Oversized tyres and trip computer inaccuracies

[educated arborist]

On a positive note numerous coppers have been polite & curtious ive given one discounts off tree work after he pulled me & gave me a producer i wouldn't be taring them all with the same brush most are real nice blokes.

My fondest cop memory was when i was pulled at 3 in the moring on a dead motorway clocked well over 110 i held my hands up & assumed i was about to loose my licence, he checked the car hadn't been stolen from cardiff audi (it was on test drive to me) said i was a niced chap, that i should lower the speed in the future & didn't even give me a producer let alone a ticket LEGEND!

[Treewolf]

The 10% + 3mph business is based on ACPO (Association of Chief Police Officers) guidelines and is not law; it is simply that they reckon if they booked everyone over the speed limit at all there wouldn't be enough time left to catch real crims. Legally you could be booked for 1mph over the limit but realistically it won't happen.

 

Vehicle speedos by law can over-read by (I think) 10% but are not allowed to under-read at all. So it would be ok to show 77mph at a true 70mph, but not to show anything less that 70mph at a real 70mph. Since it is is practice virtually impossible (and very expensive) to make a speedo that is spot on accurate, car makers always fit speedos that over-read, that way it keeps them safe from under-reading which would be illegal. Most if not all police traffic cars and bikes (but not the bog standard panda cars as far as I know) have calibrated speedos which are regularly checked and certified and can be used in evidence.

 

I have a Disco 2 and do a regular run from Dorset to Devon every week. On the standard Michelins (235/70x16) the journey was 120 miles, when I replaced these with 245/75x16 BFG Mud T/As the journey reduced to 115 miles, and when the 245's wore out I couldn't get any the same size and went up to 265/75x16 BFG Muds and the journey is now shown 110 miles.

 

The amount of fuel I use hasn't changed to any significant extent.

 

If you do put large tyres on and want to check the speedo, all you need is a motorway or trunk round with kilometre posts and a stopwatch. This will be more accurate that a satnav. My speedo is now spot on at 50, 60 and 70 mph, even though the mileometer under-reads.