Not safe or legal....

[Memorytwo]

stretch some tires :-D
youtube up "stretch tires" theres a video of some guys stretching a space saver on a wide rim. it looks alot like a really big motorcycle tire. However i do not recommend it. just get the smallest rims you can get, then get as thin as possible tires, then stretch them.

EDIT: my example is this. I drive a 94 Jeep grand cherokee. its about a 4000 pound vehicle. i bought 215s that were in the proper weight range. then i put them on my rims that are 7inches wide. 215s can stretch onto 7in wheels fine (albeit harder when you buy the tires new, you probably want to goto a drift shop.) I HAVE put older 205s on the 7in rims perfectly, but i didnt run them long enough. they were just old temporary tires. But i would also recommend getting tires that are about the same height. IE: i went from 235 75r15s to 215 75r15s, if I could find 215 80R15s i would get them. This is so you don't have to compensate the speedometer for the rpm increase etc etc.

i want to say getting thinner tires got me an extra 1-2 mpg, more on the 1 side than 2.

[theunchosen]

Originally Posted by achang1

i want to say getting thinner tires got me an extra 1-2 mpg, more on the 1 side than 2.

Ah I see what you are saying. You are talking about their horizontal surface. How does that affect the tire's lifetime?

If you drop from 215 75r15 to 215 60r13 you see much bigger gains that if you shrink the tire width. The reason for that is if you shrink total wheel-tire diameter (dropping tire profile 15 mm and wheel diamter 2 inches(at least looking at my tires r stands for diameter. . .?)) you drop a substantial amount of weight on the tire total and none of the tire is where all of your tire used to be.

On a Grand Cherokee 13s probably are not possible, but 60's instead of 75s definitely are.

Just for a feel in the change my 18 inch wheels weighed over 50 lbs each and my 13s are easily under 30 lbs. The tires themselves are about the same weight because the 18s were 40 mm and the 13s are 60 mm. The 13 is slightly lighter because its a 185 instead of 225.

Ok then we aren't really arguing about the real physics here. Unsprung weight is probably a poor term for talking about the tires because if you had something else just hanging on the car unsprung it would not heavily influence the car's mpg. adding a 120 lbs of weight at the bottom of the springs would not drop my mpg back to 25, maybe 34 ish. Adding tires that had an additional total 30lbs each would radically reduce your mpg just because of the uniform force needed to maintain uniform acceleration(25 MPG).

But also even if you can't reduce the weight of your tire but you can reduce wheel-tire diameter(on your next tire swap) you will see gains on MPG because its reducing the force needed to spin your tire(also reducing your ground clearance and making your underbelly drag slightly less).

[zjrog]

I think you would be better off using rotating mass, not unsprung weight. Unsprung weight includes axles, control arms, brakes, some steering parts and other chassis components.

[theunchosen]

There we go. Reducing your rotating mass in teh drive train radically increases mpg because you lose in the objects weight and the energy needed to keep it spinning.

This is also one of the big reasons that Motor-In-Wheel designs are heavily considered. It increases the unsprung weight but it reduces the number of components of your car that need to be kept rotating and several thousand rpms to keep the car going.

[Memorytwo]

i am afraid that if you reduce the circumference of the tire too much, you will get erroneous results.

[DifferentPointofView]

^
Thats what I was saying

[Memorytwo]

the thing to do is to get thinner width tires, but not lower sidewall.

[theunchosen]

I still disagree. . .

My dad drives the same car. He had 185 60x15s downgraded to 185 60x14s and up 2 mpg(he drives several hundred miles a week(100-120 miles per day)). I dropped from 225 40x18 to 185 60x14 and saw a jump of 10 mpg, no changes in driving route or any other vehicle aspect. Changing the tire width had an impact I agree but reducing the tire diameter meant I needed less fuel to reach an rpm because the wheel was smaller and because it is much easier to rotate a smaller lighter tire than a larger heavier one. Also the larger the rim the heavier it is going to be. dropping fro 215 to 180 you will see gains but you also see gains from 18-smaller. Another important factor is 14 inch tires are dirt cheap. I got a set of four pirelli snow tires(new) for 100 bucks, and 4 of P4 all weathers(new) for 112. After checking a couple of sites, the cheapest 18 inch tire is 90 bucks. cheapest 14 is 35.

Price gap is 55 bucks per tire. . .
you have to get more than 4 mpg gain having the 18 inch tire before you save any money. it costs 240( i assumed 2 years for tires and 15,000 miles a year and gas averaging 4 a gallon) to replace those 4 tires.

You save 55 bucks a tire, and gain the 185 width without stretching your tires, while also gaining more mpg(you end up driving slower than you think, and MUCH less rotational mass)

I save lots of gas and lots on tires.

[MetroMPG]

Originally Posted by 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.

FYI: http://ecomodder.com/forum/showthrea...onomy-189.html

[Memorytwo]

Tire Size Calculator - tire & wheel plus sizing

youre going to need to compensate the speed difference to the mileage.