QuestionThank you Mr. Smith. You're the first to link pressure change with load index for me. That's very useful to know. You also were kind enough to provide a rule of thumb measure for differences. Thank you.
Several web sites host discussions about minimizing wear and increasing mileage. I'm most interested in maximizing wear.
I'm in a '92 Caravan with P205/65R15 92T Assurance Tripletred tires (upsized from P195/74R14 92S). The plackard indicates 35PSI. Can you provide a test range for trying to either soften up, or stiffen up the ride? (ie: 32-38PSI, 35-36PSI, 30-40PSI) Safety, of course, is paramount; saving tread while risking the ditch is hardly a worthy trade off.
Of cours, if you're completely against this kind of tuning, please indicate so. "Follow the darn placard" will probably put me in my place.
Thanks again. Your help is appreciated.
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The text above is a follow-up to ...
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Hello again Mr. Smith,
This one's for everyone else looking for this same thing:
With upsizing becoming popular, I’m trying to obtain the information necessary to make my own pressure calculations.
If good advise is followed, and the original tire/wheel diameter is always closely matched; only tire width increases. If SL or XL tire designations remains the same as well, you can ignoring that change requirement. I’m wondering if more pressure is required when upsizing a tire/wheel combination. There’s a lot of information and misinformation about this out there, and most of it is believable. Can you tell me how to roughly gauge a safe range in PSI if I want to actually test the “comfy, expensive” ride vs. the “harder, more efficient” ride?
Failing that, could you give me your thoughts on the accuracy of chalk line testing? Will it provide a safe pressure range by virtue of having the tread in full contact with the road?
Many thanks in advance.
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John,
Calculating the inflation pressure when up sizing is a fairly simple calculation, but it requires the load tables, which are difficult to find. But there are several simple things:
1) The proper pressure for the original tire size is what is written on the plcarard.
2) If you are using the same Load index and have changed from SL to XL or vice versa, then the pressure is the same.
3) For every change in Load Index of 1, the pressure changes 1 psi in the opposite direction. Go from 93 to 96 and the pressure can be dropped 3 psi.
4) I don't recommend lower than 26 psi for passenger car tires, and 35 for LT tires. This is for bead retention during cornering
5) For passenger car tires, if the calculation indicates the need for more than 35 psi, then you need an XL. If the calculation indicates more than 41 psi, then you need a bigger tire size. For LT tires, don't exceed what is written on the sidewall.
6) For aspect ratios lower than 60, I think 3 more psi is needed to protect the rim from impact damage.
And lastly:
I am not a fan of the "Chalk Method". This assumes that a tire driving straight down the road will have a proper footprint. Maybe, maybe not!. Most tire wear occurs in cornering. Some tires don't have good footprints. The load on a tire has a lot to do with the footprint and this method only measures one condition - usually the most empty one, and what you really want is the worst case for durability. Plus I think more pressure gives you better wear than a good footprint does.
But the big ticket item is that inflation pressure is the key to tire durability, and too little can be dangerous. Underinflated tire failures can sometimes have tragic results.
AnswerJohn,
As you probably have gathered, I am a big advocate of not rethinking the inflation pressure specifiction on the placard. There is a lot at stake when a vehicle maufacturer specifies the pressure and safety is of paramount importance.
Put another way, if a vehicle manufacturer puts a label on a car, you can be sure it will be a target for litigation - so it had better be right!
Having said that, I have observed over the years that vehicle manufacturers used to use fairly small tires - that vehicles seemed to benefit from larger tires. Others have observed this as well, including most vehicle manufacturers - and over time, the vehicle manufacturers have made several steps toward larger tires, including the latest step after the Ford / Firestone situation.
So my feeling is that vehicles produced prior to 2004 (with a few exceptions) need to have 3 to 5 psi above the placard and vehicles produced 2004 and newer are OK (with a few exceptions). In your case, I think minivans have too small of tires before this whole rethink took place. I think they were too small by 3 sizes. But on a positive note, these vehicles are treated less like vans and more like cars, so the actual loading on the tires is less intense. This explains why minivans are not in the news for tire failures and SUV's are.
So, I don't think using pressures below the placard are a good idea, and a few psi above the placard has some benefits.
Another way to put it is that using tire pressures for tuning should be confined to the racetrack where long term durability isn't a factor. And if you don't like the harsh ride, either put larger capcity tires on it and use less pressure to get the same load carrying capacity
-OR-
Get a different car.
