How much tire can I fit?

+8 = .315", but that's just the offset from the centerline. A 17x9" with a +8 offset has 5.6" of backspace. I use this calculator, you input the offset and it will show you the backspacing. Rounds it to the tenth though. Also a good way to compare your new combination vs. your old combination to see if it will clear, because it tells you the difference in the locations. Custom rims, rim tire packages for your ride - RIMSnTIRES.com

What we should really do is drop the US system entirely and use the SI system (metric) like the rest of the world. Only 3 countries in the world don't use the metric system- the US, Liberia, and Burma. There's company for you :rolleyes:. It's not even "our" system, we adopted it from the British and just never got rid of it. Won't happen though. For the average person it's a big change with no real benefit, just confusion about how fast you're going or how far you're traveling. Anyone that works in the scientific fields already uses the metric system, as the US system is a colossal pain in the butt to use with any kind of engineering, physics, mathematics etc. Fastest way to guarantee an engineering calculation will be wrong is to put everything in US standard. Biggest culprit is our use of weight instead of mass. A pound on Earth is not a pound on the moon, because a pound is weight and includes gravity. A kilogram on Earth is a kilogram on the moon, because it's a mass measurement, a kilogram doesn't include the force of gravity. Anytime you have something listed in pounds you have to divide out the acceleration due to gravity to get a unit called "slugs", and carry around a bunch of conversion factors. Almost guarantee's you'll make a math error somewhere.