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Ideally, it should be the same size. However, there are people with two different tire heights on their cars, front and back, who keep the smaller one as a spare. Width wise, the car can't tell if the tire is narrower unless you are driving to the point of losing traction. It will eventually bother the driveline, but a size off will not hurt anything immediately. Cars were, and are, issued with donut spares from OEMs, with the understanding that you: put the spare on when you need it, go home, order a new tire or a repair, and go to the tire place, and that's it. However, there are some very dumb, very poor people rocking what we around here call Big Altima Energy, and they routinely drive on donuts. These are the same people with taped up windows, body panels from other cars that are other colors, and missing bumpers.
Best practice is to have the same sized spare as all four tires, that way you can change the tire and operate fully on the way home. The only downside is loss of trunk space, but I doubt any of us need that trunk space on a hobby car.
Big Altima Energy?
Lotsa that thing down here in Jacksonville. The police should really monitor the cars doing 75 in the left lane on donuts…
‘But they don’t. Everything is kinda laid back down here.
To add a 2X to Koda’s response…you will drive your car home, fix the proper tire and be fine.
It's a BITB, but if you do have a smaller spare, and need it. Take a front tire off and put it on the rear, and spare on the front. More work,but I would not run two different sizes in the rear.
If the posi axle has two different diameter tires on it, the posi clutches WILL wear. If you need to replace a rear tire, you COULD swap a good front tire onto the rear and put the smaller spare on the front, but do you really want to go through this trouble when it happens in the rain on the side of an interstate?
I just went through this on my 68 Cutlass. My spare rim is 7" and I wanted to keep the factory cover as the trunk looks nice. I run 255/60-15 on the posi rear and I'm not sure if that would even fit on a 7" rim. I also carry an off road scissor jack and an 18" breaker bar + socket for it. Not screwing around. Doesn't take up much room.
I used a tire size calculator: Tire Size Calculator and found that a 215/70-15 is close enough for my purposes and it easily fit on the rim. I got an Achilles Touring Sport AS All Season - whatever that is exactly - for $65 delivered on Amazon. Walmart has some pretty cheap options too, they're worth a look.
The PO never bothered with the spare - it had been flat for years and he knew it. I did this 10 years ago for a C3 and since then I've thankfully never needed it.Some guys eliminate the spare tire tub or just use it for storage but I could never mentally get over that hurdle whether it's an occasional hobby car or daily driver. I use mine as the latter.
That slight difference in size will get you home easily without damage assuming you're not driving hundreds and hundreds of miles on it.
I just went through this on my 68 Cutlass. My spare rim is 7" and I wanted to keep the factory cover as the trunk looks nice. I run 255/60-15 on the posi rear and I'm not sure if that would even fit on a 7" rim. I also carry an off road scissor jack and an 18" breaker bar + socket for it. Not screwing around. Doesn't take up much room.
I used a tire size calculator: Tire Size Calculator and found that a 215/70-15 is close enough for my purposes and it easily fit on the rim. I got an Achilles Touring Sport AS All Season - whatever that is exactly - for $65 delivered on Amazon. Walmart has some pretty cheap options too, they're worth a look.
The PO never bothered with the spare - it had been flat for years and he knew it. I did this 10 years ago for a C3 and since then I've thankfully never needed it.Some guys eliminate the spare tire tub or just use it for storage but I could never mentally get over that hurdle whether it's an occasional hobby car or daily driver. I use mine as the latter.
That slight difference in size will get you home easily without damage assuming you're not driving hundreds and hundreds of miles on it.
it’s always good to be prepared. I have a plug tire set and air pump in case of a flat but will be adding a full spare
you just never know
Very few things I find edible at Tim Hortons. Their coffee smells great but doesn't taste good. Compared to Starbucks overpriced gross ***** it is a magic elixer. Just get the same size tire. I need to take off the ancient G78-R15 that was the spare on a 76 Cutlass I bought twenty years ago. See if they will put on a 275/60R15 on a 15×7 SS3.
By all means try to run with the same size tire for a spare. Trunk space is forfeited but you want to be able to change & get on your way. Last year in my 94 Cutlass Supreme I insisted we make the 17 hour trip to the Nationals equipped with the full size spare I carry in the trunk. We were able to make due with what trunk space was left. Your donut well in the 79 will fit a 14 inch tire, I've seen this done alot, but if you are running larger tires you will also need to sacrifice trunk space. Two words for that: worth it. I've had 2 flats this year (on other vehicles), first two in over 20 years.
