right, here is some info i've been holding back on, but it may be useful for some, most likely those in a hurry and with good mental arithmetic skills.
ages ago, a friend had demonstrated that his car speedometer and his new satnav were not in agreement about his speed.
and several years ago (maybe ten, not sure), it occurred to me to look at the regulations for the calibration of vehicle speedometers.
the gist is this:
all vehicle speedometers have to display a reading that is between 100% and 110% of the actual speed the vehicle is travelling at.
what that means is that when the speedo says 70, depending on how the speedo was set up,the car might be doing 70mph but it might be going as slowly as 70x (100/110) = 63.63. the real speed will probably be somewhere inbetween.
if it says 60 it might be 54.55
if it says 50 it might be 45.45
if it says 40 it might be 36.36
if it says 30 it might be 27.27
if it says 20 it might be 18.18
in most cases it should be possible to work out how far out the speedo reading is.
my previous car gave a reading that was too high by 1/16 of what it said.
if it said 64, i was doing 60. if it said 48, i was doing 45. if it said 32, I was doing 30, etc.
but if i wanted to do 30, it had to say 32, so i added 1/15 on. etc.
if i wanted to do 60, it had to say 64. if i wanted to do 75, it had to say 80. so if i wanted to do 70, i needed to add something between 4 (60/15) and 5 (75/15). so 74 and a bit.
the way i've used to work out the discrepancy for a vehicle is this.
you need a passenger with pen and paper and a mobile phone with a gps speed app on it.
you drive at a variety of speeds (ideally setting speeds on the flat using cruise control).
you tell your passenger what the speedo says when your speed is steady.
your passenger writes down the speed you've said along with the speed the phone app says.
do this for a range of speeds.
ideally, the ratios of the speeds should all be more or less the same.
(note: if you do try this and have bother with the sums, either add them to the thread or pm them to me and i'll see what i can get from them).
my current car, being of the french persuasion, is unable to give me a consistent ratio.
in the past i've worked out what the maximum safe speedometer speed is for each of the road limits.
with the car that read too high by 1/16 of what it said, i knew that going 10% over in a 70 meant not only was i safe if it said 77, but i could add 1/15 of that, which was another 5.13, but i'd still got the plus one or two as well, and with plus 2 it was going to be 84.27. (70+10%+2 all times 16/15).
in practice, i never drove with the speedo saying that much. where the limit was 50 or less, i would push it to the recalculated maximum if it was safe to do so.
i never got a ticket using that system.
a few years back, with the climate change stuff at its maddest (early 2022), petrol prices rocketed pointlessly and needlessly. petrol was around £1.90 a litre. the knock-on effects on, in particular, food (delivery costs) stlll hasn't entirely worn off.
but these insane price increases let me to a change.
i finally managed to allow more time for journeys and set off in plenty of time. it was more relaxing. i didn't need to drive like fangio, doing 10%+1 or 2 all the time.
and then i found that i could get more miles per gallon.
the yorkshire in me clicked in. i'll still do the limit if it's 50 or less, but tend to do less if it's a 70.
less speed meant that car was capable of more than 34mpg. i even managed 46mpg - more than 10 miles per litre! - at times.
the current wagon is a diesel. that means sometimes it's less than 9p per mile. with petrol i used to get 14p-16p per mile in recent years.
this stuff was useful for me, so might be for someone else.