Track Wheel & Tire Advise needed

[devildog1679]

I do atleast 4 roadcourse and 7 auto cross events every season. Just started last summer. I bought a set of 2005 GT500 wheel's (18x9.5) with the thought of putting 275 35/18 tires on them. The though was to have two sets of tires, DD and track. I figured for the track I wanted at least 275's and tires that size are cheaper in 18's vs. 19's so I went with the GT500 wheels. The 275 35/18 will have a AOD of 25.6" vs. the stock 27".

What effect will this have other then throwing my speedo off and lowering the speed at which I hit redline in every gear. Does this change change my current 3.73 rear to an effective 3.55's?

Am I better off with 18's or 19's?

I have a 2012 5.0 Brembo lowerd with Koni yellows and Steeda Sports.

 

[jayel579]

These cars ride on light truck sized tires. Because of this, good r-compound rubber is generally going to be a smaller diameter then what is stock. So a 25-26 inch tall tire is pretty normal. That said, a shorter tire is essentially putting a shorter gear ratio in your rear end. So your 3.73s should be a bit more than that. Your Speedo is going to be accelerated to the actual speed you will be doing. I think I did the math on this a while ago for myself and found that on a 275-40/17 tire I think my 3.55 gear ratio ended up being like a 3.9xx something. I thought about going on a 3.73 but on that same size tire I was like a 4.2x or something.

Please do not quote me on these numbers, it has been a while since I did that. I could be completely off with my memory.

You will be better off, budget wise on an 18” tire then a 19”. Personally I plan on running a 285-30/18 once I purchase a set of the GT500 wheels. When you are on the track, I am not too worried about MPG anymore.

 

[Whiskey11]

I do atleast 4 roadcourse and 7 auto cross events every season. Just started last summer. I bought a set of 2005 GT500 wheel's (18x9.5) with the thought of putting 275 35/18 tires on them. The though was to have two sets of tires, DD and track. I figured for the track I wanted at least 275's and tires that size are cheaper in 18's vs. 19's so I went with the GT500 wheels. The 275 35/18 will have a AOD of 25.6" vs. the stock 27".

What effect will this have other then throwing my speedo off and lowering the speed at which I hit redline in every gear. Does this change change my current 3.73 rear to an effective 3.55's?

Am I better off with 18's or 19's?

I have a 2012 5.0 Brembo lowerd with Koni yellows and Steeda Sports.

Decreasing tie overall height shortens the gear ratio (higher ratio number) so it would be like turning your 3.73s to 3.90s or 4.10s.

I would always go with the smallest wheel diameter and the lightest wheel. Even if the 19s weigh the same as 18s, the weight is half an inch further out from the hub center which means more rotational inertia and more strain on the brakes. Tires for 18s tend to be much less expensive than tires for larger wheels.

 

[devildog1679]

Thanks to you both.

 

[Norm Peterson]

I do atleast 4 roadcourse and 7 auto cross events every season. Just started last summer. I bought a set of 2005 GT500 wheel's (18x9.5) with the thought of putting 275 35/18 tires on them. The though was to have two sets of tires, DD and track. I figured for the track I wanted at least 275's and tires that size are cheaper in 18's vs. 19's so I went with the GT500 wheels. The 275 35/18 will have a AOD of 25.6" vs. the stock 27".

What effect will this have other then throwing my speedo off and lowering the speed at which I hit redline in every gear. Does this change change my current 3.73 rear to an effective 3.55's?

Effectively, you'd be having about 3.96's with OE-size tires, if there was such a thing. Call it 3.90's for what actually does exist.

You'll be shifting at about 6% lower actual road speeds - not that you'd particularly notice except for at the ends of any straights where you aren't quite into the rev limiter with the OE tire size now, or possibly with where the first upshift coming off a corner is currently occurring.

With higher straightaway speeds likely and the last upshift occurring at a different point, give some thought to where your braking point might move to and how your perception of getting there might change.


Norm

 

[devildog1679]

Effectively, you'd be having about 3.96's with OE-size tires, if there was such a thing. Call it 3.90's for what actually does exist.

You'll be shifting at about 6% lower actual road speeds - not that you'd particularly notice except for at the ends of any straights where you aren't quite into the rev limiter with the OE tire size now, or possibly with where the first upshift coming off a corner is currently occurring.

