Club 80-90 Forums

Simon Baxter wrote:Another way I just found on the internet is..

take the vehicle weight is lbs..

divide by 100
+ 2psi
+ another 2psi for the heavy end (rear on a T3)

+ another 2 psi all around if it runs standard suspension.

So, my T3 Westy weighs 1850kgs which is 4078lbs

4078 divided by 100 is 40.78psi

+ 2ps all around is 43 psi, plus 2 psi for the heavy end is 43 front, 45 rear.

And to be fair thats what I have run them at more or less!

I was given a formula for working out the new correct tyre pressures if you increase or decrease tyre size. It came from someone at Avon Tyres who I know slightly, helping me tell some muppet that just because a tyre says "max inflation pressure of 54psi" on the tyre sidewall this is not what should be used!

Here it is... Best illustrated by example.

Example: Going up in size from standard factory size of 185/60R14 to 195/50R15. Original pressure 26psi. What is the correct new pressure?

STEP 1: Calculate rim diamter in mm: (convert inches to mm by multiplying by 25.4)
[tyre 1] 14*25.4 = 355.6mm
[tyre 2] 15*25.4 = 381.0mm

STEP 2: Calculate overall diameters including tyre (the tyre profile rating is a % and should be divided by 100 = 60--0.6 50--0.5)
[tyre 1] {(185*0.6)*2}+355.6 = 577.6mm
[tyre 2] {(195*0.5)*2}+381.0 = 576.0mm

This confirms that the diameter of the tyres are approximately the same, giving a similar revolutions per mile so as not to throw out the speedometer reading too much.

STEP 3: Calculate the volume of the tyre. 3.14159 is pi
[tyre 1] {(185*0.6)*2}*(14*3.14159) = 9746
[tyre 1] Multiply by original pressure of 26psi = 253865 (equivalent volume/pressure)

[tyre 2] {(195*0.5)*2}*(15*3.14159) = 9189
[tyre 2] Pressure required = 253865 divided by 9189 = 27.6psi

Therefore, changing the tyres from 185/60R14 to 195/50R15 will need the tyre pressure changing from 26psi to 27.6psi

Hopefully you could use this guide to work out what your new tyre pressures - all you need to know is the original tyre size & pressure and also the new tyre size.

My chum says: "This is approximate as the actual calculation is a little more complicated and requires an understanding of tyre engineering design of equilibrium pressures and rim widths that is a little too much for here"

Simon Baxter wrote:Another way I just found on the internet is..

take the vehicle weight is lbs..

divide by 100
+ 2psi
+ another 2psi for the heavy end (rear on a T3)

+ another 2 psi all around if it runs standard suspension.

So, my T3 Westy weighs 1850kgs which is 4078lbs

4078 divided by 100 is 40.78psi

+ 2ps all around is 43 psi, plus 2 psi for the heavy end is 43 front, 45 rear.

And to be fair thats what I have run them at more or less!

Simon Baxter wrote:They were commercials so I would presume so.
You know what builders and plasterers are like.....

toomanytoys wrote: And if you use the calculation from the max load at max pressure? anywhere near the same?

Simon Baxter wrote:

toomanytoys wrote: And if you use the calculation from the max load at max pressure? anywhere near the same?

I'm not in the habit of remembering such dross as what is written on my sidewalls!
The van is at work, in pieces, I'll look tomorrow if I get a minute.
Just trying to get my head around putting that other lot into the same spread sheet so there are 3 versions, was just going to do all 3 and kinda work out what I should do but I'm fairly happy that my usual 37psi 45 psi is not far away, just nice to know that there is a few pointers out there if you need them.
Plus almost every van that comes in for a service has alternative wheels and most owners are clueless (no disrespect but they like the look of the wheels and are a little flummoxed when it comes to knowing what to put in them)
Doing this kinda "pooh" on a night makes my days easier!
Same as the syncro tracking graph I did, that seems to work too.