Wheel Question

[gpsiya]

I have 17" wheels on my 964 but would like to know if one of the tires should blowout can I still mount my 16" spare and drive to the nearest repair station without harming the car in any way?

[Noah930]

As long as the overall circumference/diameter of the wheels are about the same, I don't see why not.

[gpsiya]

Meaning let some air out of the remaining tires to keep the circum/diam simular?

[dentist90]

He means that a 17" tire can have the same diameter (hence rolling circumference) as a 16" tire if it has a lower profile. eg., a 225/40 R18 has the same circumference as a 215/50 R17 and a 215/55 R16. It's called 'plus sizing'.
You just need to compare the size of your 17" tire to the size of your spare (or the original factory tire size). There are charts available to find out if the spare is close enough in size.

On a related note, the 205/55-16 on the front of my car is the same diameter as the 225/50-16 on the rear. If it still doesn't make sense to you, look up tire aspect ratios. That will explain the concept better.

[gpsiya]

Stock Tire - 205/55R16 >Search Tires Tire 1 - 225/45R17 >Search Tires
Section Width: 8.07 in 205 mm
Section Width: 8.85 in 225 mm
Rim Diameter: 16 in 406.4 mm
Rim Diameter: 17 in 431.8 mm
Rim Width Range: 5.5 - 7.5 in
Rim Width Range: 7 - 8.5 in
Overall Diameter: 24.87 in 631.69 mm
Overall Diameter: 24.97 in 634.23 mm
Sidewall Height: 4.43 in 112.52 mm
Sidewall Height: 3.98 in 101.09 mm
Radius: 12.43 in 315.72 mm
Radius: 12.48 in 316.99 mm
Circumference: 78.13 in 1984.5 mm
Circumference: 78.44 in 1992.3 mm
Revs per Mile: 836.3
Revs per Mile: 832.9
Actual Speed: 60 mph 100 km/h
Speedometer1: 59.7 mph 99.5 km/h
Speedometer Difference: - Speedometer Difference: 0.408% too slow
Diameter Difference: - Diameter Difference: 0.41%

or on the rear

Stock Tire - 225/50R16 >Search Tires Tire 1 - 255/40R17 >Search Tires
Section Width: 8.85 in 225 mm
Section Width: 10.03 in 255 mm
Rim Diameter: 16 in 406.4 mm
Rim Diameter: 17 in 431.8 mm
Rim Width Range: 6 - 8.5 in
Rim Width Range: 8.5 - 10 in
Overall Diameter: 24.85 in 631.19 mm
Overall Diameter: 25.03 in 635.76 mm
Sidewall Height: 4.42 in 112.26 mm
Sidewall Height: 4.01 in 101.85 mm
Radius: 12.42 in 315.46 mm
Radius: 12.51 in 317.75 mm
Circumference: 78.06 in 1982.7 mm
Circumference: 78.63 in 1997.2 mm
Revs per Mile: 837.0
Revs per Mile: 831.0
Actual Speed: 60 mph 100 km/h
Speedometer1: 59.5 mph 99.2 km/h
Speedometer Difference: - Speedometer Difference: 0.722% too slow
Diameter Difference: - Diameter Difference: 0.72%

The diameter and circumference seem to be within allowances on the original wheels so I think things will be OK! Thanks!

[jurhip]

A mismatch in the front will just cause handling/braking issues - so its a SAFETY concern.

A mismatch in the back will trick your diff into thinking its always cornering. Probably won't hurt it short term, but I wouldn't drive too long or fast this way. Heat build-up is the main concern, not gear wear.
Also this will adversely affect handling/braking and the car may want to "steer" during acceleration - so its also a SAFETY issue, but at slow speeds probably manageable.

A thin spare is always going to cause some dynamic issues - so its not like you avoid all problem by matching OD's.

BTW - If you have a C4 - then I wouldn't do it at all. Also, I wouldn't recommend it if you have a limited slip diff.

[KTL]

Here's a tidbit of info. for thought:

1. The compact spare is temporary in nature so they intended it to work on the car for a short period of time w/out damaging the car.

2. 964 cars were indeed available with both 16 and 17 in. wheels, so the compact spare works for both of them.

3. Taking it a step further, the 993 was available with 16, 17 and 18 in. wheels- ALL 993 variants used the same compact spare.

Bottom line is it's not going to destroy your car to use the spare in the event you really do get a flat. My Mazda 6 gets all ABS happy when I mount the compact spare. That's the only issue that's really of any concern when driving safely in a spare-equipped 964

Heck, a friend whacked his car at a DE event last year and had to use the compact spare. We put it on with some spacers because the strut was so messed up that even a good wheel/tire wouldn't clear the bent strut. He drove home that way (very carefully) and didn't die. Granted this was on the front of a '86 Carrera, not a 964. But it points to the fact that even a debilitated car can manage on the dreaded compact spare somewhat safely. As when driving with ANY car on a compact spare, you have to be careful.

[jurhip]

BTW - If you have a C4 - then I wouldn't do it at all. Also, I wouldn't recommend it if you have a limited slip diff.

To clarify - I meant the above to be the real deal breaker (at least for me - although I would likely try it if I only had a 5 or so miles to drive just to save the cost and worry of a tow).
The safety concerns are minor and things really just to keep in the back of your head when driving. Wouldn't stop me from using a spare.