The February 2017 issue of Car & Driver had an interesting test of a mini-spare ("donut") on a Ford Mustang GT. The test was trying to determine how hard you can push a "DO NOT EXCEED 50 MPH" tire donut before you get into trouble. Surprisingly, the donut held up quite well in braking, skidpad and, to a lesser extent, acceleration tests. Ford's supervisor of Vehicle Dynamics Team, Jamie Cullen, stated the the "goal with a mini-spare is to come as close to the standard tire's performance and response as possible. Mini-spares use an aggressive compound and minimum tread depth to achieve those results." (Note- Some suppliers are known to use the same compound on their mini-spares that they use on superbike tires). Here are the tests summaries: Braking Full ABS braking from 70 mph with the spare on the left front was barely more dramatic than with four Pirelli P Zeros. There was a mild pull to the left but it only took one more foot for the car to stop (i.e., 173 ft.). Skidpad When fitted on the outside-front position, the spare reduced feel, response and grip, from 0.88g, with the P Zeros, to 0.84g.with the mini. Acceleration Zero to 60 mph was most adversely affected- from 4.8 sec., with the P Zeros, to 5.2 sec. with the mini on the left rear. It was interesting that the limited-slip differential attempted to rotate the tires at the same speed, despite a 2 inch diameter difference. Predictably, the result was less grip and more wheelspin.