Speedometer Out By 10Mph

[helijohn]

I don't know by how much the speedometer should be out by but Rav4 1996 reads just about 10mph fast.

I have 215 60 16s on the front - on the back are 215 70 16s!

That is 5 inches difference in rolling circumference and I assume the 70 aspect is nearer the correct size for the Rav and I also assume the speedometer gear comes from the front.

I want to put 225 75 16s M&S all round; that gives 10" more rolling circumference. Question is will that bring the speedometer into line??

[Heidfirst]

http://www.alloywheels.com/Tyre_Calculator

[helijohn]

http://www.alloywheels.com/Tyre_Calculator

Thanks, it is a useful website.

Thing is, if 215 60 16s give me 10mph too much on the dial when at an indicated 70mph I am doing 60mph which means about 17% fast, what tyre would giving me an accurate reading?

[Heidfirst]

Most modern Toyotas (& other makes) seem to over read by ~6-7%.

This is because they allow for speedo tolerance but it's illegal for them to under read.

If you look on the plate on the driver's door frame what size does it say there (I'm assuming that you haven't changed wheels)?

If it's 215/70 16 then running 215/60 16 would make your speedo indicate 5.6% faster than previous.

http://www.autopartuk.com/tyre-calculator/

Running non-standard tyre sizes can technically give insurance companies an out in the event of an accident if they haven't been previously informed.

Plus it affects your acceleration, fuel economy etc. etc.

[FROSTYBALLS]

In the UK the speedometer must never show an indicated speed less than the actual speed.

For all actual speeds between 25 mph and 70 mph (or the vehicles' maximum speed if it is lower than this), the indicated speed must not exceed 110% of the actual speed, plus 6.25 mph.

For example, if the vehicle is actually travelling at 50 mph, the speedometer must not show more than 61.25 mph or less than 50 mph.

[helijohn]

Most modern Toyotas (& other makes) seem to over read by ~6-7%.

This is because they allow for speedo tolerance but it's illegal for them to under read.

If you look on the plate on the driver's door frame what size does it say there (I'm assuming that you haven't changed wheels)?

If it's 215/70 16 then running 215/60 16 would make your speedo indicate 5.6% faster than previous.

http://www.autopartuk.com/tyre-calculator/

Running non-standard tyre sizes can technically give insurance companies an out in the event of an accident if they haven't been previously informed.

Plus it affects your acceleration, fuel economy etc. etc.

Right, according to the calculator when the speedo is reading 70mph the road speed should be 66.2 on the tyres I have.

I am actually travelling at almost 60 so there is a 6mph loss somewhere. The tyres are worn but I can't see that accounting entirely for the discrepancy so some of it must be speedo error. Problem is what is the deviance caused by wear. The tyres have just over 4mm tread left. Before I buy new tyres I would live to ascertain the speedo's error itself so I can move upwards in overall diameter and compensate. This is because I want to fit the tallest tyre I can!

The car came with odd aspect tyres and almost at the end of their service life.

In the UK the speedometer must never show an indicated speed less than the actual speed.

For all actual speeds between 25 mph and 70 mph (or the vehicles' maximum speed if it is lower than this), the indicated speed must not exceed 110% of the actual speed, plus 6.25 mph.

For example, if the vehicle is actually travelling at 50 mph, the speedometer must not show more than 61.25 mph or less than 50 mph.

So accordingly, showing 70 when the ground speed is 60 is within limits. This must give me some real leeway to fit some more business like M&S tyres. I could achieve another good inch of ground clearance with 225 75 16s.

[Red Yaris 54]

So accordingly, showing 70 when the ground speed is 60 is within limits. This must give me some real leeway to fit some more business like M&S tyres. I could achieve another good inch of ground clearance with 225 75 16s.

Not just any tyres..........These are MARKS & SPENCER tyres ;) :)

[Heidfirst]

"Right, according to the calculator when the speedo is reading 70mph the road speed should be 66.2 on the tyres I have.

I am actually travelling at almost 60 so there is a 6mph loss somewhere. "

As I said they are setup to allow for speedo tolerances - but they can't allow the car to go faster than indicated hence why most cars under read by 5-7%.

If you go back to 215/70 16s you'll come into that zone (& yes, tyre wear will also have a minor effect).

225/75 16s would have the possibility of you going faster than indicated which is illegal.