I'm trying to confirm tire sizes for the 355 Challenge. I believe the wheels are 18 x 7.5" and 18 by 10"? For Pirelli slicks the following sizes were recommended... P-F360-F 235/645-18 Pirelli 8820 P-F360-R 295/680-18 Pirelli 8821 For Hoosier DOT R's will the following fit OK? 245/35-18 305/30-18 - or would you go - ... 225/40 265/40 Thanks, rob
Rob, I've done hoosiers in that size but forget the height, and pirellis in 305 650 18/245 645 18 and up to goodyear 25.5 11.5 18 in rear and yoko 280 645 18 in rear/235 front all on 355c wheels which are 8 front IIRC and 10 rear. Be careful of going too small on the height something about lack of support on tail heavy cars. Air supports sidewalls. Same goes for 280 yokos not reccomended for rear tire in porsches.
I did some diameter calculations... 245/35-18 = 24.8 305/30-18 = 25.2 225/40-18 = 25.1 265/40-18 = 26.3 I checked some used slicks I had in the garage, they are Yoko 280/680-18 and 230/650-18. How do those sizes convert into diameter? I couldn't find any calculators online. Seeing my current tires are 280 and 230, I'm worried the extra width of 305 and 245 won't fit. My fenders are already rolled.
The diameter is as follows 280/680/18 = 680=26.72". a 645 is 25.35" a 650 tire is 25.54" I am not rolled and running 245front 305 rears hoosier dp slicks which are wider than 245 645 18 pirelli slicks by the way. These numbers are all approximates and vary with how the maker measures. Also, note that the proper tire for the wheel will make you faster than the wrong size tire on wheel. But cost sometimes causes us to compromise some seconds for dollars. I was actually fastest running yoko slicks 235f/280r which is more appropriote for the 355c wheels. But 245/305 were most available so mostly I ran that size. I never ran the R6 hoosier so I do not know if that width is equal to their dp slicks. But I did have some minor rub with the 245 up front. What spring rate are you running up front?
I have no idea, I've been running the same springs since I bought the car and the handling has been great, so haven't messed with it. Ferrari of Houston had just rebuilt the car from ground up and set it up before I bought, no tweaking by me.
Rob, I run the cheaper +10mm "Non-Challenge" Pirellis from BobWoodman. Which are 305Rear & 245Front. The more expensive, for no particular reason, 355Challenge are 295.680.18Rear and 235.645.18Front. Mark
Thanks! Right now I'm leaning towards Hoosier DOT R's for NASA because I really don't want to be swapping tires all the time between SCCA and NASA.
Maybe you should find out since more spring = less roll = less contact from tire to fender. Actually you should know the calculated spring rate to be more acurate.
just for my own ref, think I will run the Hoosier R6's so I don't have to swap between SCCA and NASA. based on avail sizes... 245/35-18 295/30-18
Probably the same as on the 355, I guess. A popular choice is: front (8"): 235/40/18 rear (10"): 295/30/18 A quick view on my shelf showed Pirelli-Slicks "305/645-18".
Thanks guys. I'm going to use my nice set of Challenge wheels strictly for the street. I'll get slicks for the OEM 17" wheels for tracking. Ordered a set of Michelin Pilot Super Sports from the Tire Rack last night. Ordered the 235/40-18 fronts, and 285/35-18 rears. The 285's were a lot cheaper than the 295's, and they're for the street anyway. How many cycles do you guys get out of either Hoosier DOT R6 or the Toyo DOT track tires before they are corded?
