2009 stock wheel max tire size

[Norm Peterson]

Perhaps your dealer had to go by what options were available for an "08" GT coupe when ordering would be my guess?

He threw some screwy reason out, but I really do believe that he was unwilling to do any "dealer-installed" work prior to delivery.

I also noticed the slight bulge from the images and can't quite understand how a 265/40-18 tire would have any bulge at all when mounted on a 18x9.5" wheel with 45mm offset?

9.5" is only the measuring width for this size. Nothing more should be inferred.

A Michelin PSS tire in 265/40-18 is listed as being 10.7" wide on a 9.5" wheel. A 9.5" wheel is 9.5 inches between the bad seats, which are on the insides of the wheel flanges. Add half an inch (typical) for each flange and you only have 10.5".

According to TireRack and Discount Tire specs, the optimal tire size for a 18x9.5" wheel with 45mm offset is 275/40-18.

"Optimal" depends on what you want to optimize. Which could be either a single aspect of tire behavior or some "best overall average" of a combination of conflicting characteristics. Handling crispness and steering precision tend to get better with wheels slightly wider than "measuring width", but ride quality and launch bite tend to be better down at measuring width and narrower. Appearance is a strictly subjective aspect that doesn't even have a definition of 'optimum'.

Looks like you've also got a bigger straightline traction requirement to meet.

On a lighter note, the GT500 wheels definitely look really awesome on your "08" GT coupe

Thanks. That year GT500 wheel is one of only a few wheels that I feel that an OE mfr ever really got right. It's probably why I went with the 7 split spoke design in Forgestar when I went to the 11" wides. Both appear to have evolved away a bit from the split-spoke design Cosmic wheels of the early 1970's for sports roadsters and compact cars, and I always did like those.

Norm

 

[oldVOR]

Perhaps your dealer had to go by what options were available for an "08" GT coupe when ordering would be my guess? I also noticed the slight bulge from the images and can't quite understand how a 265/40-18 tire would have any bulge at all when mounted on a 18x9.5" wheel with 45mm offset? According to TireRack and Discount Tire specs, the optimal tire size for a 18x9.5" wheel with 45mm offset is 275/40-18. Until reading your post along with looking over your images, I was actually considering either the Firestone Firehawk Indy 500 or General Tire G-Max RS in 275/40-18 both front and rear as a square setup for my next set of tires. However after seeing your images, it appears that I may have to end up going with 255/45-18" tires in order to run a square setup. At present, I'm still running the stock Goodyear Eagle F1 285/40-18 (rear) and 255/45-18 tires (front). Anyhow I always liked the stock GT500 wheels as well and bought the set from a friend of mine that no longer needed them after getting his car delivered back from Shelby Automotive, as he ordered the Super Snake option package on his "08" GT500. On a lighter note, the GT500 wheels definitely look really awesome on your "08" GT coupe

Here's a few pics of 275/40's on 9.5's in a square setup lowered on FRPP "P" springs. No rubbing what so ever front or back.

 

[06 T-RED S/C GT]

Tried to fix post #23, but unintentionally double posted. Therefore request that post #23 be deleted!

 

[06 T-RED S/C GT]

He threw some screwy reason out, but I really do believe that he was unwilling to do any "dealer-installed" work prior to delivery.



9.5" is only the measuring width for this size. Nothing more should be inferred.

A Michelin PSS tire in 265/40-18 is listed as being 10.7" wide on a 9.5" wheel. A 9.5" wheel is 9.5 inches between the bad seats, which are on the insides of the wheel flanges. Add half an inch (typical) for each flange and you only have 10.5".



"Optimal" depends on what you want to optimize. Which could be either a single aspect of tire behavior or some "best overall average" of a combination of conflicting characteristics. Handling crispness and steering precision tend to get better with wheels slightly wider than "measuring width", but ride quality and launch bite tend to be better down at measuring width and narrower. Appearance is a strictly subjective aspect that doesn't even have a definition of 'optimum'.

