br />• UHP tires with other UHP tires from the same manufacturer, same model name but a different stock-keeping-unit (SKU) or part number.

Give up?Well, the answer is pretty simple: All are dangerous, perhaps lethal, combinations.

The answer to “Why?” is very similar to the reason it’s risky to place tires with significantly more tread on the front axle than those on the rear. (BT­W: If you still don’t believe that, take a look at this)

In deep-water conditions, the tires with more tread will better channel the water and stay in touch with the pavement. Meanwhile, (depending on speed, water depth and tire pressure) half-worn tires may be floating on the water, completely removed from contact with the road.

If this happens in a slight corner or on a crowned road, the driver will get to tell the nice police officer, “I don’t know what happened. It just spun out.”

(Should you ever have a chance to drive a race car, when you crash it, NEVER tell the car owner or crew chief “It just spun out.” Take a moment to replay the situation in your mind to find the answer. Or make up something, such as: “It felt like the rear was going soft.” That’s an especially good excuse if the rear tire was ripped from the car.)

Perfect Pair
Especially with UHP tires, unless the tires match EXACTLY, the odd tire(s) out will almost certainly have more or less grip.

It might be better in the dry and worse in the wet, or vice versa. It might ride harsher or be more comfortable. It may better respond to steering input or be sloppier. There’s no way to know without testing.

If the tires with more grip are placed on the front axle, the car is likely to spin out in emergency man­euvers or even spirited driving.

These are UHP tires: Spirited driving is the reason for their existence. While far less dangerous, if the tires with more grip wind up on the rear the car will not turn well.

In the ex­treme the car will plow straight off the road.

Tire companies perform “mixability testing” when, for instance, production is moved from one plant to another or a major ingredient is changed. Even when the tire company aspires to exactly replicate the product made in the previous plant or with a slightly different recipe, an unacceptably high difference in traction may occur.

The computer models and tire designer’s calculations say the tires are identical. How bad can the difference be? Bad enough that I once ruined a sales guy’s day (week? month?). “I have good news and bad news,” I said. “The tires from the new plant are really good. The bad news is they’re too good. I wouldn’t allow my wife, much less your wife, to drive with the new plant’s tires in front and the old plant’s tires in the rear.”

My rule when replacing any tire: If you’re not going to replace four tires, match the existing tires right down to the SKU or part number. Other­wise your customer is engaging in Redneck Tire Testing: “Hey, y’all watch this! Git that on video? That’s going on You­Tube tonight!”

Tire-company boilerplate says don’t mix radials and bias ply tires, or speed ratings or such. At the same time, in many states, it’s no-harm, no-foul if you’re at 0.07 blood-alcohol level. If no spark is present, that mixture of fuel oil and fertilizer is fairly safe. And I’ve poured ammonia and bleach down chipmunk holes with no harm to myself – and no apparent injury to the rodents. See the logic?

Specifically, I would not mix a Super Veloce with a Super Veloce 2 or a Super Veloce 3, unless, of course, the maker specifically says the different-named tires do, in fact, play well to­gether.

While I have direct experience with many, many UHP tires – summer, A/S, etc. – there are as many out there that I have never tried. For instance, I have no experience mixing the Bridgestone Potenza S-04 Pole Position with the S-02 or S-03 versions of that tire. If for­ced to drive, say, a Porsche 911 or Cor­vette Z06 with a mishmash of these tires, I’d take it damn easy until I figured out which end was gonna lose grip first on which roads and in which weather conditions.

I’m not saying that any tires are bad or ill-conceived. What I am saying is that you – and your customer – may think a Super Veloce by any other name is still a Super Veloce, but seemingly minor variations might hinder their ability to perform together safely.

(Race story time: In an endurance race I drove a Camaro that the owner crashed in the first lap or two of a three-hour race. None of its wheels pointed in the same direction and the Camaro had a solid rear axle. I had two hours and 45 minutes to deal with a car that, in lefthanders, initially tried to spin out (oversteer or loose) before transitioning to a severe plowing push (understeer). In righthan­ders, it first understeered before snapping loose. “You wanted to be a race driver, Mac,” I said to myself. “Be a race driver.” Things got dicey if my attention wavered, such as when the crew chief was asking for temps. “Listen, Chuck, I’m sticking my neck way out here, the engine is gonna have to stick its neck out, too.”)

Even veteran tire testers can’t determine how a tire might perform or what tire might perform better just by looking at it. I once witnessed four extremely bright tire engineers and tire test drivers unsuccessfully trying to determine why one set of tires of the same model name, dimension, tread pattern, country and plant of production had a LOT less damp grip than another visually identical tire. One of the ex-racers noticed a small star on one that didn’t appear on the other. If I recall correctly, the tire with excellent wet grip had the star, which indicated it was a BMW OE tire. The non-star tire with modest wet grip tire was OE for a General Motors product. BMW more highly values wet and drip grip, which comes at the expense of snow traction (among other things), while The General is big on snow grip.

The short answer: All four tires must match exactly, right down to the SKU or part number. And the ones with more tread depth must go on the rear axle.

What about split fitments? “You bought a Porsche, BMW, Corvette and you’re gonna cheap out on tires? That’s what Mac does with his 2003 Camry.”

Yes, yes, that’s true. And I do so with my eyes wide open and my professional race license in my pocket and test driver experience on my resume. In other words, I know what to expect. Do your customers?

And to be clear, here are the consequences of the mixtures at the top of the article.

Independently, chlorine beach is annoying. Mixed with, among other things, ammonia or vinegar, it produces various gasses that can damage lungs and eyes.

Mixing ethanol in your stomach with pretty much anything – cars, firearms, women, men, but especially the drugs listed – can cause one to awaken in jail, in a hospital, with a strange person, or not at all.

We all witnessed the explosive power of ammonium nitrate fertilizer mixed and nitromethane in 1995 when that mixture nearly demolished a federal building in Oklahoma City.

The lesson: Don’t mix things when you’re not sure what will happen.