Rear Tire Clearance (SVTPP)

[Madlock]

Forgive me for anything that may be laughably obvious to most others - that I'm only now beginning to learn, realize and understand.

 

I'm finally preparing to pull the trigger on ordering my alternative wheels and tires for my 2011 GT500 with SVTPP to give myself an alternative to the F-1 G: 2's that make the car more everyday friendly - even during non-winter months.

 

One of the biggest obstacles has been the lack of any kind of definitive reference that establishes the boundaries of wheel and tire size combinations that can be used without causing clearance issues- particularly given the extent to which there can be so many different S197 suspensions that lower the chassis to different degrees - particularly for vehicles with the SVTPP, as it affects some 3/4 of all 2011 GT500s.

 

With respect to the SVTPP, one of the reasons I decided to order it was the 3.73 rear axle - not knowing just how easily certain things can be modified after he fact. If 3.73 is what I wanted most, I could've simply swapped-out the gearing and added the FR3 pack for much less money and drama because I could've decoupled everything from the damned F-1 G: 2's. As others have pointed out to me (which I hadn't fully appreciated at the time), the 3.73 merely gives back some of the quickness lost when going from a 27" diameter rear tire to 28".

 

I'll be going to a 19" rear wheel. While I had been trying to remain as faithful to the original 28" tire diameter (both to keep the car as close to its factory stance as possible and to use the additional sidewall to soften the ride a little and gain a bit better power application) and my thoughts were largely focused on not exceeding the factory width or diameter, I'm now beginning to think about different combinations, including final drive gearing - without setting all the factors of the equation in motion against each other.

 

So, I'd appreciate any helpful thoughts about the following two propositions.

 

 

 

• On a 19" wheel, what's the smallest tire diameter I can get away with - given that any diameter reduction COULD enhance quickness by effectively increasing the final drive ratio effect - but knowing that the car has already been lowered by virtue of the SVTPP suspension setup. I don't want to be using the pavement to polish the bottom of the differential.

 

 

• I've also begun giving thought to a far more radical path. I know the FR 4.10 gearing is mostly meant for dragging and would probably make shifting during everyday driving fairly cumbersome - not to mention being absolutely brutal in terms of fuel economy. First gear would pretty-much become a bit of nostalgia.

 

 

The theoretical up-side would come from the fact that in the same vein that I could reduce the rear tire diameter to gain-back some of the quickness the 3.73 gearing loses by the 28" OE tire, I could (theoretically) increase the rear tire diameter to offset some of the huge differential between the 3.73 to 4.10 gearing - while simultaneously gaining even

more

sidewall for an even

more

compliant ride with an even-better ability to connect the crankshaft to the pavement. For this scenario, I'd need to know the largest diameter a 2011 chassis could accommodate slung on the SVTPP suspension set-up.

 

The challenge for me (particularly with the second scenario) is the difference between the math that works and the real world that sometimes gets in the way. To fully offset a 3.73 to 4.10 gear change, a tire would have to turn 676 times per mile versus the OE 724. But to recreate the same kind of giddyup a 3.73 normally gives versus a 3.55, a tire would need to turn about 690 times per mile, something that can be almost be achieved with a 29" diameter tire - for which a reasonable selection of tires actually exists, particularly if I were to consider keeping the OE 20" wheels on the rear. For example's sake, a 285/40-20, 285/45-19 or even 305/40-19's would be close enough for government work.

 

So, I suppose the bottom line is that I'd need (like) to know both how tall, and how short a rear wheel diameter a 2011 GT500 SVTPP can accommodate without shooting myself in the foot. I'm not necessarily interested in simply going wider for the sake of going wider, particularly because so many tires this wide already have a tendency to hydroplane, but I'd also appreciate knowing just how wide a person can go under the same circumstances (presuming equivalent relevant offsets to the factory setup).

 

I'd appreciate any helpful insight - even if I happen to be heading toward the deep-end.

[BDrool]

Here is a tire calculator that I've used a lot to figure sizes. Mostly for dedicated snow tire for other vehicles...

 

http://www.miata.net/garage/tirecalc.html

[dwcallaway]

Madlock, great post! I, too, am a first-timer on the Shelby front. Sounds like we have similar thinking on the extra pair. I haven't decided at all what to do yet (other than garage the car for this, it's first winter). I will be getting an extra pair of wheels and tires sometime next year though. The only plus is that by then, perhaps there will be a little more information and/or alternatives.

 

Your post is pretty informative. While there are literally dozens of posts on the tire issue, it seems that most are more oriented to drag tires. Let me know what you end up deciding. Your comment about having a round table to discuss this issue is right on point.

 

I'm planning on attending the Mid-America Shelby Fest in Tulsa, OK next year and hopefully will be able to learn a lot from different folks in regards to their setup, styles, etc.

 

For now, I really like the looks of the Forgestar F-14's, but am left wondering if they are really an option without making a ton of other adjustments/modifications. I also think I'd prefer 19's, but would entertain 18's, given the right amount of information. I plan to keep mine stock and maybe track it a little (just because), but really have no interest in a ton of modifications. I just want to be able to get in her and drive on days like today when it's sunny and clear, even if it is 28 degrees out!

 

Anyway, no help from me, but thanks for the post. It's a good addition to everything else I've read.

[Madlock]

Thanks, DWC.

 

For all the discussion about so many people doing so many things, I'm always surprised by how little bona fide reference information exists - or the lack of any kind of useful compendium, particularly about wheel and tire sizing. As you've noted, there's some disparate tire discussion by the owner subgroup that drags their GT500s and some include some very detailed information about very specific products or configurations, but remarkably little exists from the broader perspective of what's possible.

 

 

I like the Forgestar's a lot. I had initially set my sights on the RTR's because they seemed to be such a close match to the OE SVTPP wheels, but since Ford Racing has now come out with identical 19" wheels the OE SVTPP wheels, I've decided upon to stay faithful to the factory appearance. The "twins" offer sufficient caliper clearance and offsets that vary with the wheel width to maintain the same relative tire centerline as the OE spec.

 

Note: I'd love to hear Ford's pricing rationale for charging $1,300 to replacement an OE 19 x 9.5 SVTPP wheel - but only $500 for the same 19" wheel in 9" or 10" widths from Ford Racing. I dunno. Perhaps there's a special duty levied upon wheels imported from Taiwan wider than 9 1/4" but narrower than 9 3/4".

 

 

I'm sure Ford has a reasonable interest in not suggesting anything anybody might later claim to be officially-sanctioned and exposing itself to additional warranty claims or dispute. I'm also sure SVT spends a lot of time tuning suspensions within a closely controlled set of variables and refusing to deviate from the officially-published OE specifications (not even on an "off the record" basis) makes their lives much easier and insulates them from the inevitable peckerwood whose lawyer would try to turn an unofficial hypothetical tidbit into a Lotto windfall by claiming Ford had no business even trying to answer the question he asked that helped Charles Darwin hunt him down.

 

It's just a shame for the rest of us.