KDF why on earth would you carry two spares??? the only important parameter is that the OEM front+tire are the same circumference as the rest... You can use it as a temp on the rear... EVEN on a Brabus. Why do you think other cars have those bicycle looking spares? as a temp it wont ruin your car..
Not to burst your bubble, but on the Brabus the front is a 165r55 15 (15 inch) the rear is a 215r35 17 (17 inch). The front wheel/tire assembly measures 21.5 inches (approx.) in diameter, the rear is 22.5 inches (approx.). Albeit not truly significant. Yes, just one front spare wheel/tire assembly may do just fine to get you off of the road and to somewhere to get it fixed, however I look beyond that. I hit a pothole in Chicago and blew out the front sidewall, destroying the tire and bruising the rear. After some time, the tire service contract people managed to find me a front tire. It was 19 miles away at a Smart dealer. Mind you, I am 4 hours from home. The rear tire was no where to be found. The dealer only had a front tire. It is friday afternoon, 2pm - just where am I going to find any rear and expect it to be delivered, mounted balanced and ready before sometime the following week. Remember, the Brabus tires are uncommon tires. Hankook & Falken are the only makers of the rear. Hankook & Continental are the only makers of the front. If this breakdown happens in rural Pennsylvania -- many, many miles from any Smart dealer -- how much luck would I have trying to find these tires. The only option is to bring one of each along. Their total weight is 32 pounds. The Brabus wheels are too heavy and just darn too expensive (front wheel=$699, rear wheel=$878) to keep a couple of spares lying around. At the very least, if I have replacements, any tire shop should be able to help.
As a closing argument, my son took a 769 mile (measured one-way) round trip on a Suzuki VZ800 motorcycle. Fortunately, he did not encounter any tire or wheel mishaps and he did not carry a spare. But, he is only 26 years old. A mere child to me. I, on the other hand, know only too well the utter helplessness and frustration you can endure waiting roadside in the summer heat unable to move the vehicle and depending on the contract folks to help get you out of a jam [note: in this case, a 5 hour wait for not one, but two tow companies to find me]. The ordeal still, even under the best of situations, cost me a lost day stay over in Chicago. I experienced that real recently. So, on my next long trip (over 800 miles), I am traveling with at least the best odds I can bring to bear. The cost of the 2 tires came to a nudge over $200 delivered from tirerack.com. That cost of security = priceless.
[editor's note: I do not carry these two spares around locally. They are only for long trips. If the wife needs help, the cell phone is the fix (see Jetfuel's coments).]