Just based on the usual engine efficiency curves it seems unlikely that 3 extra gears would have an average ~10% efficiency/MPG gain by themselves. For that to

Just based on the usual engine efficiency curves it seems unlikely that 3 extra gears would have an average ~10% efficiency/MPG gain by themselves. For that to happen the engine itself must be falling off a lot more than 10% over a few hundred RPM outside of the efficiency peak. Which seems unlikely, look at this stack overflow question, about half way down where someone posts an efficiency map. https://mechanics.stackexchange.com/questions/2161/fuel-efficiency-and-shifting-rpm https://mechanics.stackexchange.com/questions/2161/fuel-effi... Note that the most efficient curve for a given load is frequently ~1k rpm. Which given the usual gear ratios a 5 speed transmission should be more than capable of keeping it a couple hundred RPM on one side or the other of the actual peak. Its more likely a long list of optimizations where the most noticeable was the gear count change on a car that probably wasn't particularly optimized to begin with. A good part of it might have been the transmission/torque converter, but even then it had to be a pretty crummy transmission for that large of an improvement. The usual things to look for are drag improvements, weight improvements, tire efficiency, oil/transmission viscosity changes, and an untold number of small transmission and engine tuning changes to the ECUs as well as any number of subtle mechanical changes to the intake and exhaust to ignition system changes. Most of those things aren't going to show up on a white paper at the dealership. So while the 0W-30 oils and the like have an effect, the tires things as I mentioned is huge on some cars. You can walk around in the truck area of many dealerships and see 10% deltas in MPG on some of the them that are identical except for things like offroad tires and 300Lbs of bed liner.