pcarguy911

My 997.2 C2 PDK came with optional 19" wheels having 305 30 R19 at the rear and 235 35 R19 at the front. If I get a flat tire at the rear, I am supposed to mount the front wheel at the rear. The rolling diameters (as per https://www.willtheyfit.com/) are 665.6 mm and 647.1 mm with a difference of 2.86%.

A difference of 2.86% seems a bit high. I am planning to buy a full size front wheel to be used as a spare tire with an intention to continue my journey without the limitations of space saver. My intention is to change the tire and keep driving at regular speed till I reach my destination. This difference of 2.86% is making me uncomfortable and seems my investment in a full wheel is pointless. 2.86% difference is between rear right and rear left (not between front and rear) after mounting the spare wheel at the rear.

What do you guys think?

NoTurboRequired

Never heard that. Replace the flat rear tire with a front tire? Do you have a real spare tire for the front?
I’ve never seen anything but the tire filler goop and compressor.
I’m sure it will increase tire wear, but you should use your spare very temporarily.

pcarguy911

In case the rear tire goes flat, you are not supposed to mount the space saver at the rear. It will fit but given all the weight and traction requirement at the rear, it is considered too undersized. Instead, we need to first mount the space saver at the front and then mount the front wheel at the rear.

In the case of 18" wheels (235/40 R18 and 265/40 R18), the rolling dia difference is a whopping 3.7% - won't it throw ABS/TPMS errors with this much of a difference?

I do understand the concept of "emergency limited distance at safe speed". I want to buy a full size spare wheel that will offer a more practical solution to reach my destination like the good old days - it will be an unjustified expense if, even with a full size front wheel as a spare, I am stuck with the same limitations of a space saver (limited distance at slow speed).

What % of rolling dia difference is OK between wheels of the same real axle for normal driving (say for a couple of days till I get back from a holiday)?

cgfen

you have a c2, not a c4 so ERD is not a big deal.
2.86%................................. ERD may trigger your psm warning light, but you won't damage anything and it will auto reset once you fit proper tyres.
have fun.
ps. TPMS = tyre pressure monitoring system, that does not measure ERD

Petza914

The ABS and PSM systems rely on wheel speed rotational information as well, so although you won't damage something like a transfer case, the stability management could start cutting engine power or things like that if it thinks there's slipping or skidding.

CAVU

I've put 25k miles of touring the eastern states on the 911 and no flats. So here is what I did.

I did buy a jack off ebay for a boxer, a very nice electric tire pump (goes with me in my other cars when touring), proper tools to release/tighten wheel bolts, wheel guide pin, and a tire repair kit (go heavy duty, not the cheap plastic ones), leather gloves, knee pad, and AAA card. The first time I go without it I am sure to get a flat