Rtmoore4

All,

I just recently added quite a bit of power to my 2015 Z51 A8, thanks to LMR:

https://www.corvetteforum.com/forums...r-package.html

The bad news is that the rear tires break free way too easily now at relatively mild throttle input from a dead stop. I think I need some larger rubber here to help harness this power and was thinking about moving up to the Toyo R888 tire (with larger wheels required, of course).

My concern is that the outside diameter of the tires will be too different from stock, and that this could have strange and potentially negative consequences on the track when the brains of the car are expecting a certain wheel speed and get something else due to the size differences. I know the Toyo's are close (within 2-3% of stock), BUT I was astounded by how closely all three stock options (base C7, Z51, and Z06) are to each other. This leads me to believe that the stock outside diameter matters quite a lot to the car. Here is the raw data I calculated:

Front-

Base C7 245/40-18 - 25.71"
Z51 245/35-19 - 25.75"
Z06 285/30/19 - 25.73"

Toyo 265/35-19 - 26.3" - 1.9% larger than Z51


Rear-

Base C7 285/35-19 - 26.85"
Z51 285/30-20 - 26.73"
Z06 335/25-20 - 26.59"

Toyo 315/30-20 - 27.44" - 2.6% larger than Z51

If anyone can challenge my math, or otherwise provide some guidance here, I would appreciate it. Am I worried about nothing? Do the eLSD and Active Handling capabilities care at all about the accuracy of the vehicle speed sensor? Does a 3% difference matter?

John58

The front tires are ok at 265,but the rear tire can't exceed 305, or they will be outside the wheel well... Not a good look!

BlueDevilZ51

I'm in the same boat after adding a supercharger to my z51.
I've seen to options.
go from my stock 20" rims to 19" and get Mickey Thompson drag radials 285 35 19" (26.9")
Option 2
stay with my stock 20" rims and get Toyo 888s in 285 35 20".... being the wall height is 35 and the stock tires wall height is 30 it might run being my car is fully lowered on factory bolts.

Elite275

You can update the tire size in the tune easily to accommodate different sized tires. I've run the 315/30/20 R888 on the back of mine and now have 305/40/18 which are slightly taller than that. It was lowered all the way on the 20's but raised a little on the 18's. I haven't driven it enough yet to know how or if it will rub when the car squats or tires expand at high speed.

Rtmoore4

I have another thought here that solves the outside diameter issues, but brings up it's own set of problems. I am thinking about going with the new Michelin PS4S and sticking with the stock size up front (245/35-19) and then stepping down to a 19" tire in the rear as well, but going wider with the 325/30-19. The front outside diameter is then the same as stock, and the rear OD is 26.67", which is right in between the stock Z51 and Z06 OD's.

The issues I see with this are that I would be running 19's on both the front and rear, and the rear tires are going to stick out a little bit beyond the rear wheel well. With the proper offset, it won't be too bad, and I can always get the wider rear quarter panels from the Z06/grandsport if I end up hating it.

Of course, the other big issue is that I would need to get custom wheels done to get the offsets right.

Thoughts?

Marine1965

When at the Dragstrip....I run 18" Z06 rims on rear with M/T ET Drag radials......I am 767 h.p. And have ZERO wheel spin when launching...my 60' is 1.51....ET 6.57 at 108 mph in 1/8th mi....I love these tires...

Kent1999

It's really the difference in height between F/R tires that is critical to the car's active handling computers. It expects a certain differential in speed comparing front/rear wheel rotational speed, and if it sees a differential greater than what it expects, it thinks a wheel is slipping/spinning and may take corrective action.

I believe the tiny difference between "indicated" ground speed and "actual" ground speed is non-critical for higher speed handling aids.

BrunoTheMellow

it WILL rub. I have 285/35r20 hankook rs3 on mine and it rubbed on any bump at fully lowered. rubbed at doing laps at a track at half way up. Doesn't rub a single bit at full up. With that said I can't wait to get rid of them. It doesn't rub at the top inner or outer but rather at the rear, both sides (where the cubby is specially). Also, speedo is off by ~4 mph at 70 mph. Also I lost about 1 second on a 1:25 lap at the track, not gained.

