It is actually possible to get all four tires properly into the car. I have a '94, so I have 15" wheels. I have tires that are basically stock (225/50 rear and 205/50 front). With 14" wheels and 50 series tires (stock on '91-92), they will be even smaller, so the tolerances will be looser. I've just carried my tires ~1500 miles, so it's fine for long trips (but you can't bring anybody else along in the car.)
The basic idea is to put one tire in the front trunk (leaving the spare at home) and three in the passenger seat. One of the front tires goes here since it is a much more comfortable fit than the rear tires. Next, slide the passenger seat all the way back and tilt it back as far as it will go. If there's anything you want to carry in the floor, or behind the seat, put it in now since it'll be hard to get to after the tires go in. Next, put a tarp in to cover from the dashboard down along the seat and back over the headrest. Leave plenty of overlap over the console and hanging out the door.
Now, one of the rear tires goes in (vertically) next to the console and rolled up against the dash. It won't really stay there very well, so it may need to be repositioned later. Next, take the other front wheel and put it in sideways behind the rear wheel already in the car. The outside of the wheel should be pointing back (so that the inside side of the wheel is against the rear wheel). Now, take the other rear wheel and put it in place next to the other rear wheel already in the car. Be aware that the outside of the wheel extends beyond the edge of the tire, so don't face the wheels against each other or they will scratch each other up. You may have to pull the front wheel up a little while getting the third wheel in, but it will all go. Fold up the extra tarp hanging out to protect the door and close it, careful to make sure that it isn't hitting the tires.
If you have a T-top, you can take the passenger side off to make it a little easier to push or pull the tires around, but it isn't necessary and so this packing sequence should work for non-T-tops as well. Also, be careful when closing the door to make sure that things really are in place. It *will* fit, but it may take a little bit of fine positioning to get things lined up so that everything has enough clearance. Don't just get the tires in there and try to slam the door shut. With the tires in the car, you can't see the right side mirror, but you should be able to see over the tires and out the side window, and back out the rear quarter window as well. There's probably some place that the space saver spare could be stashed (perhaps it would fit in the passenger seat floor?) if you really want to carry it as well, but I figure that if I get a flat I wouldn't really mind driving on one of the R1s for a few miles.
marcus hall '94 MR2T - Solar Yellow!