Please keep in mine that a donut tire will have significantly less traction, leading to longer stopping distances in an emergency. Also, if running donuts in just the rear, the car will be much more likely to spin under braking.
That's nuts. I'm thinking that it's not really practical to use them long-term, though. Last time I had to use a donut tire, the directions said to use it at low speed and change it back to a 'normal' tire ASAP - don't remember how many miles.
Amazing results, though. Maybe those old Ford Model T tires were a good thing.
I guess I like cornering fast (only to avoid braking and losing momentum) too much to use donuts.
Re. the FE penalty, there's also an FE gain when you can go around a corner pretty fast and stay in control. Skinny tires won't do so well at that.
I love doing this in traffic, people see me coast for 3 blocks, then turn, no brake lights. I bet they wonder WTF? I got 205 60r 15s on my Accord, nice and fat. Yea, I could probably break 40mpg with a few areo mods and skinny tires, but it wouldn't be as much fun!