Staggered or square stance..

[Pentalab]

A 255-45-18 uses a 8.5" rim for measuring width. On a 9" rim, its stretched. On a 9.5" rim, it's really stretched.

A 275-40-18 uses a 9.5" rim for measuring width. On a 10" rim, it's stretched. Roush used 275-40-18 in a square setup, all on 18 x 10" rims. Works very good. Having the fronts stretched 1/2" over their measuring width works superb for handling. IMO, you really want at least it's measuring width on the fronts...and preferably a 1/2" wider.

The GT-500 was sorta a goofy setup. Staggered tire sizes, but 4 x identical 18 x 9.5 rims. IMO, since it was staggered, and can't be rotated, they shoulda just staggered the rim widths as well. IE: 18 x 10 rears..and 18 x 9 fronts. A 9.5 is not optimum for a 285-40-18 , and a 9.5 is a bit too wide for a 255-45-18.

I used the 285-40-18 rears on a 18 x 10 for a few years...and the fronts were 255-45-18 on a 18 x 9. That setup worked very well....... until I swapped the 255 fronts for 275-40-18..on the same 18 x 9" rims. Then the handling went to hell, with the unresponsiveness and squishy feeling.

I highly suspect a 285-40-18 on a 18 x 9.5 front rim will also be slightly unresponsive. Gabe has that setup on the front, and can probably chime in.

What would work using the GT-500 9.5 rims is 275-40-18 in a square setup. Being able to rotate is a bonus, for any street car.

I measured the actual tread on my 275's at 9.75". The 285's tread is 10" wide. ( both are on 10" wide rims, and currently in heated basement storage for the winter).

 

[Gabe]

...
I highly suspect a 285-40-18 on a 18 x 9.5 front rim will also be slightly unresponsive. Gabe has that setup on the front, and can probably chime in.

What would work using the GT-500 9.5 rims is 275-40-18 in a square setup. Being able to rotate is a bonus, for any street car.

I measured the actual tread on my 275's at 9.75". The 285's tread is 10" wide. ( both are on 10" wide rims, and currently in heated basement storage for the winter).

Unfortunately this might be the best pic I have of how my 285/40 Continental DW's fit on the 18x9.5's I have:
(let me know if the pic doesn't show for some reason)

There's a pretty good fitment there as far as I can see, and the tires drive well and handle well for me, so far.
This is my street setup, until I can afford another set of wheels/tires, which is inevitable, since I'm a wheel whore, haha.
My next setup will most likely be 19x10 / 19x11 staggered, but that's for another thread, another time.

I did think that the 275/40/18's fit more square on these rims, and they were OK to rotate, something I didn't really do anyway though, since my heavy right foot always seemed to cause the rears to wear quite quickly


But the 275's did fit awesome!

And I even autocrossed in them, they were Continental DWS all-seasons, they did great!

 

[Norm Peterson]

You would think by mounting 255/45 diameter tires on 9.5" rims would stretch from not being wide enough, unless it's 255/35/18 - 255/40/18 that require between 18x8.5" - 18x9" maximum width

It's at the max end of most recommended rim width ranges.

I suppose technically it is a mildly stretched fitment, assuming you're referencing both 'stretched' and 'squeezed' against measuring width.


Norm

 

[Norm Peterson]

Unfortunately this might be the best pic I have of how my 285/40 Continental DW's fit on the 18x9.5's I have:
(let me know if the pic doesn't show for some reason)

There's a pretty good fitment there as far as I can see, and the tires drive well and handle well for me, so far.

Fortunately, this whole tire width vs wheel width thing isn't all-or-nothing, pass-or-fail. That's a min-recommended fitment within tire industry guidelines, which do have some margin (on both ends).

FWIW . . . all numbers approximate
A 255/45-18 ends up with about a 265-wide section width when mounted on a 9.5" wide wheel.

A 285/40-18 ends up with about a 280-wide section width on the same 9.5" wide wheel.

So what you see on the GT500 is only a 15mm difference in tire section, a good bit smaller than what the numbers say is there (a 30mm difference).


Norm

 

[Sky Render]

So how well do the 255/45/18's clear up front ? IIRC most recommend running 255/40/18 - 275/40/18 in the front.. Supposedly running anything above a 40 series sidewall on 18-19 inch tires will not provide enough clearance in the front due to the taller sidewall of 45/18 series tires

What? I run 255/45R18 on all four corners all day, every day. It's nearly the exact diameter of the OEM tires, like within tenths.

The sidewall number is a percentage of the width.

 

[OX1]

So how well do the 255/45/18's clear up front ? IIRC most recommend running 255/40/18 - 275/40/18 in the front.. Supposedly running anything above a 40 series sidewall on 18-19 inch tires will not provide enough clearance in the front due to the taller sidewall of 45/18 series tires

Thats the size I have up front. Stock height, but plenty of room.

