New tires (and rims?) for a 1994 Geo Metro XFI
 

I just purchased a 94 Geo Metro XFI and it needs new tires. It currently has 12 inch rims on it and 155R12 tires.

What would be the most fuel efficient option for me to get (and where to get it) that would not cost an arm and a leg more than the simple option of just grabbing the cheapest new tires I can find?

You will probably have to go to 13" tires from a later metro. 12's are getting really hard to find. 155/80-13 is your best bet but is actually about 8% taller than a 155R12 so you may have issues with rubbing, power, or mpg.

I found some new 12 inchers at a few local tire places. I was just asking to see if maybe moving to a 13" or something would be better or worse as far as fuel use. Also, if it matters to consider trying to find low rolling resistance on such a small tire or not. Or maybe if there are extra-lightweight rims available somewhere... I have not done any "ecomodding" before. But it sounds like fun and worth it, so I thought I'd tap into the collective wisdom.

I found LRR tires to definitely give better mpg. Whether it's worth the cost for your particular car and tire size, try do some math. Estimate a couple mpg increase and see what you come up with.

I can't say how much mpg increase you'd get but I know it makes a difference.

As for whether tires would rub or not -
Bigger rims don't determine overall tire dimensions. That is determined by tire size.

The deal is to get the same size outer circumference as what your car had for stock. You need a tire size calculator for that, if you are changing tire or rim size. Or a decent tire place like tirerack.com will tell you what sizes will fit. As for the width of the tire, I think tirerack will tell you that too. And/or you can try find out the biggest "performance" tire size that GM ever sold for the car. That would be your max width.

Also you can put your hand inside the wheelwells and feel around to find out how much extra clearance there is. Remember to account for wheels going up + down, and front ones turning to steer.

I'm not suggesting you put on tires of the max possible width - because skinny tires are usually better for mpg. But you might not find skinny ones available and might have to get something a bit wider.

If you can get tires rated for 51 psi pressure instead of being limited to 44 psi, go for it. Once you get a taste for highly pressurized tires you won't want to be below 45 psi again.

If you have any Neighber Hood Electric Vehicle dealers in your area give them a call and see what they have for tires, they are almost always low rolling resistance and come in some of the smaller 12" and 13" sizes.
You have limited options when it comes to rims as 99% of the metros came with pretty light weight narrow steel rims and a few came with fancy alloy rims, but the steel rims with moon disk hub cabs tend to perform pretty well.

15" tires Geo Metro