Tire sizing help
 

Our rims are 18 x 8", so 8 inches wide.

The stock tires are 225/45/R18, so they are 225mm wide, which is 8.85".

If the tires are supposedly wider than the rims, why do the stock tires look like they are narrower (slightly stretched) on the rims?

Where is this 8" measured on the rim? Is it an absolute width at the widest point?
Where is this 225mm measured? on the tread? or again at the absolute widest point of the tire (somewhere near the bead)?

A few things to remember about tires.......................everything on the tires really means nothing lol. Okay lets start from square one. The tire tread wear and temp rating are brand specific, meaning a traedwear rating of 100 on a Bridgestone could be a 60 or 160 on a Michelin. The sizing (225/45/18) is not the same from brand to brand. For example, a Hoosier 225/50R15 tire was almost an inch taller and wider than a Toyo 225/50R15.

What all this means is some brands are larger than others even though the tire size is exactly the same. If your looking at the specs of the tire you also have to look at the "measured" rim width the specs are coming from. 225mm is not just the visible area that has tread but rather the "section width" of the tire and each brands measures that slightly differently too! If you measure the 225/45/18 on a 7 inch wide rim vs a 9 inch wide rim the tread on the road or "measured" tread width will change dramatically. Hope all this helped lol

My question to you is...what is does it matter? How does this information help you? I guess the real question would be what you are trying to establish/try to do?

Starting from end point of what you want to do might make the process or need of information much easier....

Just a thought...don't blow up at me like you did with Al's comment...lolz
________
Motorcycle tires

Al,

What is "section width"? Where is that measured from which point to which point?

From some more reading, it appears the 225mm is a measurement from sidewall edge to sidewall edge. So one tire manufacturer or model specifically might be designed with a curvier sidewall whilst another tire maybe designed with a more square sidewall with reference to the tread contact patch. What you said makes sense. It all comes down to the design of the tire.

Thanks Al.

You basically answered your question: Section width = max distance from inner sidewall to outer sidewall (where the line is parallel to the axis of rotation of the tire)


Rather than checking the 2XX/XX/RXX spec on the tire sidewall, it is often more useful to check the manufacturer's website to find whether a specific tire will mount on your rims.


I'm suppose you've probably read this already, but tire rack has a pretty article on tires.

http://www.tirerack.com/tires/tirete....jsp?techid=46

http://www.tirerack.com/tires/tirete....jsp?techid=33

remember andrew if you go to wide you;ll start to notice reduce fuel mileage. i dont no how any one can daily drive with 275's

This tool can helpfull....but is just a guideline.
http://www.1010tires.com/TireSizeCalculator.asp

The bottom line with tires is that each company sets its own standards for sizing, traction, temp and treadwear ratings. Until there is a universal standard it will always be hit and miss when you try to get a bigger tire than stock on your car.

Footman,

What are you trying to achieve? Do you want to run a wider tire on the stock rim?
Or are you trying to reduce the stock tire size?
Let me remind you, the manufacturer has chosen this tire size in particular to make the car do what it was designed to do. Moving away from the stock tire size isn't really recommended.

This being said, the widest tire you can run on the stock rim is a 245 tire. Anything more and you'll be asking for trouble.

Have you seen this website before???
http://www.carbibles.com/tyre_bible.html

It tells you alot about tires and what recommended size tires should run on which wheels.

I think you should concentrate on removing as much unsprung mass as you possibly can. Getting rid of the stock wheel should be priority 1! :)

Sorry but, you're going to find a lot of people who disagree with this that have run wider tires and showed they provided better performance all around on the street, and track contrast to the stock 225 size.

From my own personal experience, I can tell you running even slightly larger than stock tire size you're compromising in other areas.
Here are 2 examples.

1. 2005 Mazda3. Usually runs 205/50/17 as stock tire. At the time, I couldn't afford new tires and the car badly needed them. So I went ahead and installed my FD wheels (16x8) with the stock rubber in place (225/50/16).
Stock wheel on the 3 is 17x7 with a 52 offset. FD wheel 16x8 with a 50 offset.

The car was significantly slower. It was slower to react, slower to accelerate, and consumed alot more fuel as opposed to the stock tire size. About the only thing that improved was stopping distance. Which of course is due to it's wider foot print. But the car didn't behave well and was not liking the overall change.
This is on a light weight econo car.

2. 2004 Mazda6. Stock tire size 215/50/17 running on 17x7 with 60 offset.
I picked up some good condition RX-8 wheels for the summer duty. I knew the overall affect on the car was probably going to be worse since the stock wheel/tire on the 8 is heavier than the stock arrangement on the 6.
Difference again was brutal. Fuel consumption was way higher. Acceleration took a nose dive. And it was harder to do quick transitions. Braking was longer despite the added tread width.
Hell, when it was time for winter again, we did the switch, and the difference was noted even by my girls daughter who wasn't used to driving all that much. They actually thought going to the winter rubber was much better than dealing with the stock RX-8 summer rubber.