I think it might be your tire choice or possibly your alignment settings. Neither of the setups pictured in post #24 do much 'pulling' at all even near the truck stops here in NJ. I haven't run tires narrower than 255 or wheels narrower than 9.5" on the Mustang in several years. Winters included.

I did briefly consider NT05's . . . until I saw Nitto's own video that showed a car running them cornering with way bigger slip angles than it should have ever needed. That told me it was really a tire for the drifters rather than anybody who wants steering precision to go with their hard cornering. Not long after I saw that video Nitto pulled it from their site.


Yeah, it would take a little effort to make 295's work up front. Still more effort if you want them to perform/behave as good as the size/appearance would suggest they should be doing.


Norm

Could be just our difference in tolerance. I still have the stock 235 Pir, 235 PSS in stock size (for the front of my "drag" setup),
and those 255's (NT555's if you did not see them in the earlier pic). Significant difference between the 255's and the stock
235's. Could be tire construction/compound, rim width, too I guess.

I do have 245's PS4S's on my new DD, Fusion Sport, and that car overall is much more tolerant of grooves and
ruts, so again, it could just be I'm used to a more forgiving chassis, day to day.

 

[06 T-RED S/C GT]

What? I run 255/45R18 on all four corners all day, every day. It's nearly the exact diameter of the OEM tires, like within tenths.

The sidewall number is a percentage of the width.

What I should've said in my previous post was according to tire rack specs, 255 diameter size tires are recommended for rim sizes between 8.5" - 9" in diameter.. Anything above 9" isn't considered as optimal.. Although as previously posted, 2007-2009 GT500 models were factory equipped with 255/45/18" tires on 18x9.5" wheels w/45mm offset in the front..

Anyhow my apologies for any post mis-wording on my part..

Thats the size I have up front. Stock height, but plenty of room.

Could be just our difference in tolerance. I still have the stock 235 Pir, 235 PSS in stock size (for the front of my "drag" setup),
and those 255's (NT555's if you did not see them in the earlier pic). Significant difference between the 255's and the stock
235's. Could be tire construction/compound, rim width, too I guess.

I do have 245's PS4S's on my new DD, Fusion Sport, and that car overall is much more tolerant of grooves and
ruts, so again, it could just be I'm used to a more forgiving chassis, day to day.

As I just mentioned above, what I posted earlier was mis-worded.. So anyhow I should be asking what rim size are your 255 tires mounted on.. Are they mounted on 18x8.5, 18x9 or 18x9.5 ? if they're mounted on 18x9.5" wheels, this was my main concern regarding what is recommended according to tire and wheel spec charts..

 

[Norm Peterson]

What I should've said in my previous post was according to tire rack specs, 255 diameter size tires are recommended for rim sizes between 8.5" - 9" in diameter.. Anything above 9" isn't considered as optimal.. Although as previously posted, 2007-2009 GT500 models were factory equipped with 255/45/18" tires on 18x9.5" wheels w/45mm offset in the front..

Anyhow my apologies for any post mis-wording on my part..

I can think of at least three somewhat different definitions of "optimal". One would be where ride comfort has top priority (down at/around minimum recommended width), another would be aimed at improving turn-in response and steering precision (up around maximum recommended width), and the third would be a "some of each/not all of either" answer with neither ride nor handling having priority (think close to "measuring width"). I'm going to guess that TR will tend to be somewhat conservative in most of their discussions intended for average consumers, and are sticking closer to "measuring width" as a result.

FWIW, 255/45-18's on 9.5" wide wheels isn't a particularly harsh-riding combination even though it's a max-recommended fitment.


Norm

 

[06 T-RED S/C GT]

I can think of at least three somewhat different definitions of "optimal". One would be where ride comfort has top priority (down at/around minimum recommended width), another would be aimed at improving turn-in response and steering precision (up around maximum recommended width), and the third would be a "some of each/not all of either" answer with neither ride nor handling having priority (think close to "measuring width"). I'm going to guess that TR will tend to be somewhat conservative in most of their discussions intended for average consumers, and are sticking closer to "measuring width" as a result.

FWIW, 255/45-18's on 9.5" wide wheels isn't a particularly harsh-riding combination even though it's a max-recommended fitment.


Norm

To be honest with you, my top priorities are ride quality (comfort) along with turn in response/steering precision (ride handling) So my question is this, if i were to switch out my current 275/35/20's on 20x9.5" wheels w/45mm offset.. Would switching to either 275/40/18 or 285/40/18 on 18x9.5 wheels w/45mm offset on all 4 corners provide a taller sidewall and improve ride comfort quality over my current 275/35/20 on 20x9.5" setup or would there be very little or nothing at all to be gained from making the switch.. Anyhow perhaps yourself along with Gabe, Pentalab, OX1 and Sky Render (Vince) can all chime in and provide feedback

 

[Gabe]

Yes, going to an 18 you'll typically gain some ride comfort, compared to a 20" tire, on the same car.
But you'll need to also consider which tire you'll go with, some tires certainly ride softer than others.

