ronp wrote:

Thehorse wrote:

Under read. I.e. You will be going faster than the speedo says.

Yep got that, it was the recording of mileage and the ave mpg.

@ choccymonster
Re the unlawful over reading of the speedo.
Wasn't aware of that.
How is this determined and by whom - is it an mot check?
Or is it a law that stands, but a vehicle isn't officially tested for?

Try a search for a Speedo healer.

Speedo re-calibration is common using these Speedo healers on motorbikes when adjusting the gearing.
It is not illegal and is just a plug and play box of tricks.

Not sure how common they are in the car world?

Have been reading back through the thread, and "overread" and "underread" seem to be used interchangeably at various points.

Just for absolute clarity, and I realise I am repeating myself:-

- Your speedo may "overread" (tell you that you are going faster than you actually are - indicated speed > true speed)
- You speedo must not "underread" (tell you that you are going slower than you actually are - indicated speed < true speed)

I believe that the speedo may overread by as much as 10% + 6.25mph. In other words your speedo may indicate you are going 50mph, but you are in fact only doing 40mph. This would be legal.

(Don't quote me on those tolerances - have dragged that up from the pit of knowledge they laughingly call my brain. May not be correct).

Billsy wrote:

Try a search for a Speedo healer.

Speedo re-calibration is common using these Speedo healers on motorbikes when adjusting the gearing.
It is not illegal and is just a plug and play box of tricks.

Not sure how common they are in the car world?

choccymonster wrote:

Just for absolute clarity,
- You speedo must not "underread" (tell you that you are going slower than you actually are - indicated speed < true speed

Yeah for me as the OP, it is the "under reading" that would possibly concern me.

As you say, generally most vehicles will "over read", and for me that's not the issue.



@ Billsy, will check out that box of tricks

Edited to add, just checked the website and as you mentioned it seems tailored to bikes, but I've just dropped them an email explaining my potential predicament on a LR2.

Bigger tyres make it appear the car is going slower, using more fuel and travelling less miles. My tyres are around 6% oversize, therefore if my speedo says 60mph I am actually travelling 63.6mph. If my odometer says I have travelled 100 miles, I have actually travelled 106 miles. Fuel consumption on the read out is 6% worse than actual.

The only thing to keep in mind is that speedos normally read slow by 5-10% anyway. You can check this with a gps. Sometimes fitting larger tyres actually makes them more accurate.

Speedos generally read FAST - not slow.

ie they say you are going faster than you actually are, which is perfectly legal (within tolerance) in the UK.

However, as "mikehzz" says, fitting bigger tyres may skew the readout more toward underreading (ie reading "slow").

As long as it doesn't ACTUALLY underread, then you're legal in the UK.

Put another way; fitting bigger tyres may absorb some of the inherent "overread" that is built into every speedo. As long as you don't got mad and actually cause the speedo to underread, then you could probably get away with it.

The key would be to see how much your speedo is overreading by with normal tyres fitted - you'd then know how much you've got to play with.

Yes you are correct mr monster. My statement should have said fast not slow. I think the manufacturers like that because it makes the car seem to go faster, have better fuel economy and more frequent services. Cheers.

choccymonster wrote:

Put another way; fitting bigger tyres may absorb some of the inherent "overread" that is built into every speedo.