Anyone dishonest enough to try this could only remove the mileage traveled since their last MOT of the car, as the odometer reading is on the certificate.
I suppose this could be useful if your using one of those little gearbox thingys to make your KPH speedo read in MPH. You could “adjust” the reading to show total miles travailed rather than having to explain to a prospective buyer that the first “X” many of the miles shown on the odometer were in fact kilometres, Or you could do this to adjust a second hand speedo head to your cars correct millage.
This info could also be used to defraud the insurance company if you had a limited mileage policy and you were getting close to your yearly mileage allowance.
I presume they have access to MOT records. Adrian Flux asked me to complete an annual declaration; like any declaration to the insurers, you are obliged to declare any material fact. If you don’t, if you have to claim, you might be refused.
I’ve got four limited mileage policies, two ask for mileage readings on renewal, two don’t bother. One of the companies increased my mileage limit from 3000 to 6000 miles for free when I phoned up and asked, another company charged me £100 more on a £300 premium to increase the mileage from 3000 to 4000 per year.
None of it makes any real sense, but misleading an insurance company isn’t worth the risk. Besides, sometimes it’s fun to use a silly car as daily transport to even out the mileage across the fleet.