It doesn't take much research to realize the efficiency benefits of electric vehicles over their gasoline counterparts. However, if you want to quantify those benefits, it helps to do a little digging. The University of Michigan's Transportation Research Institute did exactly that in its latest study.
"The Relative Merits of Battery-Electric Vehicles and Fuel-Cell Vehicles" (PDF) looks at the two distinctly different ways automakers are building electric cars. Battery-electric vehicles (BEVs) use the grid to draw power, whereas hydrogen fuel-cell electric vehicles (FCEVs) use compressed hydrogen gas stored and delivered in a method similar to gasoline.
The general advantages and disadvantages are things that have already been covered -- BEVs have a more readily available fuel source, whereas FCEVs have a longer range and lower refueling time, to name a couple -- but the study goes deeper than that. It uses technical documentation and interviews with industry experts to provide some real numbers.
At the moment, it appears that BEVs carry the best environmental benefit. In terms of cost to operate per mile, their 4 cents per mile rating is half that of FCEVs (using national-average figures). BEVs also produce the lowest number of greenhouse gas emissions in a "well-to-wheel" measurement: roughly 214 grams per mile, compared with hydrogen's 260-364 g/mi.
On the other hand, FCEVs do win in one category: well-to-wheel petroleum use. BEVs average 54 Btu (British thermal units) per mile, while hydrogen can bring that value as low as 27 Btu/mi or as high as 67 Btu/mi, depending on how the hydrogen is sourced. For comparison's sake, current gasoline vehicles average about 4,000 Btu/mi.
No matter which way you go, which could depend on a whole host of factors, it's pretty obvious that gasoline takes home the crown as the least efficient option of them all. Hydrogen definitely has its benefits, but a lack of infrastructure in 98 percent of the country is holding it back at the moment.