I have plans to go with a much wider tire and wheel and am looking at going with an 18" wheel. But determining a proper offset has been a bit of a pain. Overall my height wont really change going from a 225/45-17 to a 255/35-18. But, I think I have found a calculation that makes sense.

Tire Dimensions:
Old 225 wide, 634.3 overall height
New 255 wide, 635.4 overall height
with a difference of 1.1mm, I am not at all worried about height differences

So, Currently I have a 17x7" wheel with a +45 offset and converted I get these numbers:

Width - 7 x 25.4 = 177.8mm
Centerline - 177.8 / 2 = 88.9mm
Hub surface - 88.9 + 45= 133.9mm

With the dimensions and math from the old wheels we can continue to the new 18x9.5" wheels

Width - 9.5 x 25.4 = 241.3mm
Centerline - 241.3 / 2 = 120.65mm

Now, we know the old wheels fit so we can determine that the hub surface needs to be about 134mm from the inside of the wheel.

New offset - 133.9 - 120.65 = 13.25mm

So with this math, we can determine a wheel sized at 18x9.5 with an offset of +15 will be ideal.

Does this make sense to anybody else? Are there an major flaws in my math or how this would work?

Also, I do know that there will be clearance issues at the fender, but I will be going with either flares or full custom fenders. So, I am not worried about fender clearance, just Knuckle clearance.

I could also use the tire width and used the same math and that might make a bit more sense.

But what do you guys think?

When comparing the old wheels and the ideal new wheels you calculated, I'm seeing a 2mm less inner clearance and 62mm (2.44") extra outer extension.

The current wheels should have a lot of inner clearance, so you could go with a higher offset to tuck the wheels a bit more. I see you have a '97, so you should have 2gb rear knuckles which give you a bit more clearance than 2ga rear knuckles. I'd try to measure your current inner clearance and use that to help determine how much you can afford to lose if you tucked the wheels in more.

Here's the calculator I used to determine the offset results.
http://www.1010tires.com/Tools/Wheel-Offset-Calculator

+35 is your ideal offset with 255s. You may have to run a very small spacer or grind a bit of the knuckle for inner clearance in the rear with 2ga knuckles, it varies. I had no issues with one set of tires and did on another set.

Gotta love the cool calculators available. Your math method looks to be correct just looking at how you figured it. I didn't calculate it though to double check you.

I think paul turned me on to this one. http://www.rimsntires.com/specspro.jsp Makes a nice comparison wheel to wheel and tire to tire.

Dyesuperman said:

+35 is your ideal offset with 255s. You may have to run a very small spacer or grind a bit of the knuckle for inner clearance in the rear with 2ga knuckles, it varies. I had no issues with one set of tires and did on another set.

Click to expand...

Saying something is ideal, but saying he may have to run spacers or grind for clearance makes it less than ideal, no? What makes +35 "ideal" if you have to do something else to make it fit?

What all did you have to do to get your 9.5" wheel with 265/35 tire to fit and how much rear knuckle clearance did you end up with?


On my 2ga with a 17x8 +35 Evo 8 wheel with 245/40 tires, I have 4mm of clearance on the inside to the knuckle. This same setup had 9.5mm of clearance on my brother's 2gb.
http://www.dsmtuners.com/forums/152189873-post18.html

I plugged in my Evo wheel and the OP's wheel (with +35 offset) and it showed 16mm less inner clearance. Based on the above numbers, that'd mean he'd have to run around a 10mm spacer to clear safely. To get the same inner clearance, he'd have to get a wheel with a +19mm offset. Based on the calculator I posted above, I wouldn't run anything higher than a +25, and would err to something around a +20 to give myself a bit more room. The OP might not be able to find an offset between +15 and +27 or so and might be forced to either run a lower offset or use spacers with a higher offset.

snowborder714 said:

Saying something is ideal, but saying he may have to run spacers or grind for clearance makes it less than ideal, no? What makes +35 "ideal" if you have to do something else to make it fit?

What all did you have to do to get your 9.5" wheel with 265/35 tire to fit and how much rear knuckle clearance did you end up with?


On my 2ga with a 17x8 +35 Evo 8 wheel with 245/40 tires, I have 4mm of clearance on the inside to the knuckle. This same setup had 9.5mm of clearance on my brother's 2gb.
http://www.dsmtuners.com/forums/152189873-post18.html

I plugged in my Evo wheel and the OP's wheel (with +35 offset) and it showed 16mm less inner clearance. Based on the above numbers, that'd mean he'd have to run around a 10mm spacer to clear safely. To get the same inner clearance, he'd have to get a wheel with a +19mm offset. Based on the calculator I posted above, I wouldn't run anything higher than a +25, and would err to something around a +20 to give myself a bit more room. The OP might not be able to find an offset between +15 and +27 or so and might be forced to either run a lower offset or use spacers with a higher offset.

Click to expand...

Because running a +35 all around with a 3mm spacer in the rear (+32) is much more ideal than running something low like a +25 and having them hit the fender all the time. There is no way around rubbing with a +25 on stock body (unless you stretch tire). With a +35 and small spacer (if needed) in the rear you have a much better chance of fitting well.

With the calculations you just gave, I can guarantee that anyone with 2gb knuckles can fit a 255 with a +35 without any issue whatsoever for inner knuckle clearance. It is the exact reason that I am looking for a set of 2gb rear knuckles. As far as fitting my 265s, I ground the knuckle down for clearance. Roughly 1/16" off in the front and 1/8" in the rear. No rubbing whatsoever in front (fender or inside) and rubbing on the fender in the rear. I want to run less camber in the rear so I will be adding rear flares this winter

I"m on 17s but I run a 255/40/17 which makes fitment very similar to what the OP is looking for. my wheels are 17x8.5 +30 and i have 2ga knuckles

if you do the math with both on a 255 tire, the 18x9.5 the OP tires would stick inward my 3/16" more them my current setup.

if you get a little less aggressive offset like dyesuperman is suggesting you are better off because you can always add a small spacer to give yourself more clearance but you cannot take offset away from your wheels.

if you do need to run a spacer be sure to get a hubcentric spacer and always use hubcentric rings.

I actually had these spacer/hubcentric ring combo custom made for my G35 becasue i didn't like having a spacer and a hubcentric ring

also for reference my camber specs are F 1.5*/R 2.5*