answer:5–15% according to Wikipedia, although driver style definitely factors in. Wikihow also highlights power advantages of manual transmission in that you’ll be able to utilize the full range of rpm (whereas an automatic will shift at 5–6k rpm) and can execute a downshift (to pass, for example) more precisely vs. an automatic which generally has a lag. A Consumer Reports article from 2004 cites an average 10% savings in the small car class when comparing auto v. manual in 6 models. Only one showed worse fuel economy with the manual (the Aveo). They calculated annual gas savings (based on $2 per gallon and 15k miles per year) at $75–125 with the manual models themselves costing about $1,000 less. This article also noted a significant acceleration advantage with the manual transmission. My uncle claims that automatics are better for 4WD, but I can’t vouch for that and haven’t found anything online supporting that claim. Manuals are a pain in the a55 in stop and go traffic, especially in hilly areas, but that hasn’t stopped me from owning one. The biggest drawback as a driver with a stick shift is jumping behind the wheel of a car with automatic and then pressing down on the clutch brake at 40 mph. A cursory look on the web seems to put Continuously Variable Transmission (CVT) somewhere in between automatic and manual in terms of fuel efficiency.