Re:Tyre Pressure for 215/75/R15 ?

wrote:

As it's a gen4 the offset of the factory wheels is actually negative 22 

Factory alloys on a gen 4 have an ET/offset of positive 5

Yes we found that out a while ago. I thought I had rectified all the mention of the 22 offset.

wrote:

Yes we found that out a while ago. I thought I had rectified all the mention of the 22 offset.

You did somewhere, must have been a different topic.

Ideal tirewidth (section width) for 7 inch is 245/75 or 255/75
But 205/75 is minimum adviced by the tiremakers.
So your 215/75 is within what they allow.
Max section width " allowed" is 285/75.
Made calculator for that, after contact with a tyremaker, and is also dependend on Hight/Width division, called Aspect ratio.

Needed cold pressure for onroad use is depended on load on tyre and speed used. All to not overheat any part of tyre when driving that reference-speed constantly.
Reference-speed for Q R S T H V speedcode is 160kmph/99mph.

If you give of that tyre next, I will make a pressure/axleload list for you.
1. Maximum load or loadindex.
2. Kind of tyre to determine the reference-pressure
Standard load personscartyre AT 36psi
XL/reinforced/extraload AT 42psi.
Given on sidewall the maximum allowed cold pressure between 44 and 51 psi, and an ocacional XL// 60psi.
C-tyres and LT-tires almost always give reference-pressure behind AT on sidewall
3. Speedcode, less important, have my own system for that.

Then also give if you want it in KG or LBS, and in psi or bar or kPa.

Then you "ONLY" have to determine the axleloads acurate, the most tricky part in it all, and your responsibility.
Carmakers use max permissable axleloads for that, if they not give it higher for energy-saving or responcibility reasons.

End 2007 this " pigheaded Dutch selfdeclared tyrepressure specialist" got hold of the officially used formula and system for calculating needed tyre-pressure, and went running with it.

I have old list of Michelin, in wich pressure loadcapacity list for a specific tyre, for 110 kmph ( K speedrated) down to 20kmph onroad, and for mud/sand about 50% of pressure for 20kmph( 12.5mph) onroad, wich then is maximum allowed speed.
For on track about 80% of the pressure for 65kmph onroad, wich then is max allowed speed.
I write " about" because that is what I made of the lists, would like to know the real system the tyremakers use for that.

Sorry to necro an old thread. I got my new Jimny with Kumho LT215/75r15 they inflated to about 35 psi when they installed them. reading up a general rule of thumb was when going from P to LT tyres add up to 15 PSI from whats noted in the door.

They actually seem to run just fine at these pressures on road if I go for what it says on the door they look like they are a bit flat tbh. I dropped to 15 for sand driving and didnt go above 50kph (mostly around 40 tops)

are people really still saying use what it says on the door for LT tyres when the Jimny comes with P rated tyres normally ?

For starters, I would have tried to avoid LT tires on a Jimny, they are built for "light trucks" (which is what the LT means), so they are heavier and have stiffer sidewalls.

Moving on, I have no idea where you read that you should add 15 psi when going from P to LT, but whoever came up with that needs their head examined, if anything, you're going to need to drop the pressure not raise it - the sidewalls are stiffer, the tires are designed to carry more weight, which the Jimny doesn't have.

If you want to play with the tire pressures, find yourself some chalk and a quiet straight stretch of asphalted roadway - use the chalk to mark a wide band across the entire width of the tire and then drive as straight as you can for a short distance - get out and examine the chalk mark - where has it been wiped away? if it's the center of the tread the pressure is too high, if it's both edges of the tread, the pressure is too low, if it's one edge of the tread, there's an alignment problem.

Guess I need to do the chalk thing, will have to test Thursday as away from home for work until then.

I read about the extra 15 psi in quite a few places and it made sense considering that's what the tyre people set them up as (got the new tyres installed before took ownership of the new car)
When I aired down even as I got to around the "standard" pressure I could see the side walls bulging g a bit low down like a slightly flat tyre would show so it seemed reasonable. The ride is also fine on road at around 35psi.

