In our minds, this was going to be a lot more exciting. Our plan to test a Ford Mustang GT with a space-saver spare in place of the full-size rubber at one corner seemed not just imprudent but also stood in direct violation of the advice of the owner’s manual and our attorneys. Mom also weighed in against it. The manual was most explicit: “ . . . do not exceed 50 mph” it warned, with another warning on the diminutive donut. But we defied all that because we wanted to see how much emergency performance you can expect when your car is wearing a spare.
Some suppliers are known to use the same compound on their mini spares that they use on superbike tires. And our spare did indeed have soft rubber. Pre-test, we measured 4/32 inch of tread depth on our space saver. Post-test, though the wear wasn’t consistent, tread depth was reduced to roughly 2/32 inch. The OE fitment Pirelli P Zero Nero All Season tires offer 10/32 inch of tread depth when new.
Surely there would be fire—actual flames!—from the mini spare as we relentlessly circled the skidpad, reducing the donut to smoldering rubber in only two laps. If we weren’t handy with the extinguisher, the conflagration might just consume the entire Mustang in a matter of seconds. There would be explosions, air tankers, and rescue choppers. Monkeys and zombies would sprint to escape the inferno. Perhaps not, but you get the point. It was sure to make the typical cars-and-coffee Mustang debacle look like mere curb rash.
Results |
|||
| — | |||
| Compact-spare location |
70-to-0 mph | Roadholding, 300-ft skidpad |
Zero to 60 mph |
| None | 172 ft | 0.88 g | 4.8 sec |
| Front | 173 ft | 0.84 g outside 0.88 g inside 0.86 g average |
— |
| Rear | 174 ft | 0.85 g outside 0.91 g inside 0.88 g average |
5.2 sec |
In truth, we didn’t even need common sense to avoid catastrophe. Nor, in fact, will you. Because the most remarkable thing about driving a Mustang at its limit with a space-saver spare on one corner is just how unremarkable it actually is. The T155/60R-18 Maxxis mini spare that Ford supplies as an option on Mustangs without the Performance pack absorbed the abuse with virtual indifference when inflated to the recommended 60 psi. That it produced handling and braking results very similar to the 255/40ZR-19 Pirelli P Zero Nero All Season tire it replaced was as surprising as it was anticlimatic. There’s a lot more going on here than just a flat-tire contingency plan: “The goal with a mini spare,” says Jamie Cullen, supervisor of Ford’s Vehicle Dynamics Team, “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.” Here are ours:
Braking
Full ABS braking from 70 mph with the spare on the left front was barely more dramatic than with four P Zeros. There was a mild pull to the left, but not so much that even the most distracted driver couldn’t make the correction, Cherry Blossom Frappuccino in hand. And, at 173 feet, it was only one foot longer than with four standard tires. The pull was less significant with the spare on the left rear, though stopping distance increased another foot.
Skidpad
Roadholding testing followed our standard protocol, which is to lap a 300-foot circle in both directions and average the results—in this case, 0.88 g with the P Zeros all around. When fitted on the outside-front position, the spare reduced feel, response, and grip for 0.84 g. When we turned the other way, with the spare on the inside, its presence was barely perceptible and grip was 0.88 g, for an average of 0.86 g. Moving it to the inside rear increased the Mustang’s willingness to rotate and improved the result to 0.91 g, granting us insight into what sprint-car master Steve Kinser experiences most Saturday nights. Switching directions had the opposite effect, yielding 0.85 g. Average grip worked out to—wait for it—a very Pirelli-like 0.88 g.
Acceleration
Zero-to-60-mph acceleration suffered more dramatically than any other test, slowing from 4.8 seconds to 5.2 with the spare standing in for the left-rear Pirelli. The Mustang’s limited-slip differential attempts to rotate the tires at the same speed, despite diameters that differ by almost two inches. The result is less grip, which means launching either less aggressively or with more wheelspin, both of which hamper acceleration. Again, once the wheels regained traction, there was a slight pull to the left, and again, it was easy to correct. The resulting narrow/wide burnout marks, however, were priceless. As will be the letter we send to our attorneys, relieving them of their duties. Mom can keep her job.
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