Tezza

46 Psi is about  317 Kpa

140 Kpa  is about 20  Psi.
I would go by the makers reccomendations. 
My Hiace has  195 x 14 Light Truck Tyres   Front are 325 and Rear are 450 Kpa on the makers plate 

Usualy the Tyre Air unit at service stations is Calibrated in both Kpa and Psi Units.

Regards
Mike

G'day Terry, first up a small conversion rate for you, basically 1 KPA = 0.145 PSI tyre pressure is a much misquoted animal, two things you are looking for in relation to inflation pressure is (1) comfort and reliability of ride, pressure too high and it will cause damage to the tyre due to overheating, an overinflated tyre will also create a harsh ride doing all sorts of damamge to your vehicle, remember the tyre is the first part of your suspension sytem (2) tyre life, an under inflated or overinflated tyre will reduce your tyres life quite dramatically by uneven wearing or overheating if your tyre is wearing more on the centre of the tread then your pressure is too high, if your tyre is wearing on the outside of the tread then your pressure is too low, even wear right across the tread pattern is what you are looking for, if you achieve this then you are doing everything right dont worry about what anyone tells you, there are too many knowalls on this subject that will throw a new player right out the window a good place to start is your own tyre placard for your vehicle, then add more or less as you check your tyre wear pattern using the aforementioned wear method the actual "pressure" that you will end up with is secondary and should not be the be all and end all, as you add or deduct weight to and from the van then your tyres should be adjusted accordingly, nobody does this of course, an average is about as close as you will get if your tyres are "standing" on the tread and not on the wall then this is a good starting point after inflating to your tyre placards recomendated pressure, adjust as you check your wear rate

Tezza Had a look on my van , single axle about 1.2t, tyres 185x14 8ply light truck, and on info plate show cold pressure as 280kpa or 41psi. Just had van serviced and they put 50psi in the tyres. What Davo says make a lot of sense as usual. Regards Eddie

Read somewhere that if you inflate to a recommended pressure when cold and after driving for awhile, say your next comfort break, and the pressure has only gone up by 2 psi when hot then your on the money. I don't know I just read it in some van maintenance book

This may help maybe:- Inflation and the 4 psi rule Tyre pressures vary for every vehicle and caravan depending on weight loads, size etc. An easy do-it-yourself method for checking you have the correct tyre pressure for your vehicle and caravan is known as the "4 psi rule". Firstly you need to inflate all tyres to the pressure recommended by the manufacturer, for a "cold tyre" reading and then tow your caravan for say a distance of 100 klm on bitumen. Upon stopping, immediately recheck your tyre pressure while the tyre is still warm. If your tyre pressure reading is greater than 4 psi from the "cold tyre" reading you took at the beginning of your trip, then the tyres are getting too hot and your starting tyre pressure was too low. Under inflated tyres wear unevenly and lead to increased fuel consumption. They can also impair the handling of the vehicle in the areas of braking and handling. If the tyre pressure reading is less than 4 psi from the "cold tyre" reading you took at the beginning of your trip, then your starting tyre pressure was too high. You will need to run the test again (once the tyre has completely cooled) with less pressure until you find the right balance. It is acceptable for larger 4WD's to have a 6 psi difference between the pressure at the commencement of the trip, and after 100 klm of travel. Remember to use the same accurate gauge for all pressure readings and always carry a gauge with you when possible. Have a good one

therefore by that rule i need to have more than 46psi when cold if that increases to 56psi when hot? where do you stop?

mate it's not rocket science, inflate to placard recomendations then watch your tread pattern wear, centre wear = over inflated, shoulder wear = under inflated, it really is that simple, if you have a "bulge" when the vehicle is on level ground then inflate a couple of pounds more until it is standing "straight" tread flat on ground then adjust pressure according to wear I work in an industry that has been dealing with tyres and mechanical repairs and updates (have done for in excess of twenty years) there is no hard and fast rule governing tyre pressure, it's all an individual loading equation that the owner/driver must work out for their own needs for their own vehicle the placard will give you a "starting point" but as you add or subtract weight and as the temperature increases and decreases and road surfaces change then the tyre pressure "should" be adjusted accordingly none of us do it, so we must find a "happy medium" somewhere that the tyre will give a decent life, ride and safety start with manufacturer recommendations and watch the "wear" indicators across your tyre adjust as necesary, if it is wearing even and it is giving a satisfactory ride then DONT CHANGE IT, regardless of what anybody says most tyres that we get in prematurely are caused by underinflation due to not checking, better to have a little bit more air in there than not enough, and we here this oh so many times "my mate said" hell if we had a dollar for every time we heard that

thank you

I must admit that the way I worked the minimum/starting pressure is as a ratio of our vans actual weight to the Maximum Load Rating at Maximum Pressure Rating of the tyre.

What this means is this.

Our van is a tandem axle, weighing 2750 kg fully loaded, so based on that, the AVERAGE load on each tyre is 690 kg near enough (I know not exactly and this takes NO account of tow ball load, but gives a good start point)

Our tyres on the van show Maximum Load per tyre to be 925 kg at 44 PSI. They are 235/75R 15 AT tyres.

Thus our vans weight is 690/925 as a ratio/%age of maximum load, or 74.6% (near enough to 75%)

Thus I started at 75% of 44 psi, or 33 psi.

I run the tyres at 36 psi, and have found this to be a good pressure during our trips, but do drop a little on dirt, to around the 30 psi. This has been fine over the 30000 kms we have towed the van, and over that period we have had no problems with tyres. I used the same process on our previous van, and towed that some 25000 kms without incident.

This method may give you a place to start with your pressures.