bigger wheels/tires and loss of power
 

How much power do you lose by going to larger diameter wheels/tires? Will re-gearing get that lost power back?

Thanks!
Bill

depends on hp and rpm vs gearing. Taller rear tires will make you're gearing steeper. It's basically all percentages, so it depends on a lot of variables.
Yes and no. Re-gearing to the proper gears will put the motor back into it's previous power band. Different tires can also change the aerodynamics of a car, thus changing drag.

In addition to this, adding larger wheels/tires has the potential to increase drivetrain loss by adding rotational mass. So, in theory, even if you compensate increased tire diameter with gearing, you will still experience an increase in drivetrain loss if the wheel/tire combo is heavier.

I was looking at 18's versus 17's, is any of this really an issue with rims under the 18 inch size??

Also I know f-bodys are not caddys but is there a noticable roughness in the ride by switching out to 17's or 18's?? Like to keep what little ride quality is left in my car!

a 15" wheel can easily be overly heavy and cause a loss of power. - Also, you can find 17's & 18's that are lighter than factory wheel/tire combos.
- My 17x11 rear and the 315/35 weighs less than my factory 16x8 GTA wheel and 255/50 tire.

You can have a 15" or a 20" rim and still keep the same tire height. The height of the sidewall just changes.

As for tire height, the height of the tire depends a lot on how the car is set up and what it's intended for. Factory height is about 26" tall and a factory engine barely gets over 5500 rpm.

When set up for dragstrip racing, everything should be set up to cross the finish line at or just before the engine's shift point. If you shift at 6500 rpm and cross the finish line at 5000, you need deeper gears. If you put deeper gears on and at the same time put taller tires on and still don't cross at 6500 then you need even deeper gears. It's not uncommon for a SBC shifting at 7500+ rpm to have 30" tall tires and 5.13 or deeper gears. The key for dragstrip setup is to stuff the tallest tire possible under the fenders then adjust the gearing to best match the engine's performance level.

Now if you want highway speed and are willing to sacrifice accelleration performance, you put 30" tall tires on and some 2.73 gear in the diff. It will be a slug off the line but should do 150+ mph easily on the highway.

You can fake a deeper gear swap by installing shorter tires. Got some 2.73 gears and 26" tires? Put some 24" tires on and it will be like having 3.43 or so gears. The downside it that the shorter tire has a much smaller contact patch on the ground and won't get the same traction.

Technically, you can stuff a tall tire under the fenders for a bigger contact patch and change the gears to keep the overall performance the same a stock. Look in magazines at prostreet cars with 33" tall tires and 3.73 gears. This would be a highway cruiser.

On the other hand, every single top fuel dragster has 3.20 gears in the diff. With 8000 hp, they don't need to play with gear ratios.