As a mechanic, I avoid techniques that trade small mileage gains for larger mechanical repair bills or other costs. Mileage savings that increase the costs of maintenance have to take those external expenses into account to be valid. It is possible to make great savings in fuel without doing damage to a vehicle's powertrain - but it takes some knowledge.
EXAMPLES
John, an accountant, began adding hypermiling techniques to his daily driving. Soon he was saving a significant amount of money on gas and began telling all his friends how great hypermiling was.
TIRES: Some cars will wear tires unevenly if overinflated. (The heavier a car is, the more likely this is to occur.) If you hypermile with overinflated tires and they wear out faster than a set with manufacturer-recommended inflation, it is
de rigueur to account for that expense in your mileage calculations to present a valid result.
John got 20,000 miles out of his last set of Goodyears. When he started hypermiling on the next set, filled to maximum sidewall inflation, the center bead wore down at 15,000. 
Since his tires cost $600 to replace and he received 25% less use from them, his hypermiling cost an additional $150 over 15,000 miles. John calculated his fuel savings over the last 15,000 miles and subtracted $150.
MANUAL TRANSMISSION
Throw-out bearings aren't designed for long periods of operation. Lacking any provision for cooling, they can heat up, loose lubricant and fail.
John liked to hold his clutch pedal in to coast but after 5000 miles the transmission began to squeal. John took the car to a mechanic who told him that the throwout bearing was starting to seize up and was damaging the clutch. The mechanic resurfaced the flywheel and replaced the clutch, pressure plate and throwout bearing for $1200. John calculated his fuel savings from hypermiling over the last 5000 miles and subtracted his repair bill from it.
AUTOMATIC TRANSMISSION
Some automatic transmissions can be coasted in neutral with the engine running; some can be coasted with the engine off. Some can be coasted only for a certain distance at limited speeds. Damage to the bearings, seals and pump is cumulative and may not be noticed for hundreds of miles.
While John's car was in the shop, he drove his wife's car, popping her automatic transmission in and out of neutral to save gas. Months later, his wife complained about a whine she could hear so he took her car to the mechanic. The mechanic told him the transmission was about to fail and it would be best to replace it. This cost $3400 so John subtracted it from his gas savings over the 700 miles he had driven his wife's car.
IMPROPER TECHNIQUE
Synchromesh transmissions incorporate brass rings that rub their faces together during shifting to equalize the rpms of spinning gears. They are designed to match shaft rotational speeds that differ by small margins.
John found that he could save gas by turning his engine off and coasting in neutral. He also knew that he would use less gas if he restarted by shifting back into gear and dropping the clutch rather than using the starter. This seemed to work pretty good at first, but then his transmission started to grind during shifts. John took the car to his mechanic expecting a simple clutch adjustment but the mechanic informed him that his sychro rings were worn out and would have to be replaced. The repair cost $1700. John subtracted the $1700 from his fuel savings since he had gotten his car out of the shop the last time.