do 18's slow you down at all as far as acceleration goes? thanks!

 

No - 18's come with lower profile so overall wheel size is similar if not identical (did not check) - additional weight, if any is negligable...

 

Are you talking normal accleration or drag racing acceleration? If you are talking drag racing then you will want to use as small a rim height as you can, And a smaller rim will be lighter also. I am no scientist but let me try to explain it.. you whole car from engine pully to rear wheels is a spinning mass and you want to use as little effort as possible to get that mass spinning as fast as possible, things like smaller pullys, lighter flywheels and smaller diameter rims will help this mass get going faster and while it may not actually build you any horsepower it will allow what HP you have now to be utilised quicker. Think of it like this in the case of rims, which item can you spin easier and quicker, a 12 inch diameter frizbee or a 12 inch china dinner plate.

 

prior quotes are right. larger wheels increase rotational mass which ultimately decreases what you can put to the ground.

rule of thumb that I've heard is every pound of extra drivetrain mass (including rims/tires) is effectively another 6-7 pounds of non-rotating mass.

so if 18" wheels are 5 pounds heavier per wheel then that's equivalent to 20X 6 or ~ 120 pounds.

but again, that's a rule of thumb and I'm not sure if it really meshes with real world experience

 

yes, if they are heavier they will slow you down some. I notcied a big diffrence between my 18's and my stock wheels, now i notce an even bigger difference with my 19's

 

At the point that you are worrying about that though - you are likely popping the wheels off the track for slicks anyway...

Also, check the difference between Ford's 17 vs the 18 (aluminum) - I don't have the real weights on me - but as I recall it was not as much as 5LBs (anyone have those number handy?)

 

I have even larger wheels(stock) on my vette. They are much lighter than the crap they put on these cars.. Yes these wheels look good and would survive a curb crash, but I am looking for lightweight wheels for the stang now. Quicker with lighter wheels, not so much, but the steering feels better and the car feels more responsive. But you also feel potholes more. anyway just a couple of thoughts.

 

another factor that everyone is over looking is tire PSI...

speaking of which any PSI reccomendation on 18x8.5 255/45/18 and 18x9.5 275/40/18 nitto extreme ZR?

 

Technically yes. Torque is a product of mass and distance from the center. The more mass that you have further from the center, the more force is required to produce accelleration. 18 inch rims have significantly more mass on the outer edge than 17 inch wheels (3.1416 more inches times the width of the rim worth of metal).
This is why drag-racers NEVER use big wheels. They usually have pizza cutters on the front and small but wide rims on the back that allow the largest tire.
Each kilogram of unsprung, rotating mass, especially when it's further from the spindle, has MUCH more affect on your accelleration than a kilogram of mass anywhere else on your car. Lightweight pulleys, lightweight driveshafts, lightweight flywheels and clutches, lightweight brakes, and especially lightweight wheels are all designed and desired because they work. Many of the serious racing clutches/flywheels are designed to be as small in diameter as possible to avoid having the mass too far from the center.
Many track racing cars usually have 17 inch ultra-light wheels such as konig villians. They would probably go with even smaller wheels if they could fit those large brakes inside.

 

Yes, but don't forget the weight of the tires. You have to look at the total weight of both wheel and tire as a rotating mass. A smaller wheel also means larger profile tires, and vice-a-versa. so, in the end, nothing is for free unless you take both matters into account and look at the totals in relative terms.