For your situation, a 275/40/18 should work out great.
I loved my Continental DWS tires in that size, and they were all-season, so I could actually drive the car in cold weather and even light snow.

 

[06 T-RED S/C GT]

Yes, going to an 18 you'll typically gain some ride comfort, compared to a 20" tire, on the same car.
But you'll need to also consider which tire you'll go with, some tires certainly ride softer than others.

For your situation, a 275/40/18 should work out great.
I loved my Continental DWS tires in that size, and they were all-season, so I could actually drive the car in cold weather and even light snow.

Gabe, my current setup is Steeda Spyder 20x9.5 wheels w/45mm offset paired with Nitto NT555 G2 tires in 275/35/20.. Anyhow I'd be interested in going with either Continental extreme contact sport or Continental extreme contact DW's as my potential next set of tires.. Being as I never drive my car during the winter, I really wouldn't need an all season tire to begin with.. Therefore if the Continentals would provide a softer ride over my current Nitto G2's, I'd also be interested in them in 275/35/20 as well at least until my current Spyder wheels will need replaced, then I'll decide as to which brand and style wheel to get in an 18" size.. In the meantime, let me know what you think - Rocky

 

[desert cowboy]

Yeah I dont think this is accurate at all unless your wheels have a completely jacked offset. For example I have a 1.5" front, 2" rear drop with 285/35r19 front, and 305/35r19 rear tires. Zero issues/rubbing/etc. Your rear tires are a little tall though

Thanks for your info.
You didn’t say what wheel width & offset you are using to achieve your clearance.
FYI my front wheels were originally 20x8.5 w/35mm o/s. After the lowering it was okay but adding the roush front brake set I was getting scuffing of the tire and leaving rubber on the fenders so I lifted the front 1/2in to clear. I suspect the Roush rotors are 5mm+ thicker than OEM which reduced my effective o/s to 25-30mm.
Changing to 19x8.5 w/35 o/s wheels allowed the front to come back down. I didn’t want to go with a significantly bigger offset in case I introduced issues with inside clearance. You guys may have more experience with inside clearance but, I didn’t want to risk more money trying the hard way.

 

[Norm Peterson]

275/40's on 18x9.5 is the easy choice, as there's a little over half an inch more sidewall height vs 275/35's. I'd put 285/40's on 10's, to at least get up to "measuring width".


19x8.5 in +35 is leaving a lot of room on the inside - more than 3/4" at 18" by my estimation plus about another 1/8" from the inward slope of the strut for 19's. While tire bulge will use up a little of that, you shouldn't be putting 275's or wider on them to begin with (even though you should still have somewhere around 5/8" strut side clearance with 275's). When you trust your measurements and your math, 1/4" is more than plenty of strut clearance. The only caveat I offhand know of is the front sta-bar endlink bolt, which may need to be cut, ground down, or swapped inside to outside so that it won't catch the sidewall.


Norm

 

[stevbd]

I can't compare to 275/35/20, but I have 275/40/18 MPSS square on 18x9.5 Apex rims with 35 mm offset and the ride is great. For comparison, I swapped from the oem 19" rim with 245/45/19 Falken all season tires, which has the same height sidewall, and noticed no difference in impact harshness. I noticed a huge difference in overall ride, handling, and responsiveness, since each corner is about 10 lbs lighter, 30 mm wider, and about 1" further outboard than stock.

EDIT: And plenty of room everywhere. As Norm says, the 275/40/18 is kind of the easy button in this discussion.

 

[Sky Render]

To be honest with you, my top priorities are ride quality (comfort) along with turn in response/steering precision (ride handling) So my question is this, if i were to switch out my current 275/35/20's on 20x9.5" wheels w/45mm offset.. Would switching to either 275/40/18 or 285/40/18 on 18x9.5 wheels w/45mm offset on all 4 corners provide a taller sidewall and improve ride comfort quality over my current 275/35/20 on 20x9.5" setup or would there be very little or nothing at all to be gained from making the switch.. Anyhow perhaps yourself along with Gabe, Pentalab, OX1 and Sky Render (Vince) can all chime in and provide feedback

Theoretically an 18 with larger sidewalls will provide more comfort than a 20 with shorter ones. That being said, I really doubt you can tell much of a difference between the two, unless you're going from a super-stiff summer 20 to a really squishy all-season 18.

If comfort is a bigger priority than performance, I'd recommend those Pirrelli all-season tires that came from the factory on my '11.

