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I've been looking at wheels lately and I have finally found some I like. As the diameter increases from 17" to 18" to 19", the weight only increases a couple pounds. I was just wondering if anyone knew or could make a reasonable guess to how much your car would slow down as you increase. I know it would be some, since each time the majority of the wheel's weight(hub) would be an inch closer to the outside, but would the differences be like a second or so, or just a few tenths??? I've wondered about this a long time.

 
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Moment of inertia increases in squares. You can approximate the rim as either a thin ring I=MR^2 or as a disc I=.5mr^2
so the farther you go out, you're multiplying a larger number by a number that is going to increase and be squared.
Say you've got a 17" wheel that weighs 17 lbs, the 18 weighs 20 and the 19 is 23...
I(17)=38.14 slug-in^2
I(18)=50.31 slug-in^2
I(19)=64.64 slug-in^2
A slug is the british unit of mass... it is lb-s^2/ft or Weight/gravity
I used the ring approximation.
It slows you down by more than you think. More torque is required to start the wheel rolling, which slows you down. If you've got a physics or dynamics book handy look up equations for finding acceleration from equations involving mass moment of inertia. Assume the car puts out 50 ft-lbs of torque at the RPM that you want to launch at (its a pretty close guess). I don't have time to play with the equations right now.

 
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Thanks for the nice reply. So do you think it would slow you down like seconds or tenths of a second if I went 19" instead of 18"?

 
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Remember, there are other factors in determining acceleration when you talk about wheels/tires. Such as Side wall Height(of the tire), and The total Diamiter of the Wheel/tire combo.

Low and Ultra low profile tires are NOT good for drag racing. The help cornering, not acceleration. Drag tires have large, soft side walls that allow the tire to flex and distort for maximum accleration in a strait line. Low profile tires have Rigid side walls that reduce the amount of tire Deflection durring a corner.

the Total Wheel/tire Diamiter has a Direct Michanical effect on acceleration. If the total tire/wheel Diamiter is greater then stock it will do three things, First it will slow your acceleratin Vs. stock. Second it will Give you a Higher top speed. and third, it will give you an increse in fuel economy while on the Highway. If the Wheel/Tire diamiter is smaller then stock it does the exact opposite, 1 You will accelerate quicker Vs. stock, second, you will have a lower top speed and third you will have lower Fuel economy while on the highway.

when dealing with wheels and tires, Weight in not everything. But it seems to be the only thing people pay attention to.

 
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You're not gonna go from running 16s to running even 16.5s (I'm just making up numbers here) if you get bigger rims. If you're considering 19s though, I doubt you'll be drag racing. What boilermaker1 and Zzyzx said is true, but I really doubt you'll be able to feel much if any difference if you go to 17s. You'll probably notice something if you go from a 15" to a 19" rim, but it's seriously nothing to worry about.

My advice: go as big as you want to, but don't cut your own throat on tires. Get good sticky tires. Tires are a million times more important than what people think. If you get some insanely light forged 3 piece racing rims and then put some worthless tires on them, it'll suck. If you get ________ rims and put good tires on, it'll work out better.

 
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This post has a lot of good information in it. I really appreciate everyone taking the time to post. As far as sizes I always thought I would get 18's. The wheel I plan on getting are Centerline RPMs. The reason I started thinking about 19's are because I could get them for like not that much more than the 18's. There is no way I would run crappy tires either. I plan on getting Parada Spec 2's. Well keep the good post coming.

 
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What is your reasoning for getting 18's? Looks or performance - and why focus on wheel weight rather than design idea and/or material? There are so many factors that will adjust ET and launch velocity that just asking if wheel size and weight will noticably change performance is a bit crude of a question. If you like the centerlines and want to go for max performance with a plus size I'd say go with the 17" size - that way there is still some semblance of a sidewall and from a cost vs. weight standpoint you're getting the best of both worlds. If you are looking at the looks component of the car then what difference does it make - not to be a jerk, I love my EX - but they are a bit on the slow side in general. Now if you turbo or something radical, then ignore what I've said and buy what you want Jazzy