Increasing weight on wheels have a higher effect on increasing weight
than, say, inside the car.  The wheels and suspension pieces are
unsprung weight.  This is much more significant than sprung weight.
Consider this:  Wear a backpack with 20 pounds in it and run.  You
won't feel a big difference.  But if you put 10-pound weights on each
foot and ran, you will be much slower.

In addition to reducing gas mileage, it makes the handling worse
(other than the added mechanical grip with wider tires), makes the
ride harsher, increases braking distance, and slows down acceleration.

Lighter or equivalent wheels are indeed possible even though the rim
size increases...if you're budget allows it.  Keep in mind some tires
are heavier than others too.  That weight is actually more critical
than the wheel weight.

Anything that makes the car heavier will reduce fuel economy.  40-60
pounds extra probably won't make a significant difference though.  Cars
are a lot heavier than ATVs to begin with, so the PERCENTAGE weight gain
will be a lot less.  Cars also have a lot more power, so the effect of
the added weight won't be as noticeable.

You might want to look for lighter rims, if you're that concerned about
it.  Also, try to get a lower-profile tire that will give the same
overall outside circumference, or expect your speedometer and odometer
to read wrong (smaller circumference will make the speedo read higher
and the odo tick over faster).  Any decent tire shop should be able to
match up the new tires properly.

Remember too that wider tires will increase your rolling resistance and
that will negatively affect fuel economy, though it will improve handling.

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    Hey.... I got a 2002 Civic LX 5sp with stock 14" steel wheels and
hubcaps. Right now I do a lot of commuting and get about 39 to 42 miles per
gallon. However don't like the looks of the cheap looking hubcaps on my
Civic and was thinking of going to the nicer looking 15" or 16" Honda wheels
or nice after market wheels.

     But, I really don't want to lose my good gas mileage that I'm getting
with my Civic.

     I did some checking and my stock steel wheels, plus tires and hubcap
weight around 31 pounds. going to a new larger wheel and tire will add
around 10 to 15 pounds per wheel and tire.
     I do a lot of atv riding and no first hand at the power you will lose
by getting a heavier bigger wheel and tire, so was wondering about the mpg
that all Honda owners here have experienced when they install new sets of
rims. Thanks for any advice, Rob.