225/45/17 vs 245/40/17 Same Compound and Wheel, which is faster

[RedReplicant]

Originally Posted by Capt Spaulding

Actually, it does to me. If the wheel is laid flat it looks like the combination will be resting on the outside of the rim even though the sidewall seems to bulge a bit. And, to my eye, the tread of the tire is even narrower.

Wheel width is not measured outside edge to outside edge, it is measured bead to bead. Also, tread width is not section width.

[Pat]

True, but it's pretty obvious the tire is stretched. You can't even see the tire on the trailing edge of the wheel. Compare that to how much you can see on the leading edge and you'll appreciate how great the difference is.

[RedReplicant]

edit: nevermind

[gt5759]

Just throwing a few of my two cent thoughts in here.

For fastest lap times you match tire section width to rim width. This gives the best feedback, wear and lap times. The only time that we shove larger tires on smaller rims is when we are rim width limited in classes, such as scca since they rule by rim width versus tire width like nasa does.

Something people seem to get focused on also is the number printed on the sidewall of the tire, that number is nothing meaningful in the performance world. Like RedReplicant mentioned earlier, a radial slick's number on the sidewall is not something to use as a size reference. A 225 hoosier, goodyear or bfgoodrich is more like a 245 - section width is pretty much 10" on those sizes - and a 245 is a roughly a 10.5" section width. Now you take most street performance tires like a goodyear supercar or a Yokohama ao48 and 225 is 9" section width, a 245 is a 10" section width.

Just a FYI, I am rounding a little on the section widths for quick explanation, manufacturers vary on all of this a little like one is 9.8, another 10 or another 9.9 for a given section width, aspect ratio causes variance as well. Hopefully this helps a few making size choices.

[MX-5RACER]

Another variable to throw out that I have done some testing on with my track Miata, in low HP cars like the Miata and the Twins, I found that carrying an inch of wheel and tire around a high-speed track can hurt your laptimes. You will not make up the drag of those inches down the straights with the extra corning grip from the wider set-up. There is a balance that you have to figure out for your set-up and track. My Miata at 145WHP was faster around Sebring and Homestead on a 7" rim than 9" rim with the same 205 NT-01R's.

Also, keep in mind that the tread-wear number is almost meaningless as there is no industry standard for what it means. In other words, a 400TW Bridgestone may be a 200TW to Toyo or vice versa.

The only thing I can say for sure is to test it yourself.

[gt5759]

Originally Posted by MX-5RACER

Another variable to throw out that I have done some testing on with my track Miata, in low HP cars like the Miata and the Twins, I found that carrying an inch of wheel and tire around a high-speed track can hurt your laptimes. You will not make up the drag of those inches down the straights with the extra corning grip from the wider set-up. There is a balance that you have to figure out for your set-up and track. My Miata at 145WHP was faster around Sebring and Homestead on a 7" rim than 9" rim with the same 205 NT-01R's.

Also, keep in mind that the tread-wear number is almost meaningless as there is no industry standard for what it means. In other words, a 400TW Bridgestone may be a 200TW to Toyo or vice versa.

The only thing I can say for sure is to test it yourself.

Thats another valid thought, something I have forgotten about since I haven't played with sub 200whp cars in a really long time. Rolling resistance of the larger contact patch plus added rotating mass, be it larger tire or wheel, will have a pretty great affect on low hp cars. Good points!

[MX-5RACER]

Originally Posted by gt5759

Thats another valid thought, something I have forgotten about since I haven't played with sub 200whp cars in a really long time. Rolling resistance of the larger contact patch plus added rotating mass, be it larger tire or wheel, will have a pretty great affect on low hp cars. Good points!

Not just rolling resistance, but wind resistance too. You are trying to cut the air with two more inches of width across the entire wheel/tire. I think the wind resistance would greater than the rolling resistance.

[RedReplicant]

That is part of why you get the people working around restrictive aero rules in certain classes by using larger than usual stick on fender flares that cover some of the frontal area of the tire, the big airdams that are bowed out in front of the tires, and the front of fenders bent out with the rearmost front bumper mounts spaced out wider than stock.

