How much is too much or too little for donut spare?

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Bill's 64 Signet

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Hey Folks,
In my 64 Valiant, with a 8 1/4 suregrip, I want to put a donut in my trunk and I have found a size that's really close to the diameter I want to run at all 4 corners. After reading a bunch of info on the site, it left me with an unanswered question. How much of a diameter difference can I run with the space saver on the rear before the suregrip doesn't like it.

Would 1 inch or less between the diameter of the donut and the opposite side be OK in the rear or does it need to be tighter? Finding a donut that will be close to the same height that I can use either front or rear can be done but I want to make sure from the folks that know before I buy it.

What I want to run in the front and back as follows:

Front 215/70-14
Rear 235/60/15

According to : Tire Size Calculator
The difference is 0.2" diameter (rear's a little taller) between the front and the back.

The donut I found at the jukyard (and hid away to find it again) is 155/70-17 with a 5x4.5 lbp and the tire calculator is showing that it is about 17 revolutions per mile difference (donut smaller by 0.6"). I don't want to drive far if it's a rear tire that goes bad but at those rev's/mile am I going to have problems.

Thanks in advance

Bill
 
Remember that the temporary tire is designed to be just that...temporary. Generally only to be used for no more than 50 miles at a time. They are really designed for low speed use as they build a lot of heat. Unless your LSD is Really tight it is designed to slip, especially when turning. So no problems if you don;'t try to drive on it for an excessive distant and/or live in the mountains with lots of curves. Remember also to keep the spare filled to max pressure. Nitrogen is ideal and cheap if it is available in your area.. All tires would benefit from using it. Does not expand or contract much when heated or cooled. The molecules are larger resulting in less pressure loss through rubber over time. Has been used for decades in aircraft tires because of these properties.
 
I've got a space saver spare for my car that is .25 smaller than my rear tires. I haven't used it yet but in the event of a flat I will use it to get my car to a place I can repair my flat. I'm planning on driving as short of distance as possible and at slow speeds. The rear axle will think I'm taking a really. Long curve:)
 
Your rear tires are 26.1 inches in diameter. I would look for a 14 or 15 inch narrow
wheel (4-5inches) and mount with a 75 or 80 series tire to come close to 26.1 in diameter. Problem the its really not a space saver and may not fit the trunk recess.
 
Air-cooled VW bug tire (165 80 15) calculates out to 25.4" diameter
 
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