My question is: if it takes 15 Hp to move 100 lbs. in a car then how much can I unleash by reducing 90 lbs. of unsprung weight from the wheels and tires?

 

wheels and tires are not just unsprung weight...they are rotational mass too ..so it's even better!!!

 

Better as far as [size=2]acceleration and handling[/size], but it doesn't change the HP, BHP, WHP, SAE or whatever we are using these days

 

Sprung vs unsprung mass has no effect on acceleration. Rotating vs non-rotating mass does.

There's no accurate answer to your question though - especially as you have it worded.

 

Another perspective on this. Some folks install a lighter flywheel so the engine will be more responsive to throttle. Taking off 15 lbs per wheel/tire per corner is equivalent to going to a 60-lb lighter flywheel. Note that the mass distribution of wheel/tire combo have very high concentration at the circumference. This make a very efficient flywheel in storing kinetic energy. Note that the analogy of the engine flywheel have to keep in perspective as the transmission and differential gear ratio changes the engine torque multiplier.

BTW. I hauled down my 14" Miata wheel/tires from my garage attic and put the 16" MINI ones with RF up there. I was so shocked that the latter weight felt weighting twice as much.

Performance minded Miata owners figured out long time ago wheels and tires too big and too wide are all show no go. There is an optimum size combo for the given vehicle hp, weight, chassis dynamics, and the intended goal. Please just don't ask me which it is.

 

Fantastic disclaimer! I love it! I feel like saying things like that a lot too!