Compression can be an issue- usually lowered when adding a blower.
Cam choice is important- info from the crate manufacturer or weiand is
best.
May need to up the spark power- higher cylinder pressure, and the
higher the pressure, the harder it is for the spark to jump the gap.

If you gear the car and the blower right, and tune the engine for it
(cam choice, etc.) you can have minimal losses for normal street
driving, as long as you keep your foot out of it.

Dave

You are putting a blower on a car and are worried about fuel economy?
C'mon...

Leave it stock if fuel economy is an issue....

Your probably going to need to change the pistons to lower the
compression. Change the cam to one ground to work with a blower and you
will want the hottest ignition you can get. You may also need to change
the crank if it is a cast crank depending on how much boost you intend
to run.

Fuel economy and blower cannot be used in the same sentence.
IF you keep your foot off the pedal and set everything up to operate
properly you will minimize the loss.