I got a call back from the local Discount Tire. One of the guys has been researching tire/wheel combos for me for use as a spare. I'm running the 19" sport package wheels.

He has found an 18" wheel (I don't know the brand - it's alloy, though, not steel) and is planning on fitting it with a Yokohama 245/45. The wheel has a +20mm offset rather than the +40mm on the OEM wheels. I don't think that will be an issue.

Here's where I need the advice. The overall height of his proposed spare solution differs from the OEM by 4/10". I am advised by others on this forum that the limited slip diff can be ruined by running rear tires of different diameters. Do you think this height difference of less than 1/2 inch is enough to matter? The spare would never be driven for longer that necessary to repair or replace the tire on the OEM rim.

I wouldn't do it. If you do and you have a rear tire blow out just take one from the front put it on the back and run the spare up front. I personally would try to match them exactly. But, then again what do I know. Discount tire wouldn't even install my new wheels on my G8. So now I get to drive an hour each way to a different Discount to see what they say. Grrrrrr...

**Edit ^ I'm too slow.

It would be fine for short distances but the other option is if you get a flat on the rear, move a front wheel to the back and put the spare on the front.

Right, dodson, I was thinking the same about putting a good front on the rear if the blowout is back there.

OTOH, Tire Rack has a 19 X 8.5 ASA wheel with the correct 40mm offset on closeout for $159.00, and a couple of the right-sized tires for under $200. (Not Potenzas, though).

So for a bit more I could have an almost exact match - I'm just not sure it's worth it for the difference in price and tire height.

what's the price difference between the two?

$315 plus tax at Discount Tire for the not-quite correct one.

I just pulled the plug on that, though, and ordered from Tire Rack. I got a single ASA JH8 19" with a Sumitomo 245/40 for $380.03, mounted, balanced, and to my door.

I think that is definitely worth it. That isn't that big of a price difference and you know it's exact.

'Course, one problem is that all these tires are directional, so the tread pattern will only be correct on one side of the car, but hell, it's a spare!

The OD (overall diameter) of the 245/45/18 & the 245/40/19 are the same. I would agree that if you stayed with the same brand (Bridgestone), this would have been even less of a concern.

Compare the 2 different sizes on Tire Rack's site. Look at the 2 sizes on the same exact model of tire. BFGoodrich KDW2 (http://www.tirerack.com/tires/Spec.jsp?make=BFGoodrich&model=g-Force+T%2FA+KDW+2&vehicleSearch=false&fromCompare1=yes&partnum=445YR8GFTAKDW2&tirePageLocQty=%26partnum%3D445YR8GFTAKDW2)

Tire Rack lists some slight differences in OD in the same brands, even on the 2 sizes that come on the G8. The bigger problem is that most "temporary" tires are a couple of inches shorter the their full size tires used on the car. If it makes you feel any better, GM makes no changes to any of the electronics between the 18" package & the 19" package on the G8. They just slap them on. Again, the reason: The sizes are so close that the computer doesn't go haywire. If Pontiac was to use a spare tire/wheel assembly on the GT, it would be a P245/45R18, just as a price savings. But, leaving a spare out saves them even more.

Here is the size chart from the Commodore:
http://www.g8board.com/forums/attachment.php?attachmentid=722&stc=1&d=1208036143

I guess the point of my soapbox rant is, no, the 4/10" difference wouldn't be the problem. My problem would have been the wheel's offset. I know that Zaphod made his decision, but this is important info. Sorry for the rant. :soapbox:

Good post, sccaGTO. Thanks.