Recently bought some FR500s. 18x9 in front. 18x10 in rear. Went to go get some pricing done on tires for them, only size my local tire shop had in stock was 235/50/18 for front and 275/40/18 for rear. I have no clue how to read a tire numbering like this, an I also have no idea what to expect this to look like. I was wondering if anyone out there could give me some tips on what to look for, some pics if you have a similar setup, and some help on reading tires better. I'd love it a lot if someone did have a similar size as this because I was told once they order them from the warehouse they're stocked in and they get here I cannot return them so I'd be stuck with them. Any an all discussions are welcome!appvalleytutuapptweakbox
You're going to have some fun shifting from metric to imperial, but in short, the first number is your rim size, or the inner diameter of the tire, which doesn't have any wiggle room. the second number is the rim width, so 18x10. Easy, because we like inches.
Too bad your tires don't
Let's pick on the 235 because it's slightly easier math. So the width of that tire is 235mm, or 9.3" (275/25.2) #/mm in a inch. The 50 is the aspect ratio, which is the percentage of the side wall, measured as a proportion of the width. So 50% of 9.3" is 4.65", meaning it's 4.65" from where the rim leaves off, making your sidewall to the ground. That'll mean your overall tire height will be rim (18") + 9.3 (4.65 on the bottom, 4.65 on the top) or 27.3". For the rear, it'd be 275 (10.9") wide, slightly thinner sidewalls of 40% for 4.36" per, which then gives you a 26.7" ride height (18 + 4.36 + 4.36). Depending on how you're driving, you might want to try a different place if that's all they have, because generally speaking 50% ratio is something more for larger vehicles that need that extra cushion, but at the same time, you might not enjoy a 30% aspect which gets thinner and doesn't absorb the bumps in the same way, however, if you're doing auto-x or road racing, you want less sidewall flexing, and don't concern yourself so much with the road manners. The other magic number is 27" as I believe that's what the stock tire height was, so if you get shorter tires, your speedo will think you're going faster than you are, and if you get taller tires, your speedo will think you're going slower than you are. Being a +/- .1 or .2 here and there isn't too bad, but if you really start pushing it like 26 or 28", well, try your GPS or an App like Waze to get an idea of how far it'll drift. For instance my speedo says I'm doing 80 when it's really 78.
I'm sure others will have their feelings, but I'd think 255/45/18 and 285/40/18 might be a slightly better combo. Do remember that while it's 30mm wider in the rear, it's also a 1" wider rim.