donut tires

[GasSavers_olddad]

I put a couple donut spares on the rear of my 97 metro. Got a mile to two better milage. Only on the third tank of gas. My question to others that have done this is what causes the gain. The skinny tire?, high tire pressure? (spares are 60psi) or the lowering of the rear of the car buy 1 1/2 inches because of the smaller diameter of the donuts. The 97 sits tail end high. Has anyone ever sucked down just the rear an inch or two and checked for mileage gain?
jack

[dkjones96]

The increased mileage comes from the small spare having about a quarter's worth of contact patch with the road. It's actually very dangerous to run around like that.

The back isn't as bad as the fronts being like that but not only will the vehicle be way more likely to oversteer with those on but those spares were not designed for regular use.

[theholycow]

At 60psi, it probably does have a quarter of the contact patch.

That combined with the aerodynamic gain from leveling the car should explain it well enough. Actually, looking at a picture of a 1997 Metro, I wouldn't be surprised if it wouuld benefit from going further than leveling, down to what's called a "Cali lean" and looks (IMO) terrible.

[Improbcat]

Also, donut spares are not designed for long-term use. If you inspected them I would not be surprised if you found significant wear on them.

You would probably be better served looking for smaller tires to fit your stock rims.

[Lug_Nut]

Narrow width and high pressure don't always offset the higher rolling resistance.
I did some low speed coast down tests in advance of the 2005 Tour de Sol with the Passat I had at the time. From a sandstill on a slight slope I'd release the brakes and see how far I coasted before coming to a stop. Four Continental T135-80-15 at 60 psi did not coast as far as four 185-70-14 Michelins at their max of 32 psi. The best of the temporary tire roll-down distances was not as good as even the worst of the full size tires' distance.

[theholycow]

Lug_Nut, I hope you don't mind if I quote that in my tire threads...that's the kind of data we should all be using, and the kind of test that's perfect for the job. I can't argue with that test. They were even almost the same outside diameter. The only thing that could be argued is whether it's the width or height that had the most effect.

Did you try the 135-80-15 tires at 32psi? I know it's totally counterintuitive but if you did I'd love to hear how that compared....I'd guess that it resulted in shorter distance than at 60psi (and shorter distance than the wider tires).

[GasSavers_RoadWarrior]

Usually kicking the back of a car up (or dropping the front) helps reduce drag, so dropping it would more likely increase it. Also would tend to make the back end light at speed.

It's possible that the donuts sitting an inch further into the arch reduce drag quite a bit. There's a thread around somewhere about wheel covers/spats and racing hubcaps that has pix of a racing teams experiments with rear arch aero, where they concluded that setting back the tires in the arches a little was more effective than the hubcaps.

However, getting different offset wheels and arbitrarily changing the rear track of the vehicle may have undesirable handling consequences.

In my personal experience, the "easy to get" low rolling resistance tire is the Kumho made Walmart Marshall 791 Touring. They are rated at 36 psi, which is pretty good for RR, they take 50psi, but with the size on Marvin and his weight, the max pressure I could run without losing wet weather grip was 44psi, and had to have them at 36 in winter. On Marvin it seemed like difference between those and "bad RR"
tires was 1mpg, and difference between standard inflation and 42-44psi was another 1mpg. Which is about 5% each way. Highest pressure that maintains safe grip is a matter for experimentation on individual tire and car combinations. Handling is often much improved as long as you stay below the point where the grip tails off.

[dkjones96]

Very interesting... Makes me wonder what all is involved in that. Kinda makes me think of the mousetrap car me and another guy made in high school.

They said to go with the lowest weight body parts and that CDs were a good wheel to use because they were light and straight. It had to go as straight as possible and as far as possible. We used wood paneling, metal hooks, aluminum shafts and LPs (large records for those of you that don't know what an LP is) and it weighed in at over 3 times what any of the other cars were. However, the furthest any of the other cars got was about 55ft while our car went 175ft before hitting the end of the hallway and when it did we were only 1.5 inches off center.

Point is, we had the same amount of power available to us that everyone else did(the mouse trap) and while the car was one of the slowest of the bunch it coasted VERY well. Only the first 30-35 ft was actually powered.

[theholycow]

Originally Posted by RoadWarrior

In my personal experience, the "easy to get" low rolling resistance tire is the Kumho made Walmart Marshall 791 Touring.

Are they a badge-engineered Kumho model, or Walmart-spec tires whose manufacture was contracted out to Kumho? I can't find them on Walmart's site but wouldn't mind seeing them on Kumho's...

[Lug_Nut]

Originally Posted by dkjones96

Very interesting... Makes me wonder what all is involved in that. Kinda makes me think of the mousetrap car me and another guy made in high school.

As did I, but that was (OMG!) 35 years ago! in Jr. Hi Mine was cuckoo clock geared for revolutions, but it had no torque at all. The drive wheel spun forever when lifted off the ground but it barely moved when the entry's heavy, nearly 5 lb, weight was resting on the wheel bearings. Once the mouse trap bail approached vertical the spring force was no longer enough to move the pile across the gym floor. I think I got 8 feet or so, but the memory is foggy.