So, this season, I went ahead and installed my stock Miata wheels (17x7 with 55 offset) on new Bridgestone RE760 sport tires in stock size (215/50/17).
Difference was very noticable. Car is quicker to accelerate, can transition alot quicker, and it can brake with less drama. Not to mention fuel consumption has improved marginally.

I think Car and Driver had an article about wheel/tire sizing recently.

Generally, people who run on a wider wheel/tire size is strictly for looks. They don't care about anything other than looking "pimp".

I think doing what Kevin did (can't remember his call sign) is the best approach. He got some Enkei wheels (I forget which model) in 18x7.5 and running on stock tire size. He noticed the car behaved alot better than the stock wheel/tire combo.

I generally would agree but, this isn't the typical street car forum. Many people who get this car look into upgrading it's handling which includes tires/rims as it's a well known tracking car. Also, there a lot of options for rim/tire combos out there that are wider and weigh less or the same as stock with the tire mounted so you gain in all areas of performance including acceleration.

Granted, there are some that do it strictly for looks as you see a bit of that in the aggressive wheel thread.

However, I'd like to use the argument that the Renesis was designed by the engineers to do what it was supposed to do and no one should touch that either. Most of us know better than to believe that. Stock caters to the uninformed casual mass who don't care for anything but to get in and drive expecting their car to go past the warranty coverage. Those who are car enthusiasts will look to improve the performance of the car.

Of course, that goes without saying. Majority of us on this forum are out to "improve" the car. But do so knowing it is subjective and it fits your driving style.
No one is going to have the exact driving style as everyone else.

I just don't understand the people who go all out and change every aspect of the car just to change it. What was the point? Unless you're building a show car, there really isn't any point in "modding" the car to make it look outrageous. Unless of course that's what you're after. The Wow factor plays heavily around "enthusiasts".

Getting back to the topic, Footman, what is it you're after?
I think you're trying to improve the overall aspect of the car. I think doing what Kevin did won't affect daily driving duties. It will improve the overall aspect of the car too.

Footman, if you keep stock wheels, largest fittment is 245. My track wheels (stock wheels ie) with 245 R1 looks funny(thats a big 245 lol) but it does fit.

If your purpose is to look good, then I think a nearly flush look of a 18 x 8.5" + 245/40/18 (hint hint: EVO BBS ala 2hit6/ Chris's FAST wheels are same size) look will be a pretty good match on your driving habit and requirements - daily drive and aggressive style.

HOWEVER, if you consider to go all out street performance and economy, why don't you think of a 17" ones? Everything will be cheaper and will not negatively impact your performance at all if you choose the tire size wisely. Some ppl in the forum did that for the track and it actually improves their time!

I suggest you to go ask in the Tires section.

Good luck!

I am looking for improving performance aspect of the car without sacrificing aesthetics.

Kevin (AGENT008) ran Enkei 18 x 7.5. I remember talking to him about it, and yes he said the car handles better.

I am definitely after reducing wheel mass at each corner. I eventually will switch to DBA 2-piece slotted rotors and some Hawk Ceramic pads. As for the tires, I'm definitely due for new tires in 2011. I technically need them now, but it's not bad enough that it's unsafe. I just have lower grip, but I can live with that because money's tight.

As for tires, I'm going to try 245/40/18. It seems to be a popular tire size and I'll be 1% off the speedometer. 255/40/18 is a closer match to the total wheel diameter, but I think 255's are too wide to run all around

Another key thing I want is to keep all 4 corners the same size. I've decided the Mazda engineers designed it that way for a good reason. Correct me if I'm wrong, but Mazda chose narrow 225 tires to keep the car well balanced with very accurate/reactive steering. Going with a wider tire (more contact patch) is useful only if the car has a significant amount of torque/horsepower which the Rx8 does NOT... hence cars like the 370z have 19" fat tires in the back; while the 8 has skinny tires.

Now that I'm lowered on B8's and Prokit, I also have to worry about rubbing issues. I do not want to fender roll.

Having said all that, it's looking like

a) 18 x 8.5 rims, on 245/40/18.
b) 18 x 9 rims, on 245/40/18
c) 18 x 9 rims, on 255/40/18 (closest match to speedometer)

I'm leaning towards the A option most. I will be looking for rims that are lighter than stock to offset the extra weight of the tire size.

Some samples are the OZ Alleggerita's.... at 18 Lbs I think..

Footman, if you look at the 245/40R18 they will run up to 1/2 inch difference in diameter depending on the brand of tire. Rim width will also alter the diameter slightly.

