Lower rolling resistance: space saver spare tire donuts @70 psi, or RE92's @ 51 psi?

[MetroMPG]

Just for fun: what do you think would win a slow speed coast-down test, all else being equal:

• four Bridgestone Potenza RE92 LRR tires @ 51 psi (OEM tire installed on the first-generation Honda Insight)

... or ...

• four Goodyear space saver spare tires @ 70 psi (OEM donut provided with 1991-2008 Metros, Fireflies & Swifts.)

Today I did a comparison of the rolling performance of all the different tires in my expanding Suzukiclone fleet. I tested everything at 51 psi on one car (the ForkenSwift) in repeated coast-downs. (My life is so exciting. )

For kicks, I also threw the spare tires into the shoot-out, testing at both 51 & 70 psi (sidewall says: "inflate to 60 psi"). Why? Because these wee tires seem to regularly find their way into discussions about RR, wheel/tire aero & weight... sometimes in a speculative, hopeful way.

I will say this: I was surprised.

Will post the details/results tomorrow!

[Kodak]

I'm going with the Potenzas for a few reasons.

I know that tire pressure increases help rr with diminishing returns, and 51 is already pretty high.

Secondly, I think CapriRacer said that wider can be better for rr: Barry's Tire Tech, rolling resistance part 2

I'm also assuming that the donut spare is built heavy-duty, and that you won't be going fast enough for the improved aero of the skinnier tire to matter much.

But now you got me curious.

[gone-ot]

...I'm gonna bet the spare (doughnut) wheel probably has better aerodynamics than the stock "full" wheels.

[Ryland]

those are 5 bolt wheels... so what is the vehicle? without knowing the weight of the vehicle it's hard to say.

(EDIT: note from Darin - I changed the photo in the first post to one of the actual car - before, it was just a random one I grabbed.)

[MetroMPG]

I just grabbed that image to illustrate. The vehicle was the ForkenSwift (Geo Metro), 2070 lbs.

And the test was low speed: didn't quite reach 10 km/h / 6 mph, so aero can be safely ignored.

[gone-ot]

...I'm not a Policeman, but I voted for the 'doughnuts' (wink,wink).

...DUNKIN' doughnuts, please.

[Vekke]

I would say the donuts. If you put there 70 PSI they will contact the road only on the centre. Usually they are already so round that the contact area is very small anyway.

[Ryland]

A big part of rolling resistance is tire deflection, or the size of the flat patch on the pavement, that is why it is possible for some wider tires to be lower rolling resistance then a narrower tire, an example of this is on my bicycle, I'm a big guy and my 1 1/8" wide high pressure racing tires roll with just as much resistance at 100psi as my 2" wide bicycle tires do at 80psi because both tires need to hold my weight up and a wider flat spot ends up being less of a hill to overcome, at some point of course going wider is not going to give any gains.

[Patrick]

I vote for the RE92s.

[MetroMPG]

Well, to cut the suspense...

The donuts were by far the worst performing out of 7 sets of tires I tested on the weekend!

I was not expecting this. (Maybe, like some others, I was hoping those strange looking little donuts would have mystical properties...)

They fell short of even my crappy old Walmart snow tires, which don't roll worth crud.

I compared 7 different sets of tires at 51 PSI, and then boosted the donuts up to 70 for a second set of runs to give them another chance - they still came in last.

The RE92's came in first.

At some point I'll post the details. Later, though! It's Turkey Day weekend in Canada. And lovely outside. I'm going bike riding with my nephew.