The problem is I want other modifications on the weapons.I thought it was just (fuel) weight?
ie: Cobra MK3 with 16T tank, but with 2T of fuel in the tanks, gives the same change as just fitting a 2T tank. I just tried this with my MK3, and the speed increase seems about the same as the estimated increase of downsizing the tank (I didn't time pitch sorry).
The only real reason I thought racers downsized tanks, was because it's hard to burn fuel, they might have a poor FSD, so its just easier to start with less fuel-weight?
Ditto smaller tank means your fuel scoop wont "over fill" if you're going for speed, but essentially just scoop less, and the speed will be about the same: then you'll still have a large tank when you need it for travel.
tl;dr tank itself weighs nothing, fuel adds weight / reduces speed.
(as posted in OP's other thread) If you want more speed, try two medium lightweight MCs, 2x small efficient beams, the saving on weapons makes a difference too.
I've done this on my rat-Cobra in the past, you can still PvE while waiting for jobs, but the less weight lets you race to the clients faster