Agree, have to interpret what you read. BUT what I outlined is using different tires front and back. That is where with the ME, 60% rear weight would make the difference between Understeer (which most folks when have driven only FE cars have experienced) versus Oversteer. And that only if you make a sever change in direction like if a deer pops in front. That is where you could wind up with the rear sliding out so fast there is no time to react, even if you turn the wheel properly to correct.

Since I had the same experience as Tadge, and I'm sure like his dad knew how to deal with oversteer as I did, totally understood what he said, quoting:

"Because I came from a Porsche family and my dad was a fighter pilot and rang his cars out at the limit, “I was Paranoid and Deathly Afraid of Trailing Throttle Oversteer.” Throughout the design we did everything we could do to make the car handle benignly. That included: suspension geometry, very stiff attachment at ridged chassis to suspension attachments etc. And not have it feel like it’s getting away from you (in a high-speed turn.)"

Some of us old timers understand that early Porsches where not the great handling cars they are today. Tadge also said in an interview that it took Porsche several generations to solve the issue (with rear engine, >60% rear weight 911's) and they (GM) had to get it right the first time. AND they did but small changes will made a difference. Your Car Your Choice!

SIDEBAR
What Andy quoted said replace tires in sets of 4. Did not say can't use 4 All-Season tires, same Brand, model as what came on your Z51 or visa versa. GM Brake Engineers admitted they used smaller brakes on the Z51 than they could to have it stop faster to avoid Oversteer in very aggressive stops. That is why the C8 Z51 stops "slower" than the C7 Z51. It relates to keeping the very careful balance as Tadge said "Throughout the design we did everything we could do to make the car handle benignly."

Those NASCAR fans might relate to the small changes they make in even a few psi tire pressure or chageing one rear spring adjustment 1 turn on a screw. Drivers can say it went from Pushing (Understeer) to Loose (there term for Oversteer.) And their cars are nowhere near 60% rear weight that makes it worse!

 

You are free to interpret the Owner's Manual as you wish, or ignore it all together if that is what you want. GM "strongly recommends" replacing the tires with the same TPC spec. The Summer and All Season tires have different TPC specs. And forum members will attest that the performance does change. Is it a safety issue? The manual does not say that. I changed my Summer Tires out for the All Season tires. GM "strongly recommends" replacing tires in sets of 4, saying the braking and handling may be affected, but says if necessary to only replace 2, put the new ones on the back. I think the statement is accurate - that is - I think with worn tires on one axle, and new tires on the other axle - the handling and braking will be affected. But, when it comes to mixing tire brands, types, and treads, the wording is stronger - its in a specific "Caution" block, the words strongly recommend are missing - and it says "may cause loss of vehicle
control, resulting in a crash or other vehicle damage." This sounds like a safety issue to me. The latter two conditions are applicable to the case brought up by the OP (only replacing two of the 4, and mixing brands). The OP is asking for forum member opinions on doing that. I did not express an opinion - I cited the Owner's Manual statements that address his specific question. It says you can put new tires on the back, leave the worn ones on the front, but expect the handling and braking performance to be affected. And it says if you mix brands, you might expect to loose control of the vehicle and crash.

 

I'd do this.

 

I 100% agree with this.

I realize a lot of people are used to taking this car out of the garage once a month in the sunshine and driving at 40 mph to a parking lot somewhere and sitting on a chair in front of it.

But there are also a lot of people who bought this car to drive.

For those individuals its important to realize that the tires are the only thing that connects this car to the road.

Tires are not only part of the handling and braking abilities of the vehicle but they're also part of the suspension of the vehicle. Tires themselves provide spring rate.

Different tread compounds from different manufacturers will have different rates at which they break traction. They will have different rates at which they are able to slow the vehicle which means your front and rear are now slowing at different rates than when they had the same tread compound. They can also provide different lateral transition responses meaning how the car moves from side to side. They may even tramline differently and they may even impact spring rate differently.

If you're driving this car 40 miles an hour to a parking lot I suppose most of this will not matter too much.

If you are driving this way in any type of performance manner or at posted speed limits during rain or any other times when more than 3/10 grip comes into play I would highly recommend heading the owner's manuals advice.

 

Yep even for folks who don't get close to the edge (as I occasionally do) that transient reaction when you try to avoid hitting a deer can get you in trouble! Also many distracted drivers today who can create a similar response.

 

Personally I would keep all 4 the same but…

on the street the car is such overkill that you’ll never drive at 10/10ths so it won’t matter.
electronic Nannies are always on the job.
people change wheel diameter and nothing happens
people change tire width and nothing happens.
people change track width with add on spacers or offsets and nothing happens.
people change wheel and tire rotational mass and nothing matters

I’m guessing you could run 4 different tires and never feel the difference, whether or not the car internals (differential) would notice it is a different matter.

 

I disagree with this on an objective basis.

1. Electronic nannies should never be relied on as a primary means of controlling the vehicle.

2. Wheel diameter changes are completely different than tire compound changes. Completely. And changing wheel dismeters does not mean TIRE DIAMETER is changed. Most any wheel upgrade will take into account original tire diameter.

In fact if you change the f to r tire diameter ratio too much you may get faults and be forced to run the car with all nannies off.

3. People change tire width and things DO happen. It's actually one of the main reasons people change tire widths which is in order to invoke a change in handling. In the case of tire width that would be to improve lateral grip. But again this is very different than putting on different compounds front and rear or anywhere else. And changing tire width is a very calculating procedure where most people will increase both front and rear the same amount so you're increasing the grip by the same amount. There are other individuals who are very experienced in tracking who purposefully change tire width ratios front to rear and that is to actually invoke a change in the way the car behaves. So yes these changes can have a very significant impact.

 

Good luck... You need to READ the MANUAL!

 

that’s why I prefaced it with I would keep all 4 tires the same… but, it would be interesting if mythbusters did a segment with all these changes done. I’m guessing the C8 would still run rings around most anything south of a GT Mustang.

 

Probably BUT don't want it doing an Agressive Donut all by itself! That is what an oversteering car do in a slit second, if the rea doesn't hit a tree!

PS: I like the MythBusters. Would try to tell folks when I had a Pick-Up that with the tailgate down you got worse mpg from an aerodynamic standpoint. Even had good aero pics on my website. BUT their test with two F150s from LA toward Phoenix proved it simply! The one with the tailgate down stopped miles before the other before it ran out of gas!