Urge Designs
http://urgedesigns.com/?page_id=49

Another thing to consider is tire width. Many people believe that wider tires improve grip. However, this is not true. You need to fit your tires to your wheels so they have a slight stretch. Fitting the widest tires possible on specific wheel sizes will deaden response and cause the car to roll due to unsupported sidewalls. Additionally this will decrease the performance and function of your dampers as you introduce a major damping mechanism into your system. Extra tire width is not used as the tire is pinched; creating a muffin top close to the wheel lip that is not in consistent contact with the ground. It’s important to note that none of the racing series above use tires that are wider than the wheels. Every team has their tire widths matched to the wheel width. On TV you will noticed a slight stretch of the tire as the wheel is slightly wider than the tire.

URGE designs has experimented with multiple sets of 10.5” and 11” CCW’s to get the offsets and backspacing perfect for the S2000. With little to no front fender modification you can run 9.5” front wheels with 245mm tires. For S2000s that visit the track more often we recommend 10.5” wide front wheels mounted with 265mm tires. You’ll need to modify the front fenders with a big flare or replace them with at least 25mm+ over fenders. If you’re willing to go all in with 11” wheels, we recommend that you have at least +50mm front fenders and +25mm rear fenders and suggest running 295/30 tires
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And lastly while gearing and grip are important I think you should adjust your gearing with gearing and work on your grip with your wheel/tire combination.

Disclosure: Boosted and going to 18X9.5 with RE-71R in 255/35, Poddy mod rear end with 3.9 gears, second gear so much better for AutoX.

 

245 to 255 is at least 0.3, and maybe a bit more with the extra pinch (the 245 was measured on an 8.5" rim, the 255 on a 9" rim)

 

But the B Street rules say stock widths (AP2 7" and 8.5", offsets of +55 and +65, although the rules allow +/-7mm) and the STR rules say 9". For B Street that typically has meant 225/255 which are both specified for those size rims or 245/255, the 245 front spec'd for 8"-9.5" rim and pinched on a 7". The Z2SS has a 235/40, but the RE71r doesn't come in that size.

In the rear, both the 245 and 255 are spec'd for an 8.5" rim. There may be a slight gain in grip with the bigger tire. There may be a slight gain in balance with the smaller tire (which also won't raise the roll center). And there is a slight gain in gearing with the bigger tire.

 

My point is that it is not black and white as to whether a 255 is better than a 245. So if you want to run 245s there are many good arguments for it. Again run whatever you want my man!

 

Sorry bud, not trying to call you out or anything or start an argument, but the bolded statement is false. I think I understand what you are actually trying to say (that adding a wider tire may not neccesarily improve grip if you don't have enough wheel width to support it (i.e. as long as you aren't over-pinching it.)) But I must clarify for the sake of inexperienced thread viewers that yes, wider tires, adequately supported, most absolutely and unequivocally improve grip.

Sidebar: back to best testing on race tires in SCCA Solo, this was proven especially true when testing a 245 Hoosier A6 vs a 275 Hoosier A6 on a 7.5" rim. You might laugh at the hilarity of that pinch, but....it was faster. It made more grip on the front of the car, the lap time was lower, the datalog showed higher sustained average and peak G at the same temperature, and tolerated heat better. More grip, faster car, period.

You would be shocked at the lengths folks will go to stuff on race tires. Integra Type R's were stuffing 275 Hoosiers on 6 inch rims 5-10 years ago and going faster because of it. I know, it sounds ridiculous, but it happened.

 

Before today I didn't know of anyone running 245 rears. People do perceive a difference, but it most cases it is hard to determine if it is the tire size alone or some other factor. Tire size could effect feel, balance, grip, gearing. It also changes the roll center. However, tested cars may also have different alignment, be running different courses, running on different surfaces. And different driving styles.

 

RE: my comments about Alex running the 245 rear...

The course at the Pro required going to 3rd early going out, a drop to 2nd for the turnaround, and then going to 3rd early coming back, all the way to the finish...whether on a 255 rear (me) or a 245 rear (Alex). So, given those conditions, the 245's shorter height actually was helpful, as the thrust in 3rd was better

On a course where an extra 1.5mph "ceiling" in second would be the difference in shifting or not shifting, the taller tire would rule.

 

I know I'm one of the major reasons you're thinking so much about gearing, so keep in mind too that many manufacturers the difference between 245 and 255 is miniscule. I got both 245s and 255s in the rival S to measure and the 255 was only 3mm wider and the difference in height was also maybe 1/3 of what it should be from memory.

If you have a choice, 255 is the way to go. If it's going to mean not having tires for a while, 245 is likely not a big difference. I haven't measured the bridgestones though.

 

Yes...but there seem to be differences. Both specify the difference as 10mm, but measured on a 1/2 wider rim. On the same rim, that should be halved. But note the huge tread width different on the RE71r compared to the Rival S. On the Rival, a mere 0.2 inch. On the RE71r, a massive full inch. Is that accurate? I can't find data on Bridgestone's US or Japanese website.

What were the numbers?

My problem is that 225/245 or 245 square are configurations that never came up before. The first raises the front while not raising the rear. The second has the 1" pinch on the front that I was trying to avoid.

If some one picked 225/245 or 245 square for B Street I would love to hear their rational and results. Obviously, like 225/255 vs 245/255, both can be fast.

Another thing to keep track of is the massive load ratings of the RE71r. Where they meant for pickups? Does that mean they will be best at lower pressures? If so, will that make the smaller sizes or the larger sizes seem better?

 

My point is that while the 255 is taller, its still not enough for me on some courses.