Autoweek has an article about the mods Mazda is pursuing in its "Sky" drivetrains.
In the gas engine, the author reported an 18% real world MPG improvement over the company's conventional engined car, "during a test drive that included highway driving, two-lane roads and a long run on the autobahn".
Their mods:
- direct injection
- high-compression ratio of 14:1 with smaller bore and longer stroke
- "A cavity in the top of the piston has been redesigned to help with the efficiency"
- a 4-2-1 exhaust system is also listed as an efficiency helper
- weight reduction in the engine: "The end result was a 15 percent reduction in friction"
- expanded lock-up capabilities in the 6-speed auto - "expansion of the lock-up capabilities--when there is no slippage from the torque converter--happens in every gear at 1,200 rpm and at speeds greater than 6 mph"
- weight reduction in the transmission as well - about 10 lbs for the auto, 20 lbs for the manual
They almost certainly employ a taller final drive or top gear as well, to take advantage of the engine's increased torque, but they don't say so.
The article also details the "Sky-D" diesel mods:
- reduced compression - 14:1
- piezo injectors, variable valve lift control for the exhaust
- weight reduction (50 lbs vs. previous engine)
- fuel economy increases 20 percent over Mazda's previous diesels
Engineers suggested a 43 mpg (US) rating for a Sky-D powered Mazda6.