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Alright, so, sorry for the rookie question here. I'm thinking of getting a set of track tires for track days (and only track days of course - I know these can't and shouldn't be run on the street). I've never bought a set of track tires and have run into a discrepancy, as I understand tire sizes; I'm trying to make sure whatever I decide to pick up fits properly.

I was browsing Pirelli's, tirerack.com, and Hoosier Tire's websites. On tirerack.com, Hoosier has an A7 (or R7, different compound) tire with a size which reads:

P235/35ZR19​

Meanwhile, on their website, Pirelli has one with a size which reads:

235/645-19​

My question is, what's the difference between the two sizes when it comes to the "35" (aspect ratio) of Hoosier's vs. Pirelli's "645"? Do they translate to the same unit / ratio / measurement / etc., and if so, why the difference?
 

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The A designation is for autox (comes up to temp very fast), the R is for track (holds up under extended high performance driving), but slower to come up to temp.
 

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Alright, so, sorry for the rookie question here. I'm thinking of getting a set of track tires for track days (and only track days of course - I know these can't and shouldn't be run on the street). I've never bought a set of track tires and have run into a discrepancy, as I understand tire sizes; I'm trying to make sure whatever I decide to pick up fits properly.

I was browsing Pirelli's, tirerack.com, and Hoosier Tire's websites. On tirerack.com, Hoosier has an A7 (or R7, different compound) tire with a size which reads:

P235/35ZR19​

Meanwhile, on their website, Pirelli has one with a size which reads:

235/645-19​

My question is, what's the difference between the two sizes when it comes to

the "35" (aspect ratio) of Hoosier's vs. Pirelli's "645"? Do they translate to the same unit / ratio / measurement / etc., and if so, why the difference?
Read their websites... The 645 measurement is the nominal outer diameter of the tire in millimeters. You can do the math to figure out the equivalent aspect ratio, knowing the section width (235) and wheel bead diameter (19 inches).

ETA: That 645 mm equates to 25.4 inches. The A7/R7 is spec'ed at 25.6 inches according to the Hoosier data sheets. 0.2 inches is about 5 millimeters so not a lot to worry about in either direction.
 

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Busy-body warning: Sounds like you're educating yourself about these considerstions, which is great! I really recommend also doing similar research on picking the right tire compound for your needs if you hadn't thought of that already. I was once a diehard race compound fan for track use... but no longer.
 
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