Ask Honest John

Most recently answered speedometer questions
If the smaller of the two numbers on a tyre relates to height will remaining on the same size wheel with taller tyre not affect speedo readings?
When using Google Maps I noticed a 3mph discrepancy between my car speedometer and the phone sat-nav. Which should I rely on?
Some local authorities are proposing that drivers will be given penalty notices for exceeding 20mph limit by as little as one mile per hour. Does this match the level of accuracy of speedometers? I thought...
I have an old (1976), 24 volt ex-Army Land Rover. The speedometer needle 'waves around', and so is not reliable! Consequently, I have just had to attend a speed awareness course for going 24mph in a 20mph...
On the occasions I use my smartphone as a sat nav, I get a readout of the speed of the car on the screen. It always shows a slower speed than my speedometer by 3-4mph. Since this can make all the difference...
Cars made before 1932 do not have to have a speedometer. Are there any special dispensations for these cars when they pass through camera controlled average speed sections of road? GPS speedos have issues...
I have decided to upgrade from my 2005 Avensis to a 2017 Toyota Auris. The new car was fine but the speedo is too small for me to see easily. It is much smaller and buried away in a tube compared to my...
How accurate are car speedometers and what is the effect on accuracy of changing running radius of tyre as tyres wear?
The speed recorded on my satnav is 3mph slower than the speedometer reading, which is likely to be accurate?
How accurate are speedometers? My Toyota Auris 2016 reads about 10% lower than reality, i.e. 63mph shows as 70mph and 27mph shows as 30mph. This is unacceptable. All data on this car is inaccurate and...
 

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