Looks like you've also got a bigger straightline traction requirement to meet.



Thanks. That year GT500 wheel is one of only a few wheels that I feel that an OE mfr ever really got right. It's probably why I went with the 7 split spoke design in Forgestar when I went to the 11" wides. Both appear to have evolved away a bit from the split-spoke design Cosmic wheels of the early 1970's for sports roadsters and compact cars, and I always did like those.

Norm

I agree that more than likely your dealer just didn't want to do any dealer installed work prior to delivery? I think that at least I understand where your coming from regarding the section width of the tires on a 9.5" diameter wheel should have no more than a 10.5" section width. Therefore if this is accurate? then perhaps the ideal tire size for the OEM GT500 wheel is 255/45-18 with a tire section width of 10 inches. However I'm not quite following where your coming from when you mentioned, "Looks like you've also got a bigger straightline traction requirement to meet." So perhaps you could clarify that a bit more, as I'm running the stock GT500 wheel and tire combo from the factory both front and rear.

Here's a few pics of 275/40's on 9.5's in a square setup lowered on FRPP "P" springs. No rubbing what so ever front or back.

What's the section width of your tires along with the offset of your wheels? I just have a concern that running 275/40-18" tires on 18x9.5" wheels with 45mm offset may be a bit too wide of a section width of 10.9 inches causing the tires to bulge just a bit over the rim Other than that, I really like how well the 275/40-18's look on your GT/CS

 

[oldVOR]

The tires are BFG G-Force Comp-2 A/S



While the rims are 9.5" wide across the mounting bead, they are roughly 10.5" outside to outside.
With the tire having 10.9" section width, this means 0.2" will bulge on each side the wheel.

The offset doesn't affect bulge at all. The offset determines where the wheel sits in relation to the vertical fender line. The offset can allow the package to be inset like most factory wheels, flush or poke beyond the vertical fender line. Poke will most likely lead to tire and fender contact.

Offset refers to the location of the wheels mounting surface vs the actual centerline of the wheels width. 0 offset has the mounting surface at the center of the width. Positive offset pushes the mounting surface toward the outside or face of the wheel. Negative offset pushes the mounting surface toward the inside or back of the wheel.

The wheels have 35mm offset

 

[06 T-RED S/C GT]

You're right about offset not affecting bulge at all, therefore I now stand corrected lol. That being said, it would also appear that running 275/40-18" tires on 18x9.5" wheels will fit just fine, as I'm not going to get all stressed out over just 0.2" bulge on each side of the wheel

 

[Norm Peterson]

I agree that more than likely your dealer just didn't want to do any dealer installed work prior to delivery? I think that at least I understand where your coming from regarding the section width of the tires on a 9.5" diameter wheel should have no more than a 10.5" section width. Therefore if this is accurate? then perhaps the ideal tire size for the OEM GT500 wheel is 255/45-18 with a tire section width of 10 inches.

Running a tire size on its max-recommended width wheel would be the 'ideal' approach to take when cornering is top priority, ride comfort doesn't matter very much, and you don't care at all if straight-line launching takes a hit. It helps to be able to appreciate the mildly stretched appearance that results (easier to be OK about being different when you know why).

However I'm not quite following where your coming from when you mentioned, "Looks like you've also got a bigger straightline traction requirement to meet." So perhaps you could clarify that a bit more, as I'm running the stock GT500 wheel and tire combo from the factory both front and rear.

There's a big enough hint in your sig . . . "Saleen series VI S/C - 39lb injectors - 3.6" pulley". My car is strictly NA, so your traction requirements in 2nd probably aren't much different from what I need in 1st.


Norm

 

[06 T-RED S/C GT]

I'm going to seriously consider either 255/45-18 or 275/40-18 as a square setup for my next set of tires, as I don't think I'll be disappointed with either choice for the OEM GT500 wheels. In the meantime, thanks for clarifying about having the bigger straight line traction requirement to meet