Hi everyone. I own a great S type 3.0 litre sport with 19 inch Arden wheels. The wheels look fantastic but are not standard. Because they are not standard, it is difficult to know what pressure to inflate the tyres to.

I started with 34psi all round. This made the rear tyres wear in the middle of the tread. When replacing with new tyres, I dropped the pressures to 30psi all round. Having just had to replace the rear tyres again after only 15,000 miles, they are still badly worn in the middle, with lots of tread left on the sides.

I am nervous about running with less than 30 psi.

The tyre size is 235/35/19/91Y extra load rated.

Can anyone suggest what I should do, or what pressures to run?

Many thanks

David

 

That wear would REALLY worry me (as it clearly does you) but I also wouldn't fancy less than (or even as low as) 30psi. I hope someone has ideas - you're certainly not alone in wanting them!

Just a thought... does the narrowness (or otherwise) of the wheels (rims) have any relevance (I'm guessing it does)? The STR's 18" rear rims are 9.5 and take a 275 tyre, and I suppose the 235 you mention is a pointer but what actual width are your rims (wheels)?

Bit more data: my 2004 STR requires a 95 rating at the rear (93 front). Is 91 enough for the 3.0?

 

A 91 load rating is far too low for a 4,000-pound car. Couple that with 19-inch low-profile tires, and I'd guess that 15,000 to 20,000 miles is all you're going to get even in the best of circumstances. Huge rims and low-profile tires may give you the look you want, but you are shooting yourself in the foot if you want 1) a comfortable ride and 2) a long tread life....

 

You raise some interesting points. I have always put 235/35/19 on because that was the size of tyre on it when I purchased the car. That is not to say that these are the correct tyre size for the wheels.

How can I find out what the wheel size is (8J, 8.5J, 9J etc) and what tyre size it should have?

All I know is that it is 19 inch diameter.

As I undrstand it, the 35 part of the 235/35/19 is the tyre wall height and is a percentage of the 235. That would give a tyre wall height of
235x35% =82mm

So a 245/45/19 would have a tyre wall height of 245x45%=110mm which is 2" more tyre wall height.

Am I talking nonsence here? (probably am). An extra 2" on tyre wall height would be rubbing against the wheel arches.

Kind regards
David

 

Go to tirerack.com and read about tire sizes, including aspects and ratios. You'll learn what all of the numbers mean and how to relate them to your particular set of circumstances....

If extended treadwear life is your ultimate goal, you'll need to go with a set of 16-inch or 17-inch wheels....

 

Try calculator here http://www.tyresave.co.uk/tyresize.html

BTW, you should have a label by driver door hinge giving load ratings (as well as psi for the tyres that would normally be fitted).

 

http://www.miata.net/garage/tirecalc.html
Here is a really good calculator too. This one I would look in your door and see what size you were supposed to have stock and then put in different sizes for your 19's that way you can get the size close. It will also tell you how far off your speedo will be. You can check different widths and sidewalls.

 

Absolutely not true. You can most certainly purchase large diameter low profile tires with long treadlife. What you do give up is road holding ability due to the different tire compound required to extend tread life..

Also, remember that many of the users on this site are in the US. I don't know if UK load ratings are the same as US. If they are.. You are definitely running a tire with too low of a load rating.

Based on the back and forth... You may be running the incorrect size tire for the width of the rim you have. Tire Rack lists 245/35/19's for the S Type in an upsize fitment. There are more than a couple of tires listed that carry a treadwear rating above 400. I would definitely measure the width of the rim (usually 7 1/2 to 8 1/2 inches but may be different).

 

You're just dreaming if you believe large-diameter low-profile tires can last as long as the best long-mileage "normal-sized" tires (16-inch). Some "normal-sized" tires have been known to go more than 75,000 miles with proper care. Show me a set of monster tires that can do that. No can do....

 

There is a simple flaw in that logic. Tire wear is not determined by size. Ask a truck driver if you doubt it. Tread compound, speed rating (same brand and model tires available in different speed ratings will have different tread wear ratings) and of course the suspension settings of the car determine tread wear. 15" and 16" wheels are going the way of the Dodo. Even "econoboxes" are rolling out on 17" wheels. Also, this isn't driven by a look, like you would lead people to believe, but by a performance expectation. The engineers have discovered that if stability control and ABS technologies are going to work they require the tires to offer a level of grip that some hard compound 75 series tire will never achieve. Please list a brand and model that you feel is a adequate mileage tire and matched to the performance ratings of these vehicles and I will gladly do the look up to see what tread wear ratings and sizes are available. Yes, ten years ago most tire companies only made soft compound summer tires in 30 and 35 series profiles, and those do wear quickly. But the technology is moving on.....

 

Treadwear is not entirely determined by size (compound is important as we all know), but it does have an impact. Tread wears out for other reasons than just contact with the road surface. Continuous vibration from driving (and therefore absorbing constant shock and distortion) also plays a role. Greater mass and construction component distance helps to disburse this damaging vibration over a wider area, thus prolonging the lifespan of the tread (and ultimately the tire) in many cases. Of course, proper care of said tire is crucial throughout its lifespan as well....

I have an acquaintance who built tires at Bridgestone/Firestone for more than 15 years, then moved into R&D for them until taking early retirement to play stock options. When we're not talking market moves, we sometimes talk tires. He has postulated on the physics involved in the above paragraph during several of our conversations, and he knows that although today's large-diameter low-profile tires are somewhat better than they were a decade ago, they can never match the longevity of their greater-mass, larger-sidewall brethren due to the physics involved. I'll vote for his views on tire longevity over yours any day of the week....

