Spare tire size - Duke?

[Timothy B.]

After much delay on my part, I am finally about to order my tires using Duke's recommendation for 215/70-15s most likely the Pirelli P4000s. Question is which tire I should keep for the spare? My existing 205/75-15s are 20 year old Michelin X radials with a overall diameter of probably around 26.9 and a revs/mile of 773. The new tires will be a revs/mile of 774. My existing spare is an old Goodyear steel radial tire in a size that you can't really find anymore at all of 195/75-15 that is narrower and a revs/mile of probably about 789. It fits my spare tub quite nicely. Problem is, that tire is probably a few years older than the others but it has never really been used and been stored in the tub. No cracks on the spare. Some small sidewall cracks on all the tires that I have been running on the road but no where near worn out.

Yes, I do have a posi rearend.

Should I keep the 195 size tire as my spare or the best one of the 205/75 tires?

Thanks!!

[Wayne K.]

Re: Spare tire size - Duke?

Tim,

If the current road tires have only minor cracks I would use one of them as your spare because it is close in revs to your new tires. If I remember it was common for those Michelins to get sidewall cracking. Wasn't a big deal, besides it will hopefully just be going for the ride.

Wayne

[Duke W.]

Re: Spare tire size - Duke?

I think it's suitable as a spare. If mounted on the rear it could cause pull and wear the Positraction, but how often will you need it, and how far will you have to drive? If you have a flat on the rear you can always put the spare on the front, and the matching front on the rear.

I've been driving since 1962, and I have NEVER had to change a tire on the road. I've picked up nails, screws, and glass that cut the tires and caused leaks, but I was always able to add air to them for a few days until it was convenient to remove the tire for repair.

The spare for my SWC in a 6.70-15 tube type Michelin X radial. One of the set I bought in 1964. No cracks or bulges and it holds air!

A tire that spends most of its life in the spare tub is protected from UV and ozone, which, over time, are the two big rubber killers.

Virtually any tire that reasonably matches the installed tires' circumference or revs per mile that fits the spare tub, holds air, and does not have any serious cracks or bulges is okay as a spare including a bias ply tire when you have radials installed. Bias plys and radials should never be mixed in normal service - especially bias plys on the rear and radials on the front, which can cause oversteer, but mixing a bias ply tire with radials in an emergency to get home is okay. Just drive conservatively and don't try to explore the limits of the handling envelope!

Duke

[Duke W.]

Re: Spare tire size - Duke? PS

The difference in revs per mile you quoted is only about one percent. Keep in mind that revs per mile will increase about two percent between new and worn to the wear bars.

Even a matched set of tires can wear unevenly in service and there is a little production variation. One thing you can check is the measured circumference with the tire at normal air pressure. A 27" tire will measure about 85" circumfermence, and if the rear tire measured circumferences are less than one percent different - a little less than an inch, I don't think there will be any issues with the Positraction, and definitely not if the circumferences are within a half inch of each other.

Dukes