Tyre Pressure Question

[mattrsa]

Hi Guys

Have just got a set of 16inch merc wheels for the van running 205/65/16 on the rear and 185/75/16 on the front. Is there a way I can work out what tyre pressure I should run?

Thanks in advance

Mattt

[kevtherev]

why are the tyres in odd sizes front and rear?

[markhale10]

Rear will be wider as the vans are rear wheel drive. Could well be Porsche wheels which are wider at the back too.

[mattrsa]

Bought the wheels off someone and those were the tyres it came with them. They are load rated etc and have plenty of trend so not going to change them.

Any ideas on what tyre pressure I should go for?

[kevtherev]

they should be the same as your sticker in the van.

or draw a chalk line on the drive and drive over it (centered)
if the chalk is not on the tyre it's underinflated.

[mattrsa]

thanks for the reply kev. Was not sure if I had to adjust the pressure depending on the size of tyre etc.

Thanks
Matt

[busbuddy]

they should be the same as your sticker in the van.

thats not right, sticker only applies to the standard tyre


tyre pressures are specific to the tyre itself irrespective of the vehicle they go on

the rear tyre pressure on the sticker is 53psi* (i believe) which exceeds the max pressure on my rear tyres which are 255/55x18 109w off a discovery



*53psi only applies to the 185 rear tyre if your van left the factory with 205 tyres the pressure is a lot less, 40psi?

[CJH]

I had a similar issue when I put some winter tyres on a different vehicle. I wondered if there was an analytical way to calculate an ideal tyre pressure, at least to get a starting point that I could tweak to my preference. It turns out there is. Apparently there's a formula derived by ETRTO (The European Tyre and Rim Technical Organisation), and there's a spreadsheet that someone's put together to help you use the formula.

You can download it here

I used it on my other car, and it got me pretty close to what I felt was a good setup.

Might be worth a go. You'll need to know about the axle loads on your vehicle, and the specs of your tyres.

[kevtherev]

As I see it the weight of the van is constant.
The tyre pressure is there to keep the tyre shape correct and footprint as constant as possible to that weight.

putting less pressure in seems illogical unless the tyre can maintain shape with less
If it is logical then someone please explain it.

[CJH]

As I see it the weight of the van is constant.
The tyre pressure is there to keep the tyre shape correct and footprint as constant as possible to that weight.

putting less pressure in seems illogical unless the tyre can maintain shape with less
If it is logical then someone please explain it.

Well certainly different tyres will have different amounts of 'bulge' with the same air pressure (which is what prompted me to investigate in the first place when my winter tyres looked 'wrong'), and I guess that is down to tyre construction. Tyres designed for a heavier load will presumably have a stronger construction and might therefore bulge less under identical conditions.

[kevtherev]

I made tyres for twenty years and I can assure your vehicle tyre pressure is weight governed and OE tyres are chosen for use. Not construction.
reinforced radial and 8 ply radial are two different tyres
reinforced cannot be inflated much over 40 because of it's lighter constructon, but it's construction allows a heavy load, these tyres are cheap to produce and give less unsprung weight than an 8 ply.
8 ply tyres have stiffer sidewalls allowing heavier loading without the tyre getting out of shape

tread wear is the best indicator of underinflation.
If the tread shoulders are wearing quicker than the middle
it's under inflated.
a simple treadwear gauge well tell you that
wear should be even across the width

[bigherb]

Kev is basically right. the tyre pressure is determined by the vehicle weight and the footprint of the tyre and a few other things. 1psi pressure can support 1lb weight over one square inch. Generally you start off with the standard tyre pressure because the manufactures have worked out the calculations. If you increase the width of the tyre on the same size rim you increase the tread contact area so the pressure can be lower which is why 205/70R14C tyres have a lower pressure. But if you increase the rim diameter and keep the overall tyre diameter or make it smaller you decrease the contact patch so a 205/65R16 will have less rubber on the road than the standard 185R14 tyre so the pressure may have to be increased.
The only easy way to find the right pressure is to use a inferred thermometer and measure the tyre temperature at three points across the tread they should all be equal after a run. If the centre tread temperature is higher than the outsides then the tyre is over inflated if the outside temperatures are higher than the centre then its under inflated. But note the rear tyres are always meant to be slightly over inflated (this always applies whether it is FWD or RWD).

[pionte]

I run Merc ML wheels, with 215.60.16 Tyres , I found the best pressures for my van ( High top Camper ) to be 42 psi front and 44 psi rear. The tyres are either 97 or 99 load rated I forget at present. I settled on these by firstly looking at the tyre, did it look right .... ( all tyres have a maximum pressure stamped on them, dont exceed . I once had been standing next to someone who exploded a tyre by over inflating because he wasnt concentrating , my ears still ring 25 years later im sure )!

next I drove the van, did it feel right... right amount of roll on the flat and slight incline, then finally I looked at the tyre wear for signs of issues.

This is just what I use, and It may help you with a starting figure

[kevtherev]

I have identical wheels and tyres and run the same pressures.

[pionte]

I have identical wheels and tyres and run the same pressures.

kev , you may well have advised me in the past on where to start ... you gave me lots of help when I was deciding on wheels to fit