The jimny is.lofted with a gvm upgrade so has bigger springs/shocks a bullbar, spotlights, recovery points and the bigger tyres installed so I would already expect the required pressure to go up.

Chicken wrote:

I read about the extra 15 psi in quite a few places and it made sense considering that's what the tyre people set them up as (got the new tyres installed before took ownership of the new car)
 

Do you have any links to these places, I'd like to read about it too.

As for the what the tyre people do - if you ever get the chance, watch them work - most of them put the tyre on the rim, hook up an airline and inflate until the bead seats, disconnect the airline, insert the valve core, screw it down and they're done - let me know if you see them reach for a gauge.

I have a 2021 Jimny that has been strictly "dealer serviced", twice a year for the last three years, so six services and every time it comes back I have to reset the tyre pressures to what is on the tyre placard, you would think the dealers would at least follow the manufacturer's recommendations, but no, it's a 35 psi EVERY SINGLE TIME.

Last July I took delivery of a 2024 five door Jimny, show room stock, from the same dealership - would you like to guess what the tire pressures were set to?

There's a mathematic calculation you can do to determine the approximate pressure - based on the tyre's load rating (usually stated on the sidewall, along with the pressure it's rated to handle the load at), divide the weight of your car by four, and then divide that by the tyre's rating and you'll get a fraction, multiply that by the pressure, and what you'll get is going to be close to the pressure that gives you even wear.

heh, well if they did that without a pressure gauge then I am impressed as they were all exactly the same :dry:

This Jimny isnt stock, has a GVM upgrade which includes a lift, so has bigger / heavier shocks, springs, has a full bull bar spotlights (considering a winch also) and the bigger / heavier LT215/75r15 tyres on the stock rims, including the spare. interesting that someone said dont put LT tyres on it, the BFG K02's are one of the most popular tyre replacements, the LT gives you much tougher sidewalls which is good for offroad, the stock ones although seem fairly capable offroad considering what they are, are much more prone to puncture when you drop the pressures a bit.

The add 15 psi was something an old mate who worked in the tyre industry and liked 4WD told me (Actually think he said add 10 to 15) but I did a search for P to LT tyres pressure and found a fair few instances or this "rule of thumb" (its not an exact science just one that works in general rather than doing the maths)

There are also instances of similar statements from tyre company guides such as this from Toyo

"WARNING! Please note that size-for-size, LT-metric tires require higher air pressures to
carry equivalent loads of P-metric tires and that any failure to adjust air pressure to achieve the
vehicle’s load requirements will result in tire fatigue and eventual tire failure due to excessive
heat build-up. Due to the higher PSI requirements of LT-metric tires they may not be suitable for
replacing O.E. P-metric tires because of the ride harshness that results from higher PSI
requirements"
How to apply the load inflation tables (toyotires.com)

People suggesting that you shoudl lower your PSI for general use when upgrading to LT tyres doesnt seem right when you look at the tyre even at the door rated placard, they look underinflated.

But I guess I will do some chalk test before I go away on Friday

I applied the tyre pressure from the chart on the first page of this thread (post no. 11), I had previously been running at 25 psi.

Things improved greatly for 'A' road driving.

www.bigjimny.com/index.php/forum/6-jimny...or-215-75-r15#218419

If you have TPMS, it might start activating on a cold morning at 25PSI. As Dan says the ride is much better on the road with about 28PSI and the fuel economy will come up as well.

It doesn't surprise me the garages stick 35PSI in all tyres, I don't think they ever adjust the hose and just wack the same in regardless.

I run 27-28PSI in my Gen 4 with 215s, it seems fine, but like Dan did run 25PSI at one stage and they definately felt soft on the road.

I like Fordem's chalk test, a much quicker way than waiting to see how your tyre has worn out around the centre (or edges) after a couple of years of the wrong pressure.

If you can't do the chalk test, just run it down a road at 35, then 30, then 28, then 25 and possibly lower and see for yourself which feels better and I bet it will be around 28PSI, even with the extra weight.

Robin