Running 255s (17") vs 275 (18")

[GrandSport]

Does anyone have any experience with this in terms of lap times?
These are my two choices due to available sizes for a spec tire.

I can't help but think bumping up to a 275 on an 18 wheel but "people say" the extra weight and drag isn't offset by the extra grip for a 200hp car.

Bonus points: Who makes the cheapest 18x10 40mm offset wheel that I can trust in an endurance race?




Also, as an aside, anyone have any idea what my car will roughly dyno (I'm asking because I need to adjust weight)
93 Tune, EL headers, intake, intake manifold riser, catback, lightweight flywheel
180? 185?

[redlined600]

Searching the Nasa database could give you a pretty good power estimation. https://airtable.com/appERehieevbzLk...gWtC3AQ0QAJPI0

[ZDan]

Originally Posted by GrandSport

Does anyone have any experience with this in terms of lap times?
These are my two choices due to available sizes for a spec tire.

I can't help but think bumping up to a 275 on an 18 wheel but "people say" the extra weight and drag isn't offset by the extra grip for a 200hp car.

Bonus points: Who makes the cheapest 18x10 40mm offset wheel that I can trust in an endurance race?

Don't have any lap time data on 275 vs 255 on an FT86, but I didn't lose much if anything in terms of lap times going from 245/40-17 on 17x9 to 225/45-17 on 17x9...

I would not bet against 255/35-18 on 18x10 being quicker around some, (maybe most) tracks vs. taller and heavier and draggier 275/35-18 on the same wheel!

If you get a weight break with the 255s, no question, I'd go that route for sure.

Also, as an aside, anyone have any idea what my car will roughly dyno (I'm asking because I need to adjust weight)
93 Tune, EL headers, intake, intake manifold riser, catback, lightweight flywheel
180? 185?

Depends a lot on buncha factors. Could be anywhere from maybe 165 to 185 max?

[RedReplicant]

10 isn't enough wheel for a 275

[GrandSport]

Originally Posted by RedReplicant

10 isn't enough wheel for a 275

what makes you say that. 10 inches = 255mm.

From the tire manufacturer: Rim Width Range: 9-11". Dead middle.

[ZDan]

Originally Posted by GrandSport

what makes you say that. 10 inches = 255mm.

From the tire manufacturer: Rim Width Range: 9-11". Dead middle.

Yeah it's a fine fitment for normies. If you're looking for tenths of a second, 10s are narrowish for 275s. I've run 225/45s on 9" and on 8" and it does make a difference in terms of feel and I would bet lap times (no back to back data on that though).

In any case, at your weight and power level I would bet that even 255s on 10s is already getting past the point of diminishing returns for race track (autoX may be different). Even if you could get them at the same diameter, which you can't. If spec tires and 255/35s or 275/35s are the only choice, I'd go 255 on 10s...

[Ohio Enthusiast]

Originally Posted by ZDan

I've run 225/45s on 9" and on 8" and it does make a difference in terms of feel and I would bet lap times (no back to back data on that though).

TireRack did a fairly good test on tire width vs. wheel width and more stretch did result in better times, sometimes more than putting a wider tire.

[ZDan]

Originally Posted by Ohio Enthusiast

TireRack did a fairly good test on tire width vs. wheel width and more stretch did result in better times, sometimes more than putting a wider tire.

Great test, but I have to wonder if the 225/45-17 advantage might be partially due to gearing. 215/45-17 and 245/40-17 are the same diameter, 225/45-17 is taller. If the course favored taller gearing it would make a real difference on a short course like Tire Rack test track...

Would have been more informative to see 225/45-17 vs. 235/40-17 vs. 235/45-17 vs. 245/40-17.
But it's impossible to have a perfect test, I really appreciate all the testing Tire Rack does! I always try to buy from them because of all the testing they do.

[RedReplicant]

Originally Posted by GrandSport

what makes you say that. 10 inches = 255mm.

