Fuel Savers inc.com

As you may already be aware of, diesel engines are
more efficient than gasoline engines of the same
power, resulting in much lower fuel usage.  For an
efficient turbo diesel, the average is 40% more miles
per gallon.  The higher compression ratio with
diesel engines help to raise efficiency, but diesel
fuel also contains around 15% more energy per unit
volume than [...]

Cost
Due to the high compression ratios and resulting
high cylinder pressure in diesel engines, they
must be built to withstand a lot more punishment
than gas engines.  The parts that are spruced up
include a thicker block and cylinder heads,
pistons, crankshaft, and valves, which can be
very costly indeed.
When it comes to the price, gas wins this one by
far.  Diesel [...]

Diesel is often looked at as being smelly, noisy, and
many think the only place for it is in a tractor.  The
truth to diesel vehicles is that they are slow, noisy,
smelly, although they are cheaper to run than gas.
Diesel engines aren’t as powerful as gas engines, as
gas engines produce more horsepower than that of a
diesel engine.  [...]