With higher straightaway speeds likely and the last upshift occurring at a different point, give some thought to where your braking point might move to and how your perception of getting there might change.


Norm

Good tips, I run three main tracks. Road America, Autobahn and Black Hawk. My braking and shifting points will deffinitly change. What are your thoughts on running 17x9.5's on the new 5.0's?

 

[SoundGuyDave]

Generally speaking, the shorter the tire OD the better. The only 17" rims that I know of that will clear the Brembo calipers are the R58 Cobra wheels, and you'll have to source them used, as they're out of production. A 275/40-17 will have the same overall diameter as a 275/35-18, so at that point, I would personally go for the more easily obtained 18" setup.

The shorter tire will serve you well at BHF and Autobahn, giving you just a bit more grunt coming off the corner, and there's nowhere on those tracks where you'll run out of revs just short of the next corner; they're all very obvious 4th-gear (5th for you, I think) straights. Road America, on the other hand... RA has been confounding us club racers for as long as the place has been open. The straights there are sooooo much longer, and the speeds sooooo much higher, that you really need a completely different setup to maximize the car for that track. With budget being no object, a much shorter top gear, a taller rear gear, and aero would be indicated. For the rest of us, we just suck it up and deal when we have to hit 5th (6th?), 3-5, 11a-12, and 14-1.

Which events are you doing at Autobahn and RA this year? Good chance I'll see you there...

 

[Norm Peterson]

I'm assuming that you meant 18 x 9.5. That's as narrow as I'd ever run 275-wide tires on and narrower than I'd run 285's on barring any wheel width limitations per class rule restriction or points penalty. But quite a few people have been running on 275-wide sizes. I'm on 18 x 9.5's with 255/45's that's a really good combination for general "3-season" street use although it gives away a bit in the occasional harder use.

An all-out effort would want still more wheel and more tire - see Terry Fair's "Vorshlag Mustang" thread and look toward the end where the car was refitted to ESP where the wheel width was not so limited.

FWIW, I'm starting to look at 18 x 10's and a little more tire for autocross and track time, just having a little trouble finding available tires in the 200 treadwear required by the organizers of certain events.


Norm

 

[Random Asshole]

The shorter tire will serve you well at BHF and Autobahn, giving you just a bit more grunt coming off the corner, and there's nowhere on those tracks where you'll run out of revs just short of the next corner

Truth. My fastest recorded lap at BHF was with my stock 17's. Even with serious suspension upgrades, lightweight 18's, and sticky 285s, I haven't beaten that time. yet.

 

[devildog1679]

Which events are you doing at Autobahn and RA this year? Good chance I'll see you there...

May 4th-5th and June 29-30 at RA. Not sure about Autobahn yet

I'm assuming that you meant 18 x 9.5. That's as narrow as I'd ever run 275-wide tires on and narrower than I'd run 285's on barring any wheel width limitations per class rule restriction or points penalty. But quite a few people have been running on 275-wide sizes. I'm on 18 x 9.5's with 255/45's that's a really good combination for general "3-season" street use although it gives away a bit in the occasional harder use.


Norm

Sounds good thanks

 

[SilverMustangGT]

Truth. My fastest recorded lap at BHF was with my stock 17's. Even with serious suspension upgrades, lightweight 18's, and sticky 285s, I haven't beaten that time. yet.

I know that was on a road course, but do you think factory 17's would be faster than light weight 18's in autocross? The 18's are 8.5" wide versus 8" for the 17's.

 

[Norm Peterson]

There was an article in SCCA's "Sportscar" magazine a few years ago that addressed the matter of wheel/tire weight. 5 lbs/corner on a Miata was found to be worth somewhere between 0.10 and 0.20 seconds on a course of typical length (somewhere in the 45-ish second range, i don't remember this exactly). I think it was Jason Isley doing the testing.

On a heavier Mustang with a lot more torque to work with, I'd expect a smaller benefit.


Norm

 

[Napoleon85]

I have found these two calculators helpful:

http://www.ajdesigner.com/phpgear/gear_equation_effective_gear_ratio.php
http://www.catherineandken.co.uk/sti/tyres.html

Also, here are the ratios for the MT-82 in case you don't have the ford spec sheet:
1st 3.66
2nd 2.43
3rd 1.69
4th 1.32
5th 1.00
6th 0.65

 

[Full_Tilt]

Wheel weight is very important. You're talking about unsprung weight and rotating mass. So heavier wheels will slow down your cornering and acceleration.

