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It is all down to the vehicle you are fitting them on to and the systems it has fitted as they do use the speed the wheel rotates to run the following systems. And your new tyres are 14mm larger meaning they rotate less times over a given distance.

1 Speedo, the Speedo is driven from the gear box that then turns the wheels and this would give incorrect speeds.

2 ABS this measures how fast each wheel is going and tries to keep all wheels at the same speed stopping any locking as if it senses a wheel going slower it lets the brakes off. So all wheels would need to be the same diameter or I would expect the ABS light to come on.

3 Traction control this usually only works on the driven axle so this should not be a problem but could be.

4 Tyre pressure systems some of these measure the speed the wheel is going and if it is not correct it thinks a tyre is flat all 4 wheels would need to be the same size. I know a car that keeps saying on the dash that a tyre is flat when a new tyre is fitted this will be the correct size and pressure it will just have more tread or be a different brand but this can be re calibrated if you have the hand book and the time to sort it.

Ideally you have an old motor home with none of these systems except the Speedo by law the tyres on an axle need to be the same not all 4 if you have the clearance (7mm above and 5mm either side) then fit to the non driven wheels and you do not need to buy 2 more and it will effect nothing else.

However if you fit 2 and end up with problems with any systems you will need to have all 4 the same size if you go for the new size you will be going slightly faster than your Speedo states. This will be very slight approx 2% as your old tyres should be 682mm diameter and travel 2143mm per revolution and the new tyres should be 696mm diameter and travel 2187mm per revolution most Speedos do read slightly slower so this should not be too much of a problem.

Computer driven systems like the other 3 probably will not like the 2%

Also check the load the tyre will take this should be slightly higher on the new tyre but do not fit if it is lower.

Hi Wango

as you say the 225 is the width but most people do not realise that the height (70) is not a measurement but it is a % of the width so the side wall on a 215/70 would be 70% of 215 (150.5mm ) this is measured from the tip that you do not see behind the lip on the rim. and the 225 would measure 157.5mm