Tyre pressure question

In message , David Speir writes

The pressure does change for different tyre sizes.

be set to the

aftermarket

No, but they're a good guide even with larger diameter wheels and lower profile tyres. The rating can also affect the recommended tyre pressures. But within a couple of pounds either way, the pressure is not that critical. The best bet is to experiment a little to find the best pressure for the cars handling. Braking, cornering etc. The tread wear pattern should show if you gone too much one way or the other. Mike.

Lower profile tyres need higher pressures for even road contact.

As an example, the Owners Manual for my Saab 9000 specifies the pressures for different wheel/tyre choices:

Higher pressures are necessary for maintaining flat contact surface. If you don't increase the pressure for lower profiles, you could easily damage the alloy wheels on imperfect roads. Ride will be harder, but that is a necessary trade off for better handling.

If you don't want the harder ride, then don't go for a lower profile option. Many do just because of the fashionable looks.

I was told to keep it the same as manufacturers - when going from 165/65/13 to 195/45/15 IIRC

By whom?

People I bought the wheel and tyre package from

How can I put this gently... On the other hand better not.

OK, so standard pressures are 31 up front, 26 for rear - its a Ford Ka..

What should they be?

I don't know the figures for your car, but using the table for my Saab as a yardstick for calculations, I get 38 up front and 33 for read. Try this and see how it feels and then adjust, i might be less or more, but this can't be far off.

Hmmm.... the ride is already hard as hell.... might try it though and see how it handles - cos I can't really fault the handling at the moment it is superb - mayb a touch of understeer, but its really good.... see what its like if I pump the tyres up a bit..

If the ride is too hard, then you have to compromise. Some new cars that come with very low profiles are designed for that from the outset. The problem is that your car is probably not designed for it, so you get the hard ride. If you lower the pressures, you have to be careful with potholes and other obstacles on the road.