scut monkey

Please help out a newbie:
So I ordered some rims and tires from a vendor 17x7.5 ET48, 225/45/17 rubber. I get the rims and tires delivered today and there is no indication of the offset of the rims except for a small "43" imprinted on the inside of the mounting surface of the rim. I give my vendor a call and ask him if these were +43 or +48 offset. His response was that the rims were originally 43, but were machined down to 48 at the factory. I asked him for some concrete documentation for this fact, and he couldn't really come up with any. He also added that if the rims fit, then I have no problem.

My questions: does this guy's explanation sound reasonable, or is he trying to sell me some rims that aren't the offset that he claimed that they were?
Does anybody have a good way to actually measure the offset of a rim? I know what the definition of the offset is. My problem is being able to measure it to within 5 mm. It's a little hard with all the surfaces and the shape of the rims, especially with the tires already mounted.
Should I really care? The rims fit without rubbing on my 03 sedan. Will the 5mm really change the driving characteristics and stress on the suspension?
I sort of want to know just out of principle. I gave this vendor a good chunk of my hard earned money. I should get what I paid for.

Sorry for the long post.

Dexter@tirerack

Hmmm. I suppose that machining could be done, but it would be difficult. You can calculate the offset of the wheel. You'll need 2 yard sticks (or straight edges) and a 12" inch or so ruler with millimeter measurements.

1. Lay they yard stick on the back of the wheel and measure from that edge (using the 12" ruler) to the mounting surface of the wheel. This will give you your backspacing. Get it in millimeters.

2. Using all three of your hands, use both yard sticks placed on inside and outside edges of the wheel and measure the width of the wheel. If it's listed as a 7.5" wheel, this measurement should come to between 8 and 8.5" (get it in millimeters though).

3. Take that measurement, cut it in half, and subtract it from the backspacing measurement.

This should work pretty well. The only assumption (weakness) of this method is that the measurement from the outer and inner lip of the wheel to the bead seat is the same. This is usually true, but can vary depending on wheel geometry. Since the tires are already mounted, you will need to find a straight edge that is just the right length as not to be interfered with by the tire (cut the yard stick as needed).



As far as the "if it works don't worry" theory, it's a tough one to answer. You are adding a bit of stress to the axle/bearings with lower offset as well as changing the suspension/steering geometry, so theoretically it's not good. But we're talking 5mm here. I've heard of people running lower offsets with no problem so if you like the wheels/tires and your car works well, I wouldn't get overly worked up about it. This should be an active subject, so we'll see what others might say with relevent experience using 43mm or lower offsets on their WRXs.

scut monkey

Thanks for your help! I'll try this when I get a chance (gotta work today, Saturday).