How do you calculate the correct tyre pressures for different size wheel + tyres

[les.tansey]

I heard that it changes from the reccomended tyre pressures which it says in the manual, and that you don't just go with the pressures on the side walls of the tyres - which makes sense really as the same wheel and tyre combo could be put on various specs of the same car, all of which would have slightly different reccomended tyre pressures...

My original tyres were 175/70/13's and I've up-sized to 195/45/16's, does anyone know how to work out the new optimum tyre pressure?

Thanks for any help 

[Seanl]

http://www.milesgallon.com/tire_pressure_calculator.php

It's amazing what google does! 

[tweed]

only really matters on the weight of the car.

I go by the feel too. I once had 36 front 29 rear due to different tyres and grip.

on my 8" front 9" rear and 195/45/16 all round I have 36 front and 32 rear.

my work van has 32 all round and that's 2 ton 

[tweed]

and in karting you can win or loose with tyre pressure. Only change the rear really and it all depends on the conditions. Normally 19psi

[Wayne]

I have always stuck with standard pressures no matter what the new tyre size is.

[les.tansey]

Cheers all, Just stuck 32psi in all round for now, might put a bit more in the fronts yet but its better than what it was - it feels nippier 

[rob.043]

try a bit less in the rears. Over 30 is a lot for rears.

Adjusting pressure results in adjusting the deformation of the tyre, and thus the size of the contact patch to the road. The lower the pressure, the bigger the area that touches the tarmac, so more grip, but if the area is too big then the rubber is not pushed hard enough into the tarmac to key in and grip, so grip can be reduced. 

I would think Wayne's idea of using book pressure figures is about right. Even though a tyre may be wider, the load on it from the car is the same, so the same pressure results in the same sized contact patch.

[tshirt2k]

On a mk3 16v with 195/50-15. The recommended tyre pressure is 37psi front 35rear. The 205/50 uses around 32 front 30 rear. Iirc

[DOA]

try a bit less in the rears. Over 30 is a lot for rears.

Not really, 36-38PSI has done for me on all of my front wheel drive cars except a 106gti with a frisky rear end that needed more like 28PSI to control it. It never really becomes an issue from deformation on FWD cars either as they do nothing grip wise and have minimal weight on them. I have never actually worn a rear tyre out used solely on the rear of a FWD car due to this. They usually get put on the front when I get around 2-3mm of tread left on the fronts to get the most out of the tyre set and I have yet to see a rear tyre that wore oddly on the front after doing this.

As for what the fronts should be, each manufacturer will have its own recomendations and they used to list them on their websites. I tend to find somewhere between 28 and 32 works for most cars (FWD and RWD) with 28 being used on lighter cars and upto 32 used on heavier cars but I play around to find the best balance.

[bob23]

On a mk3 16v with 195/50-15. The recommended tyre pressure is 37psi front 35rear. The 205/50 uses around 32 front 30 rear. Iirc

Have you got any idea for mine? I've got a mk3 8v with 205/40 17's, I don't even know what size the car came with or the manufacturer recommended psi so any help would be appreciated.