Check out this link for a axle ratio and tire size calculator

http://www.snoman.com/HTML/axlecalc_5a.html

You don't say what you are working with but, changing to a
taller differential does not always improve fuel economy.
If you take the engine below an efficient operating speed
such that it has to work harder, you may use more fuel with
a 3.73 gear.  I have an F150 w/5.0 and 3.55 that gets both
better mileage and performance that it's near identically
equipped predecessor that had a 3.08.  Several years ago, we
had a 'yota 4x4 that was in the 11mpg range with 31x10.50's
and a corrected speedometer.  We change it to 4.56 gears and
corrected the speedometer because of the performance.  The
fuel mileage was immediately back the the 16-17 mpg range.
The idea is to keep the engine in an efficient operating
range for you driving needs.   IOW, if you are not using it
for much hauling or towing, you may realize an improvement.
Mathematically, you should see about a 9% improvement in
fuel economy.  If you vehicle is now getting 16 mpg, that
would translate to about 17.4.  With gas at about $2.25/gal,
you would save about $0.20/gal. after the change.  Your
caost to swap a gear if you cannot do it yourself will be at
least $400.  which means you will reach the financial
breakeven point at 2000 gal gasoline or about 34,800 miles
at the new improved MPG - assumming you actually get the
mathematical projections.  All that time, you truck will not
be performaing as it does now.  I hardy see how it could be
worth the effort in the long run.

You numbers may vary greatly since I have based my figures
on what I would expect to get with an F150 4x2 in pretty
much standard config.  You may want run some number that
more closely relate to your vehicle.

Lugnut