 

Thanks for all your replies. I didn't intend to start a controversial debate.

I think we have lost sight of my initial call for help.

My wheels are non standard.They are from a Jag after market supplier called Arden. I don't think Jag offered a standard 19" wheel option from the factory.

However my concern was the UNEVEN wear. Not the mileage covered before they wear out. My rear tyres wear out in ther middle of the tread and leave plenty of life on the inside and outside edge.

This is classically seen as over inflating the tyre. But I am only running on pressures of 30psi. Hence my initial post which said I was concerned to run with less than 30psi.

Here in the UK Jag specified that tyres used on this vehicle must be "Extra Load rated" These tryres are such rated so that is not a concern.

My biggest concern is that I may be putting the wrong size tyre on the wheel. I keep fitting the 235/35/19/91Y extra load because these were fitted when I purchased the car. That doesn't mean they are the correct size.

If I fit 245/35/91Y, will the different profile help them to wear more evenly? How much extra space do I need in the wheel arches for this profile?

Maybe this is an issue I have to live with?

I see from the replies that I need to measure the rim diameter before being able to find out what profile tyre sizes would be available to me.
If that is so, I need to go back to the tyre fitters and have one removed and measure it.

Thanks for all your help

David

 

I understood the summary was: 91 is nothing like high enough.

To decide the sizes, use the tyre calculators. But do not use 91. What does the label on the car say?

BTW, on my STR I have (over-specified) 99Y extra load tyres.

I would have hoped Arden might have a catalogue or the like to help you figure out which wheels you have. And that a wheel could be measured without removing it (though that's easy enough).

 

To put it as simple as possible.. The load rating and size you listed are both wrong and probably the cause of the problem... Please go to www.tirerack.com and read about up-sized tire options. The site will allow you to up-size to your specific car. Arden is a known brand so the assumption is that the rim size and offset are correct. BTW the 91Y you mentioned is a combination of load rating (91) and speed rating (Y). Both of those responses were early in this thread and intended to direct you to some self help.

My counter argument was meant to show that at times people interject an opinion without actually addressing someone's question. For that I apologize.

 

I have never gotten over 42,000 mi out of a set of tires. Of course, I like to squeal the tires once in a while.

 

I did some perusing on Tire Rack. For the size in question it looks like a rim width of 8.0-9.5" is the most common recommendation. Most wheels have the rim width stamped on them somewhere....sometimes on the back side.

My *personal* experience with low profle tires is that center wear is common, even when mounted on correct rim width wheels.

<shrug>

Cheers
DD

 

Adding to the general discussion......

I went to Tire Rack for some quick research on what's out there for the size in question. Tossing out snow tires there were 58 choices.

All but a a half dozen or so were "87" or "91" load rating. The few exceptions didn't have a numerical rating but were listed as "extra load". I haven't researched where the "extra load" designation fits in the grand scheme of load ratings.

Of the 58, here's how the treadlife ratings looked:

42 had a rating of less than 200
10 had a rating of 200+
3 had a rating of 300+
1 had a rating of 400+
2 had a rating of 500+

So, while some decent wear might be expected from 3 of the choices it's pretty clear that the majority will have the typical "high performance, low tread life" characteristic.

On my XJR (255/45x17) I was unable to find any tire that would give more than about 20k tread life. Between wear out and injury, the car has gone thru 18 tires in 75k miles. Ten of the 18 have been in the last 40k miles.

IMHO the allure of "high performance" tires loses its luster when faced with so much expense. I recently downgraded to 16" wheel just so I could buy a decent tire :-). I found some V-rated all season tires with a 550 (or was it 600?) treadlife rating. Wish me luck !

Cheers
DD

 

Back to the OP:

More than one good site has been suggested but try this one http://www.tyresave.co.uk/tyresize.html as follows:

Enter the values 245 40 18" (e.g. fronts on an STR).

Then, on the second row, set the wheel to 19". If you also use 245 you'll see that 35 profile is the only option within the sane overall size.

You still need to check you DO have that width (8) wheels. (And don't forget to check they have the right offset!! You're probably uninsured if not.)

And buy high enough load ratings (in case you don't you're definitely uninsured). They're going to be £150-200 each.

Personally I'd sell the wheel rims and buy standard size ones but hey.

(Note to non-UK citizens: over here it is a legal requirement to be insured and a criminal offence if you're not. There is a rather poor database which the police have that checks - badly - and they tend to impound your car on the spot if the check fails, even if it's the database that's wrong. They have ANPR - automatic number plate recognition - which does what it sounds like it does. Such fun.)

 

Same here but it is regulated from state to state.. Never heard of tires invalidating insurance. How does your (UK) insurance pull that off?

Also, Doug, unfortunately there is no industry standard for those tread wear ratings. So, they are a guide at best. In my experience, I've had "400" tires out last "600" tires. So, it is the compound that makes the difference. The "harder" the compound the less grip but longer tread life..

 

Right, it's only a guide....but when you have little or nothing else to go on, and you're comparing 58 tires, you go with the guide.

Right, too, about the compound....but I assume that rubber compound is one of the factors....probably a major factor.... that enters into the assignment of a rating.

While it's entirely plausible that some "400" tires might outlast some "600" tires, it's a fair bet that that the extremes of the rating scale are useful....and a "600" tire will almost certainly outlast a "200" tire, all else being equal.

Cheers
DD