From the tire manufacturer: Rim Width Range: 9-11". Dead middle.

Match section width (that is not the number on the side of the tire) to wheel width if you want to maximize a tire (other than Hoosier / non-DOT), going wider tire wise generally is not faster on track (autocross is probably different). It is what Andy Hollis and 949 have been saying for over a decade backed up by data and what GRM and Tire Rack have been trying to make more known over the last few.

You do you though.

[dave99rt]

Originally Posted by RedReplicant

Match section width (that is not the number on the side of the tire) to wheel width if you want to maximize a tire (other than Hoosier / non-DOT), going wider tire wise generally is not faster on track (autocross is probably different). It is what Andy Hollis and 949 have been saying for over a decade backed up by data and what GRM and Tire Rack have been trying to make more known over the last few.

You do you though.

Yep, for autocross most go wider or widest tire when restricted to narrow OEM wheels. Height for higher peak speed is also important. Bigger tires help with heat management also.
245/45/17 on 7.5" is commonly used in DS. It's the perfect blend of higher top speed and not overly pinched with the taller sidewall. 235/45/17 would be my second choice.

[GrandSport]

Originally Posted by dave99rt

Yep, for autocross most go wider or widest tire when restricted to narrow OEM wheels. Height for higher peak speed is also important. Bigger tires help with heat management also.
245/45/17 on 7.5" is commonly used in DS. It's the perfect blend of higher top speed and not overly pinched with the taller sidewall. 235/45/17 would be my second choice.

Heat and wear management are the biggest reasons as its endurance racing (WRL). 8% more tire, 8% more thermal mass. 8% more surface area to cool down the tire. Plus, less sidewall is nice.
Also, narrower tire on a wider wheel is harder to drive, even if it's faster. That's an issue for a 16 hour race.

Regarding the actual test results, it looks like 225 on 9" wheel is the fastest, but 245 on 9 is just a hair behind it and the 245 on 9 had the fastest lap (just not averaged out). 275 on 10 is basically the same as 245 on 9 (1 inch is 25, so it's almost exactly 270 on 10). Given this is enduro, it's hard to see why those test results do not support 245 +9 which would say 275 on 10.
It does support running a 245 over a 255 though.

IDK, can we fit a 10.5 or 11 inch wide wheel? 295/30/18 is really the "best" size because of gearing/height. The 275/35/18 is more than half an inch taller than the 255 and nearly an inch taller than the 245.
The 255 might be the worst size.

[GrandSport]

So they now have 275/35/17 which resolved this issue. I can't imagine a 245/40 or 255/40 is faster on a 275/35 as it is much wider and shorter than the other two.


As far as 245 vs 255, hard to tell on grip going from one day to another (I didn't even drive on them that day, another driver just did 1 session to scrub in) but he definitely noticed the height. If he noticed the bog more, it's hard to imagine the extra grip helped enough.

All on a 9.5" wheel.




BTW: It rained and we ran rains both days. So I have 2 sets of 255 and 1.5 sets of 245s (because the conti extremes are made of forged steel) to mess around with.

[Breezio]

Originally Posted by GrandSport

As far as 245 vs 255, hard to tell on grip going from one day to another (I didn't even drive on them that day, another driver just did 1 session to scrub in) but he definitely noticed the height. If he noticed the bog more, it's hard to imagine the extra grip helped enough.

Also have to consider the brand/model when making that comparison.
For example the RE-71RS 245 and 255 have the same tread width, the 255 is just a little taller. Whereas on the A052 the 255 has a good bit more tread width then the 245 and is a little bit taller as well.

[RT-BRZ]

Originally Posted by Breezio

Also have to consider the brand/model when making that comparison.
For example the RE-71RS 245 and 255 have the same tread width, the 255 is just a little taller. Whereas on the A052 the 255 has a good bit more tread width then the 245 and is a little bit taller as well.

It is absolutely the little details like this that matter. Thanks for pointing it out.