I have 17inch P1s they weigh 19lbs.
I am looking at getting 18inch Rota SubZeros, they should weigh around 18 lbs
I know youUSUSLLY lose power with bigger wheels, but since these wont weigh as much, I shouldnt lose anything correct?

4 lbs is not going to make a difference (wouldn't be noticeable at least). However, the diameter change is a different issue. I'll leave that question with someone who has made a similar change.

A taller wheel/tire combo (with a greater circumference) will result in "taller" effective gearing. It is easy enough to calculate the total rolling diameter of your combo For example: a 225/40 18 is a total of 637.2 mm tall, a 225/35 19 is 640.1 mm tall. A difference that small won't kill you at all, it is less than 1% off. Now if you run some other size (I am making these up here to make a point) like a 225/45 19 the total diameter is 685.1 mm, or 7% off.

Note: Stock diameter is 631.9 mm. A 225/35 19 is 1% off.

This means your gearing is taller (as if you started out in second, instead of first, an exageration, but I'm trying to get this across), revs at a given speed on the highway are slightly lower, acceleration GENERALLY will be slower, mileage GENERALLY will be higher.

As long as the wieght is lower, and overall rolling diameter the same, you will not 'lose' performance.

I hope this helps.

That is exactly what I thought.
According to Tire rack, a 225/40/18 is the same size as a 225/45/17, so if I can manage to keep the tires the same weight, and I know the 18inch rotas are at least a lb lighter, I will actually GAIN power(ok lol not like I would feel 1lb, but knowing its there is nice)

Dont forget about torque.

Where the weight is positioned is nearly as important as how much there is.

All else being equal the 18" combo will shift the weight farther out from the center of the wheel making it harder to spin, turn and stop.


Later,
Andy