Space saver wheels
 

I've always wondered why space savers always come with warnings not to drive on them more than 50mph or further than 50 miles. I mean, what technical reason.
I never saw anyone driving around on four of them but I bet it would be good for mpg. If not handling.

I don't know this as fact but I have always heard that they are too skinny and can't take the Gs in turning at high speeds.

my metro has 155mm wide tires on it. I think the spare was 105. big difference in footprint. not a lot of rubber on the road.

Isn't it related to their size?

They are of a very different diameter than the regular tire, so having odd sizes must destroy the differential on the car.

Using smaller diameter tires means higher engine RPMs for a given speed, so why would you expect better MPGs unless you tweak the gear ratios?

in my metro my regular tires were 12" rims where the spare was a 13". I think the spare was actually larger in overall diameter.

the spare is smaller on larger vehicles and this may just be something weird that chevy did on the metro.

Outside diameter of the tire is often different, which is definitely bad for the differential.

Really, though, the main reason it says not to drive them far is that they are not designed for general purpose driving. They are narrow, low profile, high pressure, low capacity, with tread that is not designed for great traction or long life. They were designed to be inexpensive, lightweight, and compact; and to get those qualities, they compromised other abilities.

I'd like to use a couple for my boat trailer, which was homemade (by someone else) and carries very little weight on currently huge tires, too high for easy loading of the boat. I can't seem to figure out my bolt circle correctly; I though I measured and calculated it perfectly but the wheel I bought didn't fit right. :(

I would think that you could get some pretty small trailer tires that would fit it. you did say it was home-made which may throw a monkey wrence in my idea.

check out sumittracing.com or jcwhitney.com that is where I get most of my car junk

There's a bit of a minefield in bolt circles in that there's 3 sizes that are very close, the old imperial 5x4.5 inch, then there's also 114.5mm and 115m, and then it depends how they are meant to fit on. There's hubcentric wheels which fit close to the hub and the stud holes tend to be a little oversize and you can use flat lugnuts, then there's the stud centric ones where there's lots of clearance around the hub, but the stud holes fit closer and are dished and you need conical bottomed lug nuts to center them properly.

I'm unaware of any distance limitation on these temporary tires. There is a speed limitation to limit the possibilities of weird handling characteristics and grip during turns creating accidents, but I've never seen total distance limits.
Oh, there is the expected 'replace as soon as possible with a real tire' advice, but the temporary spare can be thrown into the trunk for the next use.
These aren't good mpg tires. I did some roll-down distances tests with a set of four while preparing for the 2005 Tour de Sol event. Four 185-60-14 Michelins at their max of 32 psi coasted further from a standstill on a slope than did 4 T-105-70-15 Continentals at 60 psi. Needless to say I kept one for a spare and left the other temp tires home.

I have heard that the "doughnut" tires as I have heard them referred to should only be traveled on 50 miles at no more than 50 MPH. this may be an urban myth or just something that has been passed down and been added to as it has been passed down.

one idea (just an idea) maybe it is a limited trip amount. maybe it can't take the heat of traveling that fast for that long. I do agree that it is strange to have a range limit on a tire and I wouldn't throw the tire away after only 50 miles. it makes more sense if it were limited in a single trip how far it can go.

at the same time, I can't recall ever seeing it on the tire anywhere where it said the range limit so maybe it is just urban myth. it does say max speed and temporary tire.

crap...now I gotta check my spare.

Er...pardon the threadjacking...

It's a pretty tough situation. The axle is a front axle from a RWD vehicle of some sort, but I can't identify it.

I thought it was 5x4.5, but I got a compact spare from a vehicle with that size at a junkyard (for a highway robbery price of $60, because I lost a wheel on the highway 100 miles from home and they were about to close) and it didn't fit right. I had already done the bolt circle measurements/research beforehand or else I would have had to try to bring the other wheel and Cinderella it.

I got lucky with the hub, anyway; the wheel fit that properly, IIRC.

Here it is with the wobbly wheel:
https://lh5.ggpht.com/_oNsRR_T1Qx0/R-...8/IMG_0516.JPG

Here's a pic of the axle:
https://lh3.ggpht.com/_oNsRR_T1Qx0/R-...8/IMG_0518.JPG

And here's where insult was added to injury. No, wait, injury was added to injury...
https://lh3.ggpht.com/_oNsRR_T1Qx0/R-...8/IMG_0511.JPG

I was driving home from a "Man's weekend" getaway when the wheel fell off on the highway. I drove up and down the shoulders for a half hour trying to find it and then gave up. I found a junkyard that was open and got screwed buying that compact spare, then bought some bolts and lug nuts. I got down to it and the nuts didn't fit the bolts, and two trips later I finally had what I needed. I mounted it and the bolts didn't line up even though I had brought a piece of cardboard on which I drew a template and compared to the wheel...

I got it home driving backroads slowly with the wheel all wobbly at like 1:00 am. I parked on the back lawn and left it to deal with the next day. So, I woke up and went out to park it properly, and as I was backing up I jacknifed and broke the tongue too. :(