Kafka and I had a huge discussion about the weight of a wheel when he got his Enkei's, and I will repeat it for you lol. ITS A STREET CAR, a couple of pounds either way isnt going to make any real difference until you start looking for 1/10ths of a second on a lap time etc. lol

Wait a sec... isn't weight still weight??
You obviously want to be running the lightest equipment possible without hindering reliability, correct?
A couple of pounds of unsprung mass is still huge when you're daily driving your car.

The examples I gave are a perfect indication of that.

Moving from the stock wheel of the 6 (22-23lbs) to the Miata rim (17lbs) was a huge difference in all aspects of the car.

Yes, it's still a street car, but you should always try to get the lightest equipment possible.

You want to know difference, moving from my stock Miata wheel on the Miata (17lbs) to new Kosei K1 TS's (just shy of 14lbs) was a very big difference. It feels like the car has lost a heavy set of bricks at each corner.
It has more grip, it's easier to get up to speed, and quick transitions has increased 10 fold.
Still running the stock tire so I know the comparison would be just.

Get the lightest wheel possible for your car without compromising aesthetics as well as the original factory specs. :)

All the discussion I've read so far has been pretty relatively and subjective to debate....

Like Al has said..most of ppl on this forum do not race regularly if at all on track to notice the difference between a couple of pounds of difference on the tires + wheel combo...

at the end of the day...the only thing that matters is if you as the driver notices a difference and even that could be subjective as it could be a placebo effect of the change.

I can't speak for anyone else, but my new wheel are much lighter then the stock ones and I've notice no difference in performance or fuel economy...I do notice it has more grid around the corners but it could be due to the fact of the difference in tire grip and pattern...too many variables at play...my suggestion is to choose sometime you truly like period...

if your thinking of going the same setup as mine... the offset could be a bit more aggressive...mine is +38..you could go +35 keeping in mind that your ride is supposedly lower then mine...lolz
________
Ship sale

All that is great until you hit your 1st big pothole. Light weight wheels are usually far more expensive and fragile that the cheaper off shore alternatives. Everything you stated above is 100% true, except for the cost/replacement of the wheel.

Again, 100% correct! The "butt" dyno, gforce meter etc is not a great indication of true performance gains. Its like getting your car dyno tuned for peak horsepower, the number is really cool until you take your car back to the drag strip and realize that it is a few 10th's slower because you gained maximum horsepower but lowered your average horsepower through the entire power band.

A brand new tire is going to have more grip than any 2+ year old tire, tires get harder even if they are just sitting there. Slicks usually get recycled if they sit for more than 12 months after the production date if they are not sold for that very reason.

Here is the C&D article Neo referred to: http://www.caranddriver.com/features...sted-tech_dept

As someone who has put a bunch of different tires/rims on a number of different cars, I think this is being over thought. Clearly putting on the widest tires you can will result in the greatest amount of grip (assuming you're not doing it with high profile tires, etc) and creating a lot of tire roll. Obviously one needs to consider the actual cross section of the tread and the overall height as well as fitment/practical considerations. Also, a 245 tire of one type of compound may not be as effective as 225 in a better compound.

Weight matters, but the C&D article shows a 2/10ths and 2 mph difference in the 1/4 mile for 14 lbs! That's a huge difference in weight. I'm sure it would effect the suspension performance significantly as well. I doubt many of us are going to do anything that will result in more than a few lbs of difference.

Get thinner Footman for stage I weight reduction! Oh wait you are already pretty light...

Neo, this is a nice article about weight vs strength ON THE TRACK...actually the strength win this time lol!

http://zilvia.net/f/chat/175847-high...true-test.html

Kafka,

That was an interesting read. There were a few things that brought up red flags, but an interesting read non the less.

Footman,

There is more than enough advise in this thread to help you make decisions.
Here's the next question, what type of wheel brands are you looking at?
O.Z.'s are nice. But do they come in the required offset??
I think you said you didn't want to roll fenders? And now that you're lowered, deviating too much from the stock offset is going to be a problem.
Even going to a 45 offset, you might have to consider rolling fenders.

My 8 is on H&R springs (which gives a lower stance then your eibachs) and my wheels are 18X8.5 with a offset of +38...so I'm sure if you go between an offset of 38-48 you should be fine...
________
Bonecraft Download

Thanks everyone. There's a lot of GOOD ADVICE HERE... I have a few last questions.

1) Does anyone have the definitive weight of the stock rims? I've found several different measurements posted all over the forum and the internet. Has anyone actually put the rim on a scale by itself?

2) If I were to stick with the stock size of 18 x 8, running 225 tires, but I go for a wider stance with a lower offset i.e. 18 x 8, +30. Essentially the track has been increased by 40mm. Does that make any substantial difference to the car's steering response and handling? Take into account that I have lowered the car.