Not safe or legal....

[JQmile]

Not that this would be safe or legal, but has anyone tried running four space saver spares? I'd think a 155-series tire or whatever they are combined with 60+psi ought to show a significant increase in mileage.

[SVOboy]

Space saver tires? Some people have tried running donuts, but I just don't think it's worth it. Get appropriate LRRs and keep caution to the max psi rating and don't push the envelope.

[theunchosen]

lol, yeah I'm pitching my vote with SVOboy. Get the smallest diameter tires that are made for your vehicle(I think I can fit 12's but 13 is as far down as I want to go) and put the pressure up, but inside of its threshold.

In otherwords, buy tires but carefully read all the information about when the tires failed or when they are not responsible for your actions or tire performance. Outside those numbers is somewhat safe to say there is risk

[lunarhighway]

they would save gass, as they'd have less frontal area, and perhaps less rolling resistance, and they're rated for low speeds only! BUT
these tires are not intended to be ran more than once at low speed, so likely they'll wear very fast,

also since theyre only intended to replace one wheel, they likely rely on the remaining 3 good tires for a minimum of traction and handling.

so a test with these little tires would likely prove that their small frontal area and perhaps lower rolling resistance would be a big advantage, but they where unsuited for higher speeds and wouldn't last very long, give the car an indeed very dangerous handling and have a much longer brakeing distance.

if you take that space saver and try to alter it's negatives, giving it more traction, durability etc, you"d likely end up with a normal slightly skinny tire

so while it would be a theoreticaly interesting test, i doubt it would have any practical use, unless your a tire developer...

so as a test this would be interesting. but i dont think anyone in its right mind should considder these space savers as a valid fe friendly alternative for real tires

[DifferentPointofView]

Get the smallest diameter tires that are made for your vehicle(I think I can fit 12's but 13 is as far down as I want to go)

Do you mean rims?

Also, smaller diameter tires may equal less FA, but your speedometer and odometer will be off, and your tires will have to spin more to go the same distance as the stocks, therefore RPM's are higher and increased gas usage occurs.

I'd just go with narrower LRR tires. no messing with ODO or Speedo, same RPM's, less FA, less RR, and higher PSI! only problem is less traction and gripping when needed.

[theunchosen]

Yeah, sorry.

^_^ yes my Speedo is actually substantially off in two ways. . .

The person before me had massive rims(18s) which biased the system slightly low. Once I removed that set and replaced with 13s gas mileage went up drastically because the uniform motion sucks up massive amounts of energy on bigger tires.

The difference in RPM is kinda small. at 1:1 engine wheel spin you lose 6-7(104-97) mph at 2500 rpm, while the MPG change was from 25-33.

[DifferentPointofView]

yea, unsprung weight deals a toll on the car, much moreso than the spin of the wheels.

Are your tires the same diameter?

[theunchosen]

Originally Posted by DifferentPointofView

yea, unsprung weight deals a toll on the car, much moreso than the spin of the wheels.

Are your tires the same diameter?

?

My tires are. . .old on gotta think. I wanna say 60 MM off the tire for the profile. Its the standard profile size for a 13 inch tire in all aspects.

The 18 inch tires were definitely heavier and taller. The Del Sol sits much lower now because the tires are thicker(taller) but the wheel is much smaller(5 inches in diameter smaller). I think the wheels are 60-65 mm which is less than 3 inches. so its an overall decrease of at least 2 inches wheel diameter and a slight change in weight.

I'm not really sure how the unsprung weight influences the MPG?

I know that not having more air cushion in your tires gives you a really rough ride that can't possibly be good for the car, but I am not sure whow it would affect FE.

It's late and my brain is not functioning. I'm not saying you're wrong, just I can't put it together right now lol.

The uniform motion does create a heavy draw on powerthough because its the moment of intertia for a donut. Spin your car wheel and then spin a bicycle wheel. Your car probably has less friction and should spin faster but doesn't because the inertia of the tire resists the moment applied.

[DifferentPointofView]

Unsprung weight, or weight not held up by the vehicle, (tires, and specifically, the rims) effects it because its like trying to spin the bike tire vs. the car tire. in order for you to spin your cars tire the same speed as your bike tire from them in a stopped position, it takes a lot more force to spin the car rim than the bike rim, even though the car's bearings might have less friction.

Larger rims heavy rims are harder to get spinning than smaller lighterweight rims, so more unsprung weight takes a toll on your engine when you are accelerating to speed. Thus you use more gas to get the larger heavy tire spinning.

[theunchosen]

I think there is a mosconception about unsprung weight.

Unsprung weight typically causes issues with wear and tear because any impact the tire takes is translated to those unsprung components as well.

You also see a large reduction in MPG if you reduce the unsprung weight. However, in most cars the only real unsprung pieces that change weight are the tires. That said If you ripped everything off thats not sprung you only get the efficiency of less weight. If you shrink the tires you gain in two directions. You gain because you are not having to move that weight forward and also you gain because you are not having to overcome as much force to make the tire rotate at speed.

More mass further from center means more energy required to make it move. Tire wind resistance is small compared to the amount of energy it takes to overcome uniform resistance. The center of your wheel most likely only makes up 1/5 of total wheel weight where the rotation is free and 4/5 are allocated along the edges where its most expensive. The tire is entirely on the expensive side.

The further from the point of force the mass is the harder it is to turn it. Push a door open from the hinge instead of the handle. It's like that for your tire. If you can make the door narrower(less distance from hinge to handle) it becomes easier, or if you reduce the mass of the door. It's also the reason racing components are very lightweight especially any component that spins along the drivetrain(clutch).

You could replace standard wheels and tires for much lighter models of equivalent size and that would allow for no alterations to speedo or rpm/speed ratios and that would increase MPG as well.

If you really want to go big I'd recommend small diameter tires and wheels(standard profiles for wheels mud tires drastically reduce FE because of rolling resistance and extra energy needed to rotate them.)