This is why I'd like to get some RPF1s in 18". I even thought about getting some 6ULs but they only come in 17" at the largest. I'm not sure if they would clear the standard brakes on my '11.

It pisses me off how so few people list the weight of the wheels they sell. Especially for the domestics it seems.

 

[Random Asshole]

I know that was on a road course, but do you think factory 17's would be faster than light weight 18's in autocross? The 18's are 8.5" wide versus 8" for the 17's.

I don't think I could give you an accurate answer even if I knew all of the other variables such as class, rules, track configuration, etc. People tend to go wide as possible no matter what it takes in autocross it seems.

 

[Mountain]

I have found these two calculators helpful:

http://www.ajdesigner.com/phpgear/gear_equation_effective_gear_ratio.php
http://www.catherineandken.co.uk/sti/tyres.html

Also, here are the ratios for the MT-82 in case you don't have the ford spec sheet:
1st 3.66
2nd 2.43
3rd 1.69
4th 1.32
5th 1.00
6th 0.65

This one is pretty good too:
http://www.rimsntires.com/specs.jsp

 

[devildog1679]

So I found a place I can get Toyo R888's for $145 shaved to 4/32 with one heat cycle. Tires were used in qualifying runs. Others swear by these? Should I bite? Or should I go with new ones? Either Hankook Ventus V12 evo K110 , Kumho Ecsta LE Sport , Sumitomo HTR Z III for about $180 a tire. I'm stilling learning to drive the stang so I think I'm leaning towards these plus I think I may get more track days out of them. Thoughts?

 

[Norm Peterson]

Have you narrowed down the size or sizes you're looking at? That might narrow the choices down, or might open up other possibilities.

The R888 is an entry-level R-compound tire, and is at least two steps up from the other three that you named. I don't think I'd cross-shop over that wide a range; I'd either be ready for an R-comp tire and not interested in tracking on true street tires or I wouldn't (and shouldn't be considering R's in the first place). New, expect the R888's to run at least double that used price but not likely to provide twice the track time. Used condition is another matter entirely.

For the "other three", read this comparison test paying at least as much attention to the test comments as to the charts/graphs.

All four of the value-priced tires in our test deliver sporty handling appropriate for the Max Performance Summer category. The Sumitomo HTR Z III proved to be refined by delivering excellent road manners and capable handling. The Dunlop SP Sport Maxx TT is the nimble handling leader, with excellent response to driver inputs. The Kumho Ecsta LE Sport and Hankook Ventus V12 evo K110 are capable, but don't have quite the refinement or poise of the others.

FWIW, I'd be inclined to look somewhat higher up on this chart. Maybe the Michelin PSS is too much of a financial stretch, but the Contis and the S-04's are only a little over $200.


Norm

 

[devildog1679]

Have you narrowed down the size or sizes you're looking at? That might narrow the choices down, or might open up other possibilities.

The R888 is an entry-level R-compound tire, and is at least two steps up from the other three that you named. I don't think I'd cross-shop over that wide a range; I'd either be ready for an R-comp tire and not interested in tracking on true street tires or I wouldn't (and shouldn't be considering R's in the first place). New, expect the R888's to run at least double that used price but not likely to provide twice the track time. Used condition is another matter entirely.

For the "other three", read this comparison test paying at least as much attention to the test comments as to the charts/graphs.




FWIW, I'd be inclined to look somewhat higher up on this chart. Maybe the Michelin PSS is too much of a financial stretch, but the Contis and the S-04's are only a little over $200.


Norm

Thanks Norm, I already have 18x9.5 wheels so more than likely I’m looking for 275 35/18. Since I’m still learning how to drive this beast and am probably still going to be in the novice groups I’ll probably skip the R-Comps for now until I can really take advantage of them.

 

[DocB]

Since you have an extra set of wheels, and you AutoX and Open Track the car a few times a year, I would suggest you consider a set of R-compounds.
I believe you will find that your track days will be more enjoyable and probably safer, even if you are in the novice group. AutoXing is even more fun on a stickier tire.
275/35-18 is a good size tire for the wheels you have.
If you look around, you can find some great used tires if you don't want to buy new.
Consider using the right tire for the right application.