 

[Pentalab]

Thanks for your info.
You didn’t say what wheel width & offset you are using to achieve your clearance.
FYI my FRONT wheels were originally 20x8.5 w/35mm o/s. After the lowering it was okay but adding the roush front brake set I was getting SCUFFING of the tire and LEAVING RUBBER ON THE FENDERS so I lifted the front 1/2in to clear. I suspect the Roush rotors are 5mm+ thicker than OEM which reduced my effective o/s to 25-30mm.

Changing to 19x8.5 w/35 o/s wheels ALLOWED the FRONT to COME BACK DOWN.
I didn’t want to go with a significantly bigger offset in case I introduced issues with inside clearance. You guys may have more experience with inside clearance but, I didn’t want to risk more money trying the hard way.

This makes no sense at all. If the Roush rotors are say 5mm thicker, your offset would have been reduced from 35mm, down to just 30mm..which is bucket loads. You didn't say what size tire you were using on either your 20 x 8.5 rims..nor your 19 x 8.5 rims. You also didn't tell us how much you lowered the front end.

IF you lowered the front end by 1.5 to 2.0 inches AND were also using tall (big diam tires)..and NO negative camber, AND soft springs on the front, then I can see rubbing issues arising.

 

[Pentalab]

Theoretically an 18 with larger sidewalls will provide more comfort than a 20 with shorter ones. That being said, I really doubt you can tell much of a difference between the two, unless you're going from a super-stiff summer 20 to a really squishy all-season 18.

If comfort is a bigger priority than performance, I'd recommend those Pirrelli all-season tires that came from the factory on my '11.

Assuming same tire diameter (and also width) in both cases, the 20" will have a 1" shorter sidewall vs a 18". Having been in stangs with 18-19-20 rims, the difference between the 18 + 20 is noticeable... on lousy roads. ( all had aprx same stiffness front+ rear springs) The sidewall is a portion of the overall suspension. On the smooth hwy, there is no difference. Then the struts / shocks play into all of this too. Same lowering spring with a stiffer strut /shock is smooth on the hwy..and really rough on the shit side roads.

My Roush front struts were super stiff, IMO, over damped. ( rear roush shocks + roush springs are fine). I pushed down as hard as I could on either front fender, didn't budge. I sat on the front fender with my 151 lbs, doesn't deflect at all, none of the roush setups do. Eventually one of my front roush struts snapped clean in two. Replaced both front roush struts with oem struts. Now I have a bit of deflection, slightly more than I would want, but a compromise for now.

On a side note, the roush front struts are exactly 1/2" shorter than oem. (between bottom of strut assy...and spring perch). Roush front springs are 1/2" shorter than oem. The 1" front drop came from the combo of the two items. With oem front struts installed, ride height increased by 1/2".

To measure clearance between tire and front strut, I stuffed the back end of a drill bit in there
(round shank types, not the 3 sided version). I started with a .375 bit ( 3/8)..and worked down in 1/64" increments. A 21/64" ( .328") just slides through. This is in the worse case, between tire bulge + strut. BUT I also had a 3mm (.118) spacer in there. With spacer removed, gap is down to .210" . Front rims are 18 x 10 with a 45mm ET. ( which is reduced to 42mm with the 3mm spacer added). If you try this, do it with the wheels pointed straight ahead, and also extreme CW..and CCW.

On my 2010 auto, that sta bar end link bolt was a non issue. That may be just the difference between my 2010 and norms 2008, dunno.

 

[fourdegrees11]

Thanks for your info.
You didn’t say what wheel width & offset you are using to achieve your clearance.
FYI my front wheels were originally 20x8.5 w/35mm o/s. After the lowering it was okay but adding the roush front brake set I was getting scuffing of the tire and leaving rubber on the fenders so I lifted the front 1/2in to clear. I suspect the Roush rotors are 5mm+ thicker than OEM which reduced my effective o/s to 25-30mm.
Changing to 19x8.5 w/35 o/s wheels allowed the front to come back down. I didn’t want to go with a significantly bigger offset in case I introduced issues with inside clearance. You guys may have more experience with inside clearance but, I didn’t want to risk more money trying the hard way.

Sorry, fronts are 19x10 +40, rears 19x11 +50. Front brakes are 4 piston Brembos

 

[Pentalab]

Sorry, fronts are 19x10 +40, rears 19x11 +50. Front brakes are 4 piston Brembos

Ok, what size is the rubber on each end ? Lemme guess, 285-35-19 on the fronts...and 305-35-19 for the rears ?

 

[Norm Peterson]

On my 2010 auto, that sta bar end link bolt was a non issue. That may be just the difference between my 2010 and norms 2008, dunno.

I've heard of other people having the problem, but it was a non-issue for me since I'd swapped the front sta-bar and endlinks before getting involved with wheels wider than GT500.


You can use drill bits when you think the clearance is getting tight???? I've waited until it's a couple of feeler gauge blades in the 0.02x" range


Norm