Here’s the latest reader question, along with my reply!
Derek asks: I enjoyed your article about big rims vs. MPG (here). But I thought that larger diameter tires gave less rolling resistance, all other things being equal. Your article takes as a given that larger wheels provide more rolling resistance. A quick google search seems to reinforce my belief that smaller wheels have more rolling resistance. My understanding is that rolling resistance is essentially tire deformation – primarily sidewall deformation. Of course all other things are never equal. The larger diameter wheels are heavier, which is bad three different ways: mass, unsprung mass, and rotational mass. (That’s why racy rims are so lacy – getting that triple-threat mass out.)
Air pressure changes rolling resistance dramatically, which makes sense if sidewall deformation is the source of rolling resistance.
Do low-profile tires typically run on lower pressures, since the sidewall is so stiff? (Trying to maintain ride comfort?) Does that account for the MPG result with the larger diameter wheels? Am I missing something obvious?
My reply: Most of the larger-diameter “rims” installed on most new cars are also wider than they used to be. For example, most current 17 inch diameter wheels are 8 inches wide.
The tires most cars wear are also pretty aggressive relative to what was common in the past, even on performance cars. For example, my ’76 Trans-Am came with (and still has) 15×7 wheels and the factory tire size was 225/70-15. A current Nissan Versa has a larger (and wider) wheel/tire package than my V8 muscle car!
Also, tire pressures are generally much higher now than they used to be. 40-plus PSI is pretty typical.
As a result, most new cars have a much firmer ride than the cars of the past did – and road noise is higher, too.
The upside is they do handle better, are more controllable and stop faster.
. . .
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“my ’76 Trans-Am came with (and still has) 15×7 wheels and the factory tire size was 225/70-15”
Funny, my Jeep Cherokee has the same size tires and wheels!
Theoretically, the most efficient wheel/tire would be something like the old 1920s and 1930s cars had: tall narrow spoked wheels and narrow tires.