Look at tirerack's specs for different tires. the ZP treads are actually much wider than non-zp tires of the same "size". If you don't do track days 305/30/r20 will likely be just fine. I hook pretty good on my hankook rs3s but likely due to the extra height and softer sidewall.

I plan on going to 295/30r20 and 255/35r19 on the stock z51 wheels. I likely tracking my car so I probably will try a more aggressive tire but definitely learned my lesson to stay very close to the stock size on these cars. Nothing like mustangs and camaros that can accept much bigger tires.

Does anyone make a 19x10 z51 replica?

BrunoTheMellow

I wouldn't run 325s in the rear, would stick out too much. which wheels would you run? change all 4? PSS prices should drop with the PS4S out now. The PS4S is only marginally better. Likely no better than the corvette ZPs, those are some seriously good tires.

Rtmoore4

I would run Cray Spiders. 19x9 in front and 19x11 in the rear. And yes, I would change all 4.

I understand the rears would stick out a little bit from stock(0.8"). I'm just not sure I care about that minor aesthetic detail.

Every review I have read about the new PS4S has said they are a major improvement over the PSS, in every way measurable, including non performance areas like road noise, comfort, and lifespan. It also sounds like they will phase out the PSS as they build more sizes in the PS4S.

Rtmoore4

I just ordered the PS4S in OEM sizes. We will see if they are any better than the OEM PSS ZP's before I buy a dedicated track tire, likely the Toyo R888R in 265/35-18 and 305/30-19 on 18x9 and 19x11 Cray Spiders. I would go with the 9" front's instead of the 9.5", just to get the right offset (50 vs 56).

tblu92

Problem with any super car is that if you start adding HP you risk excessive tire spin at launch----REMEMBER a Corvette is a sports car and NOT a drag car--It is mostly designed for handling and road racing----This is why GM installed the "launch control system" on all the C7 cars----Which limits tire spin from a dead stop--Have you tried using this feature ?
It works very well but does slow down your 0-60 times somewhat---but even then these cars are so fast they make up the difference quickly
Yeah maybe a 4500 lb Hellcat may leave with less tire spin but soon with their weight they fall behind even if they have more HP
I have tried 325 tires and with little success in reducing tire spin---Either use the launch control system OR you have to make many many practice runs trying to figure out exactly how to feather the throttle on launch just perfectly to reduce excessive tire spin---
JMHO

thrilled

I've heard this many times.The car with a lot of practice can do better without launch control.But for all of you that haven't raced much from sitting still you might want to use launch control.

tblu92

I agree That's why many mustang and dodge guys even guys with older C5 and C6 cars are so annoyed by the C7 because even a C7 driver with No drag experience when using the launch control system can make his car very competitive with all the above mentioned cars by letting the computer drive the car

Elite275

This may hold true for an auto, but the launch control on the M7 sets the RPM's too high and will blow the tires off. Better off just disabling TC and launching yourself at the track.

Rtmoore4

2 days on the PS4S at this point and I love them. They are quieter and stick a LOT better than the PSS ZP's. I'm still going to get the dedicated track tires, but these are WAY better for the street.

I threw a can of Fix-a-Flat in the trunk as an insurance policy, and of course, there is always AAA.

tblu92

One fact most everyone does not know the quickest Z06 corvette of all time is NOT a manual trans Z06 BUT an Automatic trans Z06 car-----The new 8 speed clutch to clutch auto trans Z06 is for the 1st time a faster and quicker car than ever before than a manual trans car-----This pisses off most all old school car guys as almost everyone in the past was correct in assuming a manual trans was quicker Not so any more--Wake up and smell the coffee---These new A8 trannys are far more efficient and have less drive train loss HP that now the z06 auto trans car is the quickest Corvette of all time
I'm not talking about majorly modded cars but just stock bolt on modded cars
But al things being relative Now most any C7 auto car can run equally with a manual trans

themonk

I'm going with Cup 2's front and rear 265/35/19 and 295/30/20, probably the best tire for grip outside a drag radial.