Space Saver Spare O.K. With Different Tires Than Original?

Hi,

Not sure I understand how a 195-60 and a 215-70 in the same wheel size can have the same diameter or rolling radius. Did you actually mean your

195-60s are on 16" wheels?

My understanding is "plus" sizing, properly done, results in a larger wheel size than stock, with a correspondingly "shorter" tire so the overall diameter and RR remain as close to identical as possible. Many manufacturers now show "revs per mile" for their different tires so you can compare. So, for example, you have a stock 15" tire that gives, say,

812 rev/mi, and you replace with a 16" that gives 810 or a 17" that gives 809 (all numbers made up for comparison only), you're about "perfect" in your "plus" sizing. This assumes, of course, you don't create other fit problems, such as going so wide on your "plus" wheels that you experience chassis interference and such. If you achieve that, your compact "donut" spare should be fine (with the usual considerations for using a "donut.")

Rick

On Fri, 18 Nov 2005 18:29:33 GMT, Jim wrote in news: snipped-for-privacy@4ax.com:

Plus 1 conversions usually refer to increasing the wheel diameter by an inch and sizing tires to be the same effective diameter.

The numbers give the effective diameter for the 215s as 26.9 inches and the 195s as 24.2 inches. That is a whopping 10% difference. The diameter of the new tires is quite a bit smaller than the previous ones. It lowers your ground clearance by 1.5 inches.

The distance travelled per tire revolution is only 90% compared to the previous tire. I would NOT use the spare.

Did you give the incorrect wheel diameter for one of the numbers?

Sorry I read the Owner's manual wrong. On my Brighton the stock tires were 185x70x14. The new ones are 195x60x15. So it appears there should be no problem if I ever need the donut for a few miles. Have not had a flat in 8 years (knock on wood).

Jim

Yea, misread the manual. The original on my Brighton was a 70X14" tire, the new is 60x15. According to the previous post, I should be ok.

Jim

Is the car an automatic or stick shift? It matters for the correct answer.

Automatic...

Most Subaru automatics are capable of 2WD (Front wheel drive) with the insertion of a fuse. Check your manual.

If your car has this capability, you have no worries using the stock spare.

Barry is correct. If you must run the space saver, or a significantly different sized tire on one corner, put it on the _back_ (I know . . . this is a pain if you have a flat on the front, nevertheless . . .), and disable the AWD. Keep speeds below 50MPH, and replace ASAP. This is acceptable for cars with LSD rears (WRX, some older Legacys, ???) as well, but realize that you are abusing the LSD by doing so. The space saver spare is only designed to get you to the nearest service station in any event; usage beyond that is contraindicated.

ByeBye! S.

Steve Jernigan KG0MB Laboratory Manager Microelectronics Research University of Colorado (719) 262-3101

I just wanted to point out that the WRX is *NOT* capable of being put into

2WD by moving the fuse. Absolutely not.

I know the auto is an option on the 'limited' WRX but i have no idea what the owner's book recommends. maybe the lsd rear diff prevents use of a FWD fuse? I dunno

Carl

Hence the "check your manual" statement.

There are at least THREE distinct Subie drive trains. Since most Subarus with automatic trannys aren't WRX's...

Wait... You have an AUTOMATIC WRX? Why? That's like an automatic TR6!

Barry

Wrong: The automatic WRX is *not* capable of being put into 2WD by moving the fuse! Only those vehicles that have the fuse mentioned explicitly in their manuals can be put into 2WD like that.

I drive a 2004 STi. I did have a 2002 WRX; however, the only reason I know for sure that the auto *can't* be put into 2wd mode is because this was already hashed out about a year or more ago in this very newsgroup, and I called up the local Subaru Master Mechanic to ask about it, whereupon he told me very specifically that the fuse slot does absolutely nothing in an auto WRX.

Nobody's debating that.

The real question is WHY buy an automatic sports car?

Oh. I dunno: maybe they drive in the city and don't want to screw around with the clutch all day long? Or--maybe they don't want to learn to drive a standard on a car like a WRX? :-)