Tyre Pressure

[Dazmundo]

Hi I am trying to work out what my tyre pressures should be. I have found the little plaque and it says

185 R14C 6PR Front 39 PSI Rear 48 PSI
205/70 R14-97R reinforced Front 30 PSI Rear 40 PSI

However I am running
225/45 ZR17.94W XL
so what pressure should I set them too??

(All these letters and numbers mean nothing to me! )

D

[Dazmundo]

Nobody got any ideas?

D

[pionte]

ok, I am no expert but pressure isnt really governed by size, more by weight. so if you run standard 185/14c tyres then ( depending on where you read ) 43 psi Front 48 psi Rear so I would be lead by that, as long as your tyres are rated to take the weight? putting 48 psi into a fiesta tyre would worry me!

[Cruz]

I run 35 front 45 rear on 225/55/16 even wear after 12 months

[CovKid]

On 195/60/15 I inflate to 40 on all four - not a bad starting point.

[Red Westie]

With a load rating of 94 (690KG) you are pretty border line, I'm guessing the XL means 'extra load' ?
35psi F
45psi Rear sounds somewhere about right.

Martin

[CycloneMike]

You can't assume that the pressures printed for the original tyres are correct when you change the size.
I'm not aware of any simple formula that can accurately dictate the correct pressure for a given size/load.
There is a relationship between pressure and volume of the air in the tyre to carry the same load, but the load capability is also dictated by the construction of the tyre. When you change the type or size of tyre, this also has an effect on what pressure is required.
Low profile tyres have stiffer side walls and a completely different footprint to standard tyres even if the volume in the tyre remains about the same, so pressure requirements for the same load are normally lower.
For those of you who are sceptical the Haynes manual (as an example) shows vastly different pressures for similar sized tyres but different construction for the same van/load. But the wider lower profile tyres have much lower pressures.
175R14C = 41(F) 48(R)
185R14C = 38(F) 55(R)
185SR 14 Reinforced = 33(F) 42(R)
185SR 14 C 6PR = 39(F) 48(R)
205/70R14 = 30(F) 36(R)
As a rough guide you can use the max pressure and max load printed on the side wall of your tyre to make an estimate of a safe pressure.
Example
Assuming you don't overload your van:
Front Axle Max load = 1200kg, (600kg each tyre)
Rear Axle = 1300kg. (650kg each tyre)
You now need to find the load for your tyres in your case (if correct) 94 which = 670kg (as Martin says this does not give much excess capacity particularly on the rear)
The Max pressure you will have to read off the tyre but as a guesstimate lets say 45psi
So for a fully loaded van a starting point would be
Front 600/670 x 45 =40 psi
Rear 650/670 x 45 = 44psi
These could be lowered for less weight.
As I said this is a guide but should ensure that you are safe. You then need to check it is correct and that you don’t have excessive wear etc. You can do this by watching the wear pattern, but a much quicker method is to cover an area of the tyre tread with chalk or similar. Take the tyre for a short gentle ride in a straight line and check to see the how the chalk has worn off. If it wears off the middle first they are over inflated, if the edges (both) go first then it is underinflated. If the one or the other edge goes then you have geometry problems, which is a whole new subject.
Hope this helps,
Regards
Mike