Rep Power: 350

if i have 17" tires would the spare really be worth having? id like to take it out for weight purposes.

 
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yes, if your tires blow, you wouldnt want to be driving around on bad tires. it'll damage the rim

 
Rep Power: 350

but if i was riding 17"s, with one 15" wouldnt the 15 just hang off the ground?

 
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no, although your car might be lop-sided, your suspension will adjust and all 4 wheels will touch the ground

 
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17" rims come with low profile tires, but the overall diameter of the wheel is still pretty much the same. 15" rims has 4" thick tires, 17" has 2" tires (just an example), total diameter still 19". ^_^ of coures that's not exact, and you're car will be lopsided so the steering will be quite off to one side.

 
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Your suspension will do its best to adjust, even with a little space-saver donut spare (Unfortunately I know from experience -- I have had 2 rear tires go out on this Civic)

It's not worth killing your nice new rims or being stranded in the middle of nowhere just to save the weight of your spare tire. You can take it out after you get to the dragstrip

 
Rep Power: 376

I plan on buying a spare 17in VX8 and fitting it in the spare spot just in case i get a flat tire that way im in and out in a few mins and still rolling. Since i have AEM brakes up front i cant really use the spare, it requires a least a 16

 
Rep Power: 425

Simple solution... Take a rim off the back, put the donut on, put the rim you just removed from the rear and put it on the front. Done... I just saved you $275 for an extra rim and tire.

Anyways... As long as the outside diameter of the 17" wheels + tires remains the same (i.e., you used the tire calculator and picked the right size), then it's no different than putting the spare on with stock wheels. It will look just as lopsided. The problem is if the car is dropped... cuz usually the spare drops the corner its on another 1"... so you might want to watch for that. If you're worried, get a real 15" steelie and a full size tire.... thats much cheaper than a new rim, and you'll rarely have to use it. I always leave the tire in, just in case, then I take it out at the track... where weight makes a difference.