C-street Tire Diameter question
 

Being relatively new to the sport, and having only run in RTR, I have limited experience with various tire options. Now with C-Street, I am dropping to a 17 inch tire. I'm considering running a 235/40-17 because it has an overall diameter of 24.4, where my current 245/40-18's are 25.7 (when new). I also thought about a 235/45-17, but the diameter isn't much less than the 18's.

I run mostly local events where we rarely break 60mph, so I'm not too concerned about running out of 2nd gear, but it could occasionally happen. I'm in a series 1, btw.

So I guess I'm asking for some opinions on whether dropping to the 235/40 would be a good idea or would a diameter drop of more than 1" actually be detrimental.

Howdy,

I ran RTR last year and am probably accurately described as "one of those idiots that spends way the hell too much time & money to try and win a $5 plaque". I'm going to start out the season on the 235/40-17 and see how it goes. Most likely shaved Toyos (which will be legal only this year), but we'll see.

I think the cars can use the gearing help. Last year other than on nationals east course I was never near the top of 2nd. And on that course, I still went to 1st three times.

Mark

With 235/40/17 you'd still be able to do 67mph in 2nd. With that in mind I wouldn't think that would be an issue because even if you have to shift, everyone else did too.

https://docs.google.com/spreadsheet/...rive_web#gid=0

My math shows the rx8 should only achieve 64.8 with that tire. And the fudge factor brings it down to 62.5 mph.

http://www.precisionsite.com/auto-x/tools/

Hey Mike,

What value are you using for the rev limiter? Even with that tool and its fudge I'm getting 65.3. Is 9400rpm not correct?

Tom R

My bad, I did not scroll sown far enough and I assumed you were using 9K for the rpm limit.

Yes, your math appears to be correct for that rpm.

I do encourage you to use the fudge factor for these gearing calculations as the real world experience in my MX5 have shown it to be quite accurate.

Mike

You guys do keep in mind 9K is not really 9K on our cars.

That's a good fudge factor. I've been making these spreadsheets for different cars (for SAC and personal reasons) and I always find its optimistic by 1-2mph depending on the car, that fudge, which I will now call Gear PAX, is a good value. Thanks.

Meaning indicated 9400 is really a lower RPM? I was going by data values I found in specs for the RX8 in a few publications. If its lower, I'd love to know that.

OE tach is off dramatically as RPM increases, for example during data logging my tach reads 5.7K and my Cobb logs 5.5K off the computer. As RPM increases the amount the tach correction factor increases as well.

9.5K is realistically 9K

Yeah, my car was closer to 2 mph.

Regardless, the -1 change appears to be a wise choice for the OP.

Is anybody going to try the old CS logic and pinch a 285 (Z2, RS3, Rival, etc) on the 8in wheels?

I know when running Hoosiers, biggest tire possible was always the answer. I know the general consensus is saying that the same logic doesn't apply to street tires, but has anybody tried?
Are the sidewalls on Z2 stiff enough to handle the pinch?

People have already squeezed 275/35/18 Rivals and Z2s on the 18x8. The problem is no one I know that I would trust the data/feedback is among them. YMMV. I'd bet on the Z2 over the rival in this application. The issue is the weight and gearing. I think I'd be more tempted to run a 245/40/17 if I was running my RX8 in C Street.

It has also been tested and proven that putting anything larger than a 245 on an 8'" wheel does nothing but add weight while adding no extra grip and actual decreases the performance of our car.

That is situational.....

Howdy,

Last year I ran the 255/35-18 z2 and the 245/40-18 Rival. Started on the Rivals. When I switched to the z2's I compared them both at our local lower grip asphalt lot and also at Nationals on the test course.

Not a scientific test of course, because the Rivals had 100+ runs, the tires were different brands, and there was a size difference.

They were, however, both black. :-)

Anyway, I found next to no difference on the clock at either site. I could feel the Dunlops better (particularly on the Nationals test course) and was therefore more consistent & comfortable on them, so I went with those.

With the +/- rule, I want the gear & weight improvement I can get going with a shorter tire, which is why I'm going to try a 235/40-17. I'm going to do that regardless of pixie-dust tire or not, but the R1R being one of the "good ones" in a 235/40-17 means that I'm going to at least try it out.

I'd love to test the z2 in particular with pinching more on the wheels, but I'll let other folks blaze that trail.

Mark

I'd be less tempted in the RX8 for the gearing consequences to over stuff the tire. In my DS car, I'm running 275s on a 18x9 for some heat dissipation that worked on my last DS car. My budget for testing is a lot more limited this year than last due to home remodeling so hopefully someone experienced tests in similar cars.