Oversized spare tire stowage on C3

[Bill E.]

I replaced the original spare on my '74 with one matching the four BFG 255/60SR15's on the car but found that the new spare was too big to allow the carrier to close. I replaced the front carrier bolts with standard bolts about an inch longer and was able to secure the spare. I am not happy with this since the stock front bolts are designed to fit the strap assembly and the rear bolt does not seat against the bracket. I don't think the spare is going to fall out but it just isn't right. I checked the archives and found reference to similar problems and a recommendation to using front and rear carrier bolts for red line tires. I don't know what this means. Any help or other ideas? Thanks, Bill

[KEN BUTCHER]

Re: Oversized spare tire stowage on C3

Hi Bill,I don't know what to say,hopefully without hurting your feelings, but the original tires were GR 70-15 which in metric 215-70-15R and also to keep it stock looking 225-70-15R which would fit in the tire carrier with no problem. So if you use the 225-70-15R as a spare and need to use it, the tire height should be close to exact with your 255-60-15R.

It may not look as good driving down the road but it's a spare.
Ken.

[Joe L.]

Re: Oversized spare tire stowage on C3

Bill-----

Some 1978-82 Corvettes were originally equipped with the 255-60-15 tire size and those cars used the same rear tire carrier tub as used for 1974. The rear bolts used for ALL 68-82 applications were the same, although they are adjusted a bit differently. The front bolt varied by model year ranges. For 78-82 the bolt was longer and was of GM #471903. This bolt is now GM-discontinued, but available in reproduction.

One might think, then, that if one were to use the 1978-82 front bolt as described above and re-adjust the rear bolts, then one could use the 255-60-15 tire in the spare tire tub. Unfortunately, that isn't the way it is. You see, in 1978-82, Corvettes used a "space-saver" spare tire. With the 255-60-15 road tires, the spare tire carrier would accomodate the road tires only in the DEFLATED condition. In other words, with the 255-60-15 road tires, the carrier would accomdate a road tire only on a "one-way-trip" (after deflation caused by puncture or other tire failure).

The spare tire tub and carrier system was originally designed for F-70-15 tires. The tub, itself, was never changed in configuration over the 68-82 period, although the "V-strap" support did receive a few very minor changes over the years but none which significantly affect the spare tire size stowage capability. The original spare tire tub will accomodate GR-70-15 tires, especially if front bolt GM #345476 is used. It will also usually accomodate the 225-70-15 tire size if the GM #471903 front bolt is used, although some deflation of the tire may be necessary. However, as I say, the 255-60-15 tire size is just too big to be satisfactorily accomodated by the tub.

No other tub to fit a larger tire size was ever available from GM in SERVICE as an accessory item and, as far as I know, there is not available in the aftermarket any over-size tub for 68-82 models.

If you use 255-60-15 road tires, your best option is to use a 215-70-15 or 225-70-15 spare tire. That tire will serve ok as a spare and, certainly, better than using a "space saver" spare.

[Bill E.]

Re: Oversized spare tire stowage on C3

Ken, no hurt feelings here. The 255-60-15's where installed by the previous owner and have too much tread left to toss. I have displayed the car in the Sportman Class but have not had it NCRS judged. It's a driver and shows well in local shows where originality isn't a factor. Bill

[Bill E.]

Re: Oversized spare tire stowage on C3

Hello Joe, I was able to get the 255-60-15 secured in the carrier by using longer front bolts. As you know, the stock bolts have a special head to accommodate the tub strap. The hex bolts I used bind on the tub.

I could lengthen the stock bolts but was hoping that a longer one might be available. You mentioned two front bolts, #345476 and #471903, do you know the length of these bolts. As always, thanks for your help. Bill

[Joe L.]

Re: Oversized spare tire stowage on C3

Bill-----

I may have confused things a bit here. By "front" bolt I was referring to the front of the carrier as it's viewed from the rear of the car. Let's just call this the lock bolt, since the spare tire lock is attached to it. That's the bolt of the part numbers I mentioned.

The bolts I referred to as "rear" are the ones to the rear as viwed from the rear of the car. These are the special bolts of which TWO are used per car. The ones that you have are the only ones ever used for all 1968-82 Corvettes. No longer or shorter ones were ever available from GM. These bolts can be slightly adjusted by adjustment of the stop nut found on the threaded portion of the bolts. The use of a thinner stop nut will allow just a slightly longer adjustment, too. However, the threads on these bolts are not very long, so you can't increase their effective length by very much.

Frankly, I don't recommend trying to "reconfigure" the spare tire carrier to accomodate larger tires. In doing so, I think that you'll create other problems that will eventually "surface". I would use a 215-70-15 spare tire (the size that will comfortably fit in the spare tire carrier). The 215-70-15 is almost identical in OD to the 255-60-15 so, while narrower, it will work perfectly on the road in combination with the 255-60-15s. I mean, how often are you going to use a spare tire, anyway?

Also, I'd use the GM #471903 lock bolt so that you can stow the DEFLATED 255 road tire if you have a flat.

[Bill E.]

Re: Oversized spare tire stowage on C3

Joe, O.K. now we have our front and rear straitened out.

My old spare was a GR70-15 which fit the tub perfectly. I thought that since the GR70-15 was replaced by a 225-70-15 that I would have no problem with a 255-60-15, obviously I was wrong. Taking your suggestion to use a 215-70-15 sounds like the way to go. The 1.57" difference in width between a 215-70-15 and 255-60-15 will solve the problem. Hopefully my friendly tire dealer will let me exchange the tire. Thanks, Bill

[Joe L.]

Re: Oversized spare tire stowage on C3

Bill----

The 225-70-15 might fit ok in the spare tire tub. That size is "touch-and-go" as far as fitment is conncerned. It may fit with some deflation (lower than usual but still good enough to get you to someplace with a compressor). No 225-70-15 tire was ever originally installed in a Corvette spare tire carrier. By the time that this size became the standard size for Corvettes (1978), the "space-saver" tire came into use.

The 215-70-15 should fit with no problems, at all. Like I say, it's also very similar in OD to the 255-60-15. So, what you end up with is a "space-saver" spare that can be driven on at normal freeway speed for an indefinite period of time and at any position on the car. No real "space saver" tire will offer you even ONE of those characteristics and capabilities. With this sort of tire, you can complete a road trip using the spare. Or, you can have the road tire repaired or replaced along the way, AT YOUR CONVENIENCE AND PRICE.

Also, the GR-70-15 will fit in the spare tire carrier with no problem, at all. That was the size used on Corvettes from 1973-1977. Of course, the GR-70-15 is an obsolete size these days.