The competitive BMWs and Miatas running in NASA usually have one or more of these so they're easy to find examples of on ye olde google images.

[jflogerzi]

Good discussion guys. Ran a 2:04.6 at Buttonwillow on CW13. This is my PB. The weather was very favorable. Really enjoyed the kumhos. Next test will be the same 225 on my 17x8 wheels. I think the car has some time to loose in the right hands but this size might be go forward to use.

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[jflogerzi]

I will be at BW at the end of this month. Plan to put my 225 kumho v730s on my lighter smaller 17x8 rims and will do my best to compare things.

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[RedReplicant]

Tirerack did some testing, as posted by nissanfanatic over in the tire subforum

https://www.tirerack.com/tires/tests...y.jsp?ttid=296

Summary
So what did we learn? The key takeaways are that in the dry, a wider tire does not automatically mean faster lap times. Using a wheel width that properly supports the tire will provide faster laps than a wider tire with very little wheel support. We saw this in our testing - the 215/45R17 tire on the 9" wide wheel was noticeably faster than the much wider 245/40R17 tire on the 7" wide wheel. That's not to say tire width doesn't matter, though, as the 225/45R17 combinations were all faster than the 215/45R17s. The 245/40R17 on the 9" wide wheel was very close to the 225/45R17 on the same wheel. If we could fit a 10" wheel width on our test cars, the pattern seems to indicate a 245 on a 10" would have been the fastest combination of all.

In the wet, a narrower tire is generally faster than a wide one, and once again, more support from the wheel is better. The 215s on the 8s and 215s on the 9s were the fastest combinations in the wet. The 215s on 7s, 225s on 8s, and 225s on 9s were a step back from there, and all the 245s, along with the 225s on 7s, set the slowest wet laps of the test.

[RT-BRZ]

Originally Posted by RedReplicant

Tirerack did some testing, as posted by nissanfanatic over in the tire subforum

https://www.tirerack.com/tires/tests...y.jsp?ttid=296

All very interesting data. The only thing that's tough to get past is that the 225 tire at the same aspect ratio is taller than the 215 tire. While the difference is pretty small, there is a difference there and it does make the results suspect. I'm not saying I doubt them but there is enough variability to make more questions.

The wet results are not surprising. Water creates drag and a wider tire is going to have more of it.

In a straight line the narrower tire is going to be faster because of less drag. The wider tire has the opportunity to make up for this in turns where the lateral grip is greater. In the wet the water acts as a lubricant and therefore reduces lateral grip for everyone which effectively nullifies the advantage.

It would be kind of neat to see a track map of speeds/times comparing these tires against each other.

[nissanfanatic]

Originally Posted by RT-BRZ

All very interesting data. The only thing that's tough to get past is that the 225 tire at the same aspect ratio is taller than the 215 tire. While the difference is pretty small, there is a difference there and it does make the results suspect. I'm not saying I doubt them but there is enough variability to make more questions.

The wet results are not surprising. Water creates drag and a wider tire is going to have more of it.

In a straight line the narrower tire is going to be faster because of less drag. The wider tire has the opportunity to make up for this in turns where the lateral grip is greater. In the wet the water acts as a lubricant and therefore reduces lateral grip for everyone which effectively nullifies the advantage.

It would be kind of neat to see a track map of speeds/times comparing these tires against each other.

Agree. I kind of wish they had posted the usual charts like they do for other tests. But, I also try not to be a choosing beggar lol because it's nice to finally have this tested _and_ on our platform nonetheless.

[cmiovino]

Speaking in terms of autocross only, it's common that everyone is putting as wide of a tire on as possible. All our second gen guys (and there's a lot of them) are running 245's and even 255's on a 7.5". They do extended studs, spacers, and max offset per the rules to achieve this. Note that I'm just talking about autocross, but our courses are very much like smaller tracks similar to the TireRack testing facility.

I personally run 225/45/17 because it's what you can get away with without going the spacer/stud route... and because having Brembos limits your wheel choices heavily. Interesting to see what 225 on the proper wheel generally is even with 245's on the same width... at least per TR's testing.