ok - I gotta put this to rest in my own mind. Here is a pic by Gsedan35 showing approx. 5/8" - 3/4" clearance from the front suspension to his 18x8 (+45) wheels w/ 245/40-18 tires.
Using that info to compare a few options:
Option 1.) a 245/45-18 x8.5 (+45) adds 0.25" (6.35mm) additional wheel width and zero tire section width to each side, leaving 3/8" - 1/2" suspension clearance*. Enough?
*It should be noted that THX723's original Fitment Chart showed 8.5 (+45) clearing the front suspension, but the revised Chart does not. Pretty certain several confirm running 245/45-18 x8.5 (+45) and stock spring height with no issues.
Option 2.) a 255/40-18 x8.5 (+45) adds 0.25" (6.35mm) additional wheel width and 0.195" (4.95mm) tire section width to each side (total .445" or 11.3mm), still within the 5/8" min.), but leaving only about 1/8" - 1/4" suspension clearance. Too tight?
Option 3.) a 255/40-18 x8.5 (+40) adds 0.5" (6.35mm) additional wheel width - 0.053" (1.346mm) inboard and 0.446" (11.3mm) outboard - while decreasing inboard tire section width by 0.001" (0.025mm) and increasing outboard section width by 0.391" (9.93mm). That totals 0.052" (1.32mm) more to inboard, and 0.837" (21.26mm) more to outboard. If we take Gsedans35's 0.625" (15.88mm) measure minus 0.052" (1.32mm) increase to inboard = 0.573" (14.55mm) clearance. IMO, a much more comfortable amount of clearance. Just right?
Ok, what about fender clearance? Even if they clear the front suspension, will 255/40-18 tires on 18x8.5 (+40) wheels rub the fenders? I refered to the Sedan Fitment Chart thread on g35driver, where Will measured clearance from the outside of the stock 17x7 wheels and the inside of the front and rear fenders and got about 1.575" (40mm) for each.
Since an 8.5" wheel adds additional width of 1.5" to the 17x7 OEM wheel clearance, you have to add 0.75" each side to the increase/decrease of the 255/40-18 on 18x8.5 +(40) setup in Option 3. Therefore, using Will's measurement of 1.575" (40mm) stock fender clearance minus 0.75" (19.5mm) to compensate for increased outside width of the 8.5" wheel = 0.825" (20.96mm). Plenty, even when lowering.
To summarize, based on Gsedan35's and Will's measures, I think a 255/40-18 tire on an 18x8.5 (+40) wheel will result in approximate:
Front suspension clearance of: 0.573" (14.55mm)
Front fender clearance: 0.825" (20.96mm)
Rear fender clearance: 0.825" (20.96mm)
Is this consistent with other folks computations? Somebody please double check the math? Sorry, didn't bother with rear inboard clearance as I'm not planning to run staggered.
[edit] note to self: 255/40-18 tire on an 18x8.5 (+43) wheel will move wheel/tire combo back inward by 0.118" (2.3mm) and result in approximate:
Front suspension clearance of: 0.455" (11.56mm)
Front fender clearance: 0.943" (23.95mm)
Rear fender clearance: 0.943" (23.95mm)
2004 | 6MTs | Diamond Graphite/Graphite
Upstate NY
Summer rubber
- Dunlop SP9000 245/35-19 
