Why do car manuals never give this? Logic seems to indicate that 10% less front and rear would be appropriate (50 kg less in boot 80kg less in front means 130kg lower axle weight on an 1100kg car with 2 person +luggage load(200kg). ie 0.2 bar lower.As this appears to be how most cars are driven most of the time it seems a bit odd not to list it.

Vulcan4 said:

Why do car manuals never give this? Logic seems to indicate that 10% less front and rear would be appropriate (50 kg less in boot 80kg less in front means 130kg lower axle weight on an 1100kg car with 2 person +luggage load(200kg). ie 0.2 bar lower.As this appears to be how most cars are driven most of the time it seems a bit odd not to list it.

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Hi, and welcome,

:idea: if you've got a 7 stone (@45Kg) driver

or a 25 stone (@200kg) driver the variance is HUGE.

Imagine all the variables with "Duckling and Hippo" passengers..

TBH - I tend to go for the mid-laden + High speed pressures,
then it'll cope with most eventualities

Charlie - Oxford

The figures given are standard, with anything from just the driver, up to a maximum, then a higher figure for when more heavily loaded than 'normal'. The standard figure is right for most general use.

DO NOT start guessing your own figures as this could be dangerous.

Check the make of tyres, and have a good search on the tyre manufacturer's website. Many will have their own pressure recommendations. Use the higher figures of tyre manufacture or Fiat.

My Panda used Continental as standard, and I have now got Vredestein on the front. Both recommend higher pressures than Fiat. These higher pressures give a smoother ride and less understeer.

Pressures required are dictated by axle loads.A car painted in antigravity paint(Cavorite?-part number?) would only need enough pressure to seal the tyre to the rim- less if it were tubed.Many cars now list a comfort setting of around 0.3 Bar lower which does give a more comfortable ride over patched roads. There must be some reason why we are not given a lower pressure for a 10% reduction in axle load but so far i cannot find one.All opinions welcome.

What are you trying to achieve here? The vehicle manufacturer has given you tyre pressures. The tyre manufacturer may give slightly different ones. Use either of these. Anyone else's guess is likely to get you into trouble, or a ditch.

Of course, tyre pressure will also change dependent on temperature. Your tyres will be softer after a cold dark night than after a day in the sun.

Then consider that there may be a case for one wheel being in direct sunlight and one not therefore a further difference.

Add temperature increase as the tyre flexes during driving or add a bit of hard cornering on a dry road and the figure given in the manual is fairly arbitrary.

A decent 'personal download' will have an effect too!!!!

Even using Nitrogen rather than air will not negate all the above effects.

Cheers

D