Kenbob speak truth.
However, I would like to point out another technology with gas engines. I will soon be taking delivery of a Passat wagon with the 2.0T FSI engine. The FSI stands for "fuel stratified injection". The beauty of this technology is that a gasser behaves more like a diesel; no longer is a stoichiometric ratio required for combustion. The intake valve ingests pure air like a diesel and the injector injects fuel as required to get the job done, as in a diesel. Compression ratio is less though, and spark ignition is still used.
Of course gasoline needs a stoichiometric ratio to explode when ignited. However the FSI engine works by having an overall air/fuel ratio that is non-stoichiometric (lean), but having stratified areas in the cylinder where there are localized stoichiometric ratios, so it's a "lean-burn" engine. When full power is required, the engine runs with a stoichiometric ratio, but in highway cruise, in max. lean burn mode for excellent efficiency.
Apparently sulphur content in our gasoline is an issue and does not allow full FSI benefits, but when this is addressed (where have we heard this before!) the ECU can be remapped to run in lean-burn mode. Already the 2.0T turbo gets fairly decent highway economy: in the Passat, about 6.7 L/100 km on the highway; I've seen numbers from France that suggest 6.3 in the lighter Jetta (rated a bit higher in N. America).
To put it into context, the closest TDI match in performance in N. America would be the B5.5 Passat (134 hp/247 lb-ft vs 200 hp and 207 lb-ft), rated at 5.7 on the highway. If you think about it, given that diesel contains about 10% more energy than gasoline, the gas engine should have a rating, all other things being equal (weight, etc) of about 6.3 L/100 km. However even FSI is a little less efficient, at 6.7.
So good efficient gas engine design is getting close to diesel in efficiency. Still a few drawbacks though: need premium, and you're still carrying around 70 liters or so of an explosive liquid in your back end, and of course gas is less efficient to refine than diesel.
Still, one wonders if the future isn't in more efficient gas and diesel engine design, NOT hybrids. Take VWs TSI engine, 170 hp an rated at 5.9 L/100 km on the highway, out of just 1.4 liters of displacement. Without the hassles of hybrids (battery packs, underpowered gas engines when the batteries run down, etc).
It's odd how VW has some great gasser designs (2.0T, normally aspirated FSI, TSI), but here in N. America, we only get the 2.0T, the others being the piggish 2.5, the awfully outdated 2.Slow (still sold in Canada in the "City" models), and the "who the heck needs 280 hp in a midsize" 3.6 (although it is FSI).
If you ask me, apart from the Touareg, VWs only need the 100 and 140 hp TDIs, and the 2.0 FSI (normally-aspirated, 150 hp), 1.4 TSI (170 hp) and 2.0T (200 hp) for more than adequate motivation in the Golf/Jetta/Passat/NB/Eos lineup. Actually in Europe the base engine in the Passat is a 1.6L, 102-hp gasser...slow but it can still cruise at European speeds...we in N. America need to rethink our love of high displacement inefficient engines.