 miles and should take me 15 minutes when walking. I was doing this when sat on the plane on the way to Milan, then I noticed that despite the phone being on "plane" mode, GPS was still on, and recording my current ground speed

A good fisherman's story Dave

GPS can be on on a plane, as it's a radio receiver with ideally no broadcast component. Though there is an agreement that domestic receivers are limited to stop working if either above 60,000 ft or moving faster than 1,200mph...

My Coupe has the most accurate speedo of any car I've used GPS in, depending on tyre tread depth it has been absolutely spot on sometimes. I did notice recently that with brand new R888r 205/15/16 it was actually slightly under-reading, saying I was doing 70 when GPS said 71.

Mine originally had the 15" wheels with 205/50 15 tyres and it was quite a bit optimistic. Those wheels now have the winter tyres on and I have a set of 20VT 16" wheels with 205/50 16 tyres. This combination is also about spot on against the GPS value.

The thin-feeling floor of the Coupe might help make one feel like one is going faster than one really wants too Note to self - must add some sound deadening in the foot wells!