Tire Diameter Reduction, cha-cha-cha

[GT_Roadcourse_Newb]

So Ive gone from 27.7" rear tire diameter to 26.9" with the Kumho Ecsta PS91A 285/35/19. No issues, at least from driving feel perspective.

However, for track wheel/tire setup I am considering R888s in 295/30/19 which have a diameter of 26.0"...

Any concerns with this much of a gearing / diameter change that I (likely) am not aware of?

Thanks..

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[Brent Dalton]

With a stock redline of 7000, it will put your top speed in 5th at 145mph. Since you have a bama tune, I assume they adjusted your redline... maybe 7500? If that's the case, you'd redline at 156mph. If you are doing track days, 145mph is plenty. If you are time trialing and are on a larger track, you may need a little more than 145mph.

You can play with the numbers here:

http://www.apexgarage.com/tech/gear_ratios.shtml

 

[GT_Roadcourse_Newb]

With a stock redline of 7000, it will put your top speed in 5th at 145mph. Since you have a bama tune, I assume they adjusted your redline... maybe 7500? If that's the case, you'd redline at 156mph. If you are doing track days, 145mph is plenty. If you are time trialing and are on a larger track, you may need a little more than 145mph.

You can play with the numbers here:

http://www.apexgarage.com/tech/gear_ratios.shtml

Much appreciated Brent!

-John

 

[Norm Peterson]

Without a compensating increase in rev limit - or extended usefulness of actually running up there - all of your shift points will be coming at lower speeds and at different points on the track. Possibly by enough to have you adding a couple of shifts. That's what I noticed right away when going from ~27" tall 255/45-18 tires down to 25.9" 285/35-18's.

There should be a little less air getting under the car.


Norm

 

[CoastHighway]

Interesting. I feel that the GT PP gearing is a bad match for Thunderhill and Laguna Seca. I short shift to 5th on the front straight at Laguna, and on the back straight at Thill to stay off the rev limiter. I also have to lift going into 11 to stay off the limiter. Moving the shift points might be good. Some extra RPM would be nice...

 

[GT_Roadcourse_Newb]

Interesting. I feel that the GT PP gearing is a bad match for Thunderhill and Laguna Seca. I short shift to 5th on the front straight at Laguna, and on the back straight at Thill to stay off the rev limiter. I also have to lift going into 11 to stay off the limiter. Moving the shift points might be good. Some extra RPM would be nice...

in agreement with that comment, I enjoyed being able to go to 7k rpm in 4th and hold it 1 sec rather than shift.

 

[shanethealmighty]

Without a compensating increase in rev limit - or extended usefulness of actually running up there - all of your shift points will be coming at lower speeds and at different points on the track. Possibly by enough to have you adding a couple of shifts. That's what I noticed right away when going from ~27" tall 255/45-18 tires down to 25.9" 285/35-18's.

There should be a little less air getting under the car.


Norm

What width wheel are you running your 285/35r18 tires on? I was thinking about picking up a set of NT05s for my 18x9.5 wheels, but my 555s in 275/40 have been wearing pretty well at the track. I like the idea of going just a touch smaller to use 4th gear a little more at the track I go to.

-Shane

 

[Norm Peterson]

18 x 11 (max recommended) x all four.

I've lowered the car about half an inch on somewhat stiffer springs since this picture was taken.

Norm

 

[NvrFinished]

I've been looking at the pitfalls of running a shorter tire as well. This is where the GT350 manifold can be a big help for those looking for performance upgrades. It pulls hard past 7500 rpm and the power does not drop off past 6200 rpm like the O.E. intake does. It could make up the difference for going with a shorter tire.

 

[GT_Roadcourse_Newb]

18 x 11 (max recommended) x all four.

I've lowered the car about half an inch on somewhat stiffer springs since this picture was taken.

Norm

Ive never seen tires stretched in that direction! Wow, they must be tenacious!

 

[Brent Dalton]

I've been looking at the pitfalls of running a shorter tire as well. This is where the GT350 manifold can be a big help for those looking for performance upgrades. It pulls hard past 7500 rpm and the power does not drop off past 6200 rpm like the O.E. intake does. It could make up the difference for going with a shorter tire.

I agree. I think the GT350 is going to be the ticket for the road course. I'm more concerned about the valve train though, but it's supposed to be the same as the boss and the boss has a stock redline of 7500. I do have a GT350 intake and throttle body to put on. I just have to find time to try to figure out tuning it.

 

[Norm Peterson]

Ive never seen tires stretched in that direction! Wow, they must be tenacious!

Turn-in response is pretty sharp, and noticeably crisper than my current "3-season" setup which isn't too shabby itself (265/40 MPSS on 18x9.5; Michelin claims the same tread width for both sizes at 10.2").

After 6 track days, they'd gone "off" a little, guessing half a second a lap or so out of 1:38-ish, but it wasn't until their 7th outing that I had any datalogging capability. Fully warmed up to track temperature; you wouldn't see these lateral numbers at street driving tire temperatures.

Norm

 

[shanethealmighty]

18 x 11 (max recommended) x all four.

I've lowered the car about half an inch on somewhat stiffer springs since this picture was taken.

Norm

http://imgur.com/CJqxHyM

This is my car on 275/40r18. I am considering dropping down to 275/35r18 since 275 is the optimal width for my 18x9.5 wheels. I wouldn't mind the extra stiffness, less weight, and lower stance. From my quick math, that would drop my top speed to about 143 mph redlining in 5th which shouldn't be a problem. I guess the question is whether I want to have that extra shift into 4th in some of the straights or stick with the same size and hold 3rd.

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