Sure-grip & spare tire size questions

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DartGTDan

DartGTDan

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I've heard using 2 different tire sizes will damage the sure-grip unit in my 741 case 8 3/4" rear end. But, I've never heard how much tolerance, in tire diameter, there is before the damage occurs.

Per an online tire comparison calculator, my rear tires are 245/60R15 (26.6" diameter) and my spare tire is 195/75R14 (25.5" diameter). Is this 1.1" difference, in diameter, enough of a difference that it would cause damage to the sure-grip? If so, is there a "safe distance", and/or a "safe speed", the car could be driven until the flat tire could be repaired/replaced?

As always, thanks in advance!
 
No, your options are towing to tire shop for repair or replacements, have 2 thin tall spare tires to install and drive away, you can also have a tire plug kit for roadside repair with 12-volt air pump. now if your good rear tire will fit in the front without damage do that with the spare and move front to back. All options suck, pick your poison
 
One-inch diameter difference is quite a bit (multiply diameter by 3.14 to get the circumference — the difference is 3.5 inches). The problem is that it makes the two axles revolve at different speeds, and the whole function of the Sure-Grip is to prevent that from happening. So you are wearing the cones with every revolution, instead of only when cornering. So while I have never seen any data on this, I suspect there is no "safe" distance beyond "get off to the shoulder and call a tow". Speed is irrelevant — the entire effect is based on distance traveled. Rebuilding the Sure-Grip is more expensive than a tow, that's for sure.

What size are your front tires? Because if you can just shift them around to get two matching tires on the back, you could deal with mismatched tires in front. Except, oh yeah, no way a 245/60-15 is gonna fit in the front.

This is why have I matching tire sizes all around. I have the same issue with the small diameter spare, but I can always shift a front tire to the back and run the spare on the front.
 
I have been through this before. If you are worried about the difference in diameter you can remove air from the taller tire and drive far enough to get another tire with no damage. It's not like you will be driving across the state...
 
I have the same deal as you. I gave up on it and use the spare tire hole for storage and ditched the spare.
 
HemiSSDart said:
I have been through this before. If you are worried about the difference in diameter you can remove air from the taller tire and drive far enough to get another tire with no damage. It's not like you will be driving across the state...
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Oh, yeah, I have heard that recommendation before. I suppose you just let out air until the car sits level, and that should be about right. Should get you to the service station, at any rate. But don't drive fast in this situation, as a tire with low pressure will heat up rapidly, which increases the tire pressure, defeating the scheme.
 
I view this differently. I have 27” tall rear and 26” tall front tires and max tire in the well is 25.6” which I did. Why carry at all is because if it’s a front you can still use the spare so 50% chance of home bound. If it’s the rear I still need a tow yes, but I will put the spare on anyway because I want the thing to roll on the tow truck and off without dragging anything. Never not carry a spare just because of the rear tire size though, you need it in emergency only and depending on what this emergency is you may decide life is more valuable than the sure grip replacement cost.
 
Needs to be the same or as close to the diameter of the regular tires as possible.
 
I have a short spare 155x14. All my tires are the same size.

If I get a flat on the rear I swap a front to the rear and put the spare on the front. Pita but it works
 
HemiSSDart said:
I have been through this before. If you are worried about the difference in diameter you can remove air from the taller tire and drive far enough to get another tire with no damage. It's not like you will be driving across the state...
Click to expand...
I’m pretty sure that doesn’t change the revolutions per mile of the taller tire vs the shorter. This doesn’t work in my opinion.
 
4mulas said:
I’m pretty sure that doesn’t change the revolutions per mile of the taller tire vs the shorter. This doesn’t work in my opinion.
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Well I did it a couple of times back in the day, so just maybe I know. I had a 3.55 sure grip at the time and tires were never new as I couldn't afford them so I went through a few tires...
 
HemiSSDart said:
Well I did it a couple of times back in the day, so just maybe I know. I had a 3.55 sure grip at the time and tires were never new as I couldn't afford them so I went through a few tires...
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Not saying you can’t do it, you can put a different size tire on and do it. But one still has to do more revolutions per mile than the other or: the sure grip has to take up that slack and slip itself inside.

In a pinch for limited miles I’d probably do it too, vs paying for a tow
 
It'll take some research and looking, but consider two space saver spares, one at each diameter. Because they're thinner, they might both fit in the spare tire well. At least as well as a D70-14, which doesn't really fit. The space savers are cheap at junkyards because many cars had them and demand is low.
 
In all my years of driving, this kind of stuff never crossed our minds. If you had tire trouble, you put on the spare, went on your way, and fixed the tire when you had the bread. Of course we always ran used tires and looked for a replacement the same size, but we always drove on the spare until we got a replacement. Getting to work and back, or the party on Friday or Saturday was paramount, and the thought of Suregrip damage never crossed our minds. Makes sense tho, just never thought of it before, and never had any rear end trouble on anything.
 
Mike69cuda said:
I have the same deal as you. I gave up on it and use the spare tire hole for storage and ditched the spare.
Click to expand...
. This , probably jinxing myself but I haven't had a flat in 40 years. For Example the spare on my 2004 Dakota has never been used it's original tire hanging under it. That being said my Dart is a specialty car not a daily driver. As far as two different size tires you'll be fine for emergencies as said but continuing to drive will damage the rear end.
 
I'd carry a spare as close to the rear tire height as I could. If it's a little off, I'd still use it on the rear if I had to, and get the tire repaired or replaced as soon as possible, with as little driving on the miss-match as I could. BUT.....
(Axle speed differences occur CONSTANTLY when driving, and don't kill suregrips in a few miles. A little driving with miss-matched tires won't either)
 
Tire size will not effect a suregrip if you keep the rear tires spinning. So if you get a flat tire put the spare on Let-er-rip like this and don't worry about the suregrip.
 
After further investigation, and because I can't remember anything anymore, my spare tire is a 215/75R14 (26.7" diameter) which is .1" taller than the rear 245/60R15s. So I should be good.

I got my current front tires, which are the 195/75R14s (25.5" diameter), mixed up with what I thought was my spare tire size.

Homer D'oh.jpg
 
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