I had a question from a customer that surprised me the other day. After doing this for as many years as I have, I tend to think that I've heard it all. We were talking about an older BMW, and he asked me if the original tire sizes were staggered. I replied that they were not. His follow-up question was, "Well, why not? It's not like the car is all-wheel drive."
There are a couple of assumptions in that simple question, and they are a mixture of true, false and revealing statements.
Assumption #1: All-wheel drive cars do not or can't have staggered tires.
Assumption #2: If a car can have staggered tires, it should or is expected to.
Assumption #1 is somewhat accurate, but isn't set in stone. Many AWD systems will wear prematurely if there is too much difference in the rolling diameter of the tires. And, it's hard to get staggered tires with perfectly identical diameters. They are usually close enough that it looks identical, but the actual revolutions per mile may be too different than a given AWD system can tolerate.
With that said, there are some vehicles that come with staggered tires from the factory. And in fact, the first example off the top of my head comes from within the BMW marque that we were discussing. The X5 is AWD (or xDrive in BMW-speak) and many of them have staggered 20" or 21" sizes.
Assumption #2 is the one that really surprised me. Staggered wheels and tires have become more common in recent years, and it seems they've become ubiquitous enough that some people think of that as the default configuration. This still blows my mind!
The default configuration for a car or truck is to have all four tires the same size. This set-up has a number of advantages. First, It is easier to find tires when one does not have to look for something that is available in two different sizes. The tires can be rotated from front to rear to help maintain even treadwear.
Next, there is less chance for confusion. I've seen staggered set-ups installed with the back on the front and vice versa; and I've seen customers assume that if they check the size of one tire, that all four tires will be that size. Also, a full-size spare or extra tire can be used on any position of the vehicle.
With that said, there are a few reasons to go with staggered tires. One is weight bias. If you have a car that has a large majority of its weight over one axle, that axle should have bigger tires. See Porsche 911s with their rear-engine weight bias as an example.
Two is power. If you have a rear-wheel drive car that has a lot of horsepower, you may need bigger tires on the rear to help put that power to the pavement. Why not just make all of the tires bigger in that case? The size of the front tires is often limited by the fact that the front wheels have to steer left and right. Too wide of a tire may rub when the steering wheel is turned. Super-wide tires in front can also aggravate the tendency to hydroplane when hitting puddles, and may make the steering feel unduly heavy.
Three, and often the most relevant, is style. The staggered look, with big tires on the back, is generally considered to be cool. I think part of that is driven by the fact that it's usually the more exotic cars that need staggered tires: rear or mid engine, high horsepower, RWD. These attributes describe some of the most iconic cars in history, from the Ferrari F40 to a Top Fuel dragster. Such is the cachet of wide rear tires, that I will often have customers with FWD cars ask for a RWD-style staggered set-up, causing Isaac Newton to spin in his grave at 8,500 RPM. (For the right way to stagger FWD, take a look at "Automotive Oddity: Correctly Staggered Tires on a Front-Wheel Drive Car")
In the case of the old BMW, it had balanced weight distribution and horsepower, that by today's standards is moderate, and the staggered-tire style trend was not nearly as widespread in the late 1980s. Therefore, there was no reason for it to diverge from the normal tire configuration. I didn't bend my customer's ear with all of these musings, but a highly condensed version was enough to give him the general idea.