Peakster's Experiment #6: Mmm... DONUTS!

[GasSavers_Ryland]

what size are they? how wide are the spare tires? alot of them that I've seen are either 125mm wide, or 135mm wide, insted of 155mm wide, and that, with the 60psi recomended presure that they have, of course they are going to role easyer.

[Lug_Nut]

This is the opposite of my results.
For the 2005 Tour de Sol a rule stated that the tires had to be OEM and inflated to OEM pressure. I tried the four temporary tire idea as a way to skirt the intent of the rule. I did three roll down tests each with four 195-60-14 Goodyear Eagle GA inflated to the max sidewall presure of 44 psi, 185-60-14 Michelin (I forget which) at their sidewall max of 32 psi, and four (three Continental, one Firestone) T 135-80-15 at their 60 psi limit.
The test consisted of rolling, engine off, from a standing stop, down a slight grade, across a brief flat and then up a slight incline. The start was an imaginary line in the street at the end of my driveway aligning two trees in my yard. The stop was wherever the car stopped forward motion and was about to roll back.
The four temp tires had the shortest distance, The wide Goodyears were second by quite a bit further, and the Michelins were the best of the three by enough to not have been a fluke. All three Michelin roll tests were slightly further than the Goodyear best. The best of the temp runs wasn't as far as the worst Goodyear.
<edit> max speed was about 20 mph so the difference is almost all CRR and less due to aerodynamics.<end>

[Peakster]

Originally Posted by Ryland

what size are they? how wide are the spare tires?

I calculated the circumference to be 91&#37; of the 'real tires'. From the treadwear, It looks like there's about 1.5 inches width of the tread that touches the ground.

Originally Posted by Lug_Nut

This is the opposite of my results.

I'm sure the results would be different for each automobile. Like I said, I was absolutely shocked at the results.

[red91sit]

I would imagine the fact that it now has a slightly lowered stance also contributes a little through aerodynamics.

[kwtorbe]

Cool. I was thinking about getting some skinny lightweight drag wheels from a 4-lug fox body mustang drag race car.....I need to see if they are 4X100mm but the size would be similar to donuts and I think I would only drive on tires meant for constant road use....

[Peakster]

Originally Posted by kwtorbe

I think I would only drive on tires meant for constant road use....

Yeah. I've decided the only time I'll use the donut tires again is when I go on a long cross-province road trip (with the normal tires in the back seat, just in case). The ~9&#37; FE increase sure is tempting.

[Bill in Houston]

There's gotta be a way to do this that doesn't endanger your life or limbs. Not sure if trying to go a long way on donuts is something I would want to try. I am wondering if maybe there are trailer tires or mailtruck tires that are tall and skinny but still have a nice high load rating and high inflation pressure.

[Lug_Nut]

Originally Posted by kwtorbe

Cool. I was thinking about getting some skinny lightweight drag wheels from a 4-lug fox body mustang drag race car.....I need to see if they are 4X100mm but the size would be similar to donuts and I think I would only drive on tires meant for constant road use....

The Ford Superbird (3.8 six with a blower) had a four bolt pattern identical to the Fox-stang AND a skinny aluminum spare wheel. These were sought after by the 5.0 drag racing crowd. I think they are 4x4 or 4x4.5 so they aren't useable on the 4x100 Honda.

[brucepick]

Originally Posted by Peakster

... From the treadwear, It looks like there's about 1.5 inches width of the tread that touches the ground...

I think the FE gain is mostly from the 10 psi difference plus the narrower tread, particularly the only 1.5 inches that's contacting pavement.

I recall econocars from years ago always had skinny tires. Nowadays I think you just can't market a car with skinny tires but with less rubber contact there's less squirm. That is, it's not fighting anything to keep going staight in coordination with the other three wheels.

Wider tires are nice to have (mine are 20 mm. wider than OEM) but I think there's a FE penalty. I'll bet the Prius and Insight come with skinnier tires than other cars of equal weight from same model years, in order to avoid a wide tread that would hurt FE.

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.

[Bill in Houston]

Originally Posted by brucepick

Wider tires are nice to have (mine are 20 mm. wider than OEM) but I think there's a FE penalty. I'll bet the Prius and Insight come with skinnier tires than other cars of equal weight from same model years, in order to avoid a wide tread that would hurt FE.

Wider tires do feel nice. But, like you say, they hurt the mileage.

It may be a combination of reasons, but cars in Mexico come with much wider tires than their US counterparts. I expect that one of the main reasons is that US cars have to meet the CAFE standards, while in Mexico, the manufacturers put on a tire that feels better, handles better, etc.