I gave up on my book because after a couple decades I didn't have a good answer. Sure I could brag about the one full-up where I got 57mpg, and I knew about the one where it was only 36 - who cares.

Back in the 1930s the book was probably useful because changes in mpg were a sign you needed to do a tuneup early. But back then everybody knew a tuneup was every few months and once in a while something went off in between. Cars are a lot more reliable.

1. To monitor gas mileage primarily. Any sudden decline in gasoline mileage can warn of impending engine component failure. [I haven't experienced it but I suppose it could also warn of a repair error, e.g., if you failed to put a new air filter cartridge in to replace the old one (a major oversight but nonetheless possible) then it is likely that your mileage would go up suddenly. One would have to question whether the better mileage was b/c, say, you just switched to Shell gasoline or instead did something wrong when you recently changed the air filter?!]

2. keeping receipts ensures that if there is a problem and your engine is damaged, then you can show where you bought the fuel,oil or component and get recompense. This is rare but does happen and it can be a costly repair. Examples are:

- bad gasoline - usually debris or water. Can stop fuel flow and cost you replacement of a gas filter at the least.

- incorrect oil change - you pay for an oil change but they don't put on a new oil filter or they add the wrong kind of oil or they add too little oil or they add too much oil. Don't discount the last one: too much oil can damage some engines seriously. I once had a dealer change my car's oil. Prior to service I explicitly warned the mechanic that the engine was a rare model and required less oil than most. Nonetheless he overfilled it. Luckily I spotted the difference immediately b/c the engine ran like shite right out of the garage. I made a freeway U-turn,drove back in, re-explained the car's needs and insisted they drain off the overfill and let me check it. The mechanic tried to cover his ass by treating it as another "crazy customer" but I just told his companions that a) he'd fucked up and b) I had specifically warned him about the problem earlier so c) they'd better keep an eye on him - he wasn't as good as they were.

3. If you sell a car, buyers love getting the records proving that you bought good oil/gas, performed maintenance, etc. FWIW most buyers do not continue to maintain such records in my experience.

I don't maintain a log book and I know only one person who does. But I file receipts for gas, parts, fluids, filters and major maintenance for each vehicle.

To figure out MPG you were getting.