The diameters of the two setups are virtually identical. The OEM 18 inch is 25.579 in, and the BBS 19 inch is 25.496 inches, a difference of less than .3 percent. I'm using 245/35 19's vs 245/40 18's in front and 275/30 19's vs 275/35 18's in the rear. So gearing is virtually identical.

What I'm finding out is wheel/tire weight has two contributions to acceleration. First, you use HP to accellerate the rotational speed of the wheel. Imagine the car is on a lift, it takes a certain amount of HP to accelerate all four wheels at a given rate even though you're doing nothing to move the car. Then you also have the actual weight of the wheels, that you carry as if they were a suitcase just riding along with the car. The two combined make wheel weight pretty important in acceleration. If you ask the folks on the racing forums, the general consensus is that 1 lb weight saved on the wheels equals 3-5 lbs saved on the car.

 

The heavier tires/rims will definitely slow down the car (even if the diameters are virtually identical) due to the greater rotational inertia. Acceleration is the rate of change of momentum (momentum is the product of mass and velocity) and the heavier tires/rims have a greater mass that needs to be rotationally accelerated. The slower acceleration is not caused solely by having a 40lb suitcase in the trunk because the rotational inertia of the extra 40lbs on the wheels/rims must be overcome to accelerate the car forward.

It is pretty simple to understand if you consider that a merry go round accelerates slower if you add extra weight to its outer edge.

 

Rotational inertia is the issue where the two wheel/tires in question have different net mass and different weight distribution radially. The wheel/tire with all the mass in the center will accelerate and brake better than the same diameter wheel/tire with the same mass but placed on the outer radius. Now, add the difference in mass and the placement of that mass, the two setups can differ considerably. The change in rotation inertia goes as the 4th power of radius. That is why, in general, larger wheels place more mass to the outer radius and raise inertia considerably even if no overall mass was changed. This applies to brake rotors too. Bigger rotors may stop better but also require more horsepower to get them up to speed.
One can calculate the difference in rotational inertia but requires a map of the wheel/tire cross section to allow an integration of mass with radius. I haven't seen anyone do that except the designers with finite element algorithms.
I wonder who reads these messages to the end?

 

I did

Can't wait to get rid of my runflats...

 

Well, I actually did all the calculations, and I won't bore you with them. Of course, I used some simplifying assumptions - wheel weight distributed 2/3 on the rim, constant acceleration over a 1/4 mile, 1/4 mile of 13.8 at 103mph.

I used actual wheel and tire weights for the OEM runflats and my BBS setup. The BBS setup is 54 lbs lighter than the OEM. According to the calculations, this equates to a 140 lb weight reduction, or if you put it another way, about about a 14 horsepower increase. I'm suprised at how much it affects the performance.

 

Wonderful wolverine! Can you give a little more details on the calculations and assumptions - especially about the wheel structure? I won't be bored... promise!

 

Did you pay attention to gas mileage?

 

Wheel weight affects acceleration not gas mileage...unless you litteraly always are accelerating and braking and never cruising.

 

Do you think wslam really meant how much gas was in the car?

 

sg350 got me right actually.
here in HK, yes you are constantly stopping and starting. cruising is a luxury i do not take for granted.