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If you're trying to save money by getting better gas mileage, it doesn't make sense to spend $150-200 on a scangauge or similar - same logic of buying a new car to "save" money on gas, vs keeping your older car with less gas mileage. The economics just don't make sense.
A new car almost never makes sense, just to get better fuel economy. However, a new car costs many thousands of $$$.
But a meter like this is a LOT cheaper than a new car, so the payback time should be a lot shorter. Remember, with today's prices, you would only have to save around 50 gallons of fuel (over the life of the device) to break even on the cost (and everything after that would be "money in your pocket"). So a little money for a decent "fuel usage" meter, could easily make economic sense. It won't pay you back overnight, but it could easily pay you back several times over during the life of your car (or if you trade-in your car, unplug the meter, and take the meter with you for use in your next vehicle).
For example, even if your car already got 30MPG, and such a device "only" let you get 5% better gas mileage (and 5% is a very low/conservative estimate from what I've heard from people who have such gauges), you would still have saved those 50gal of gas after only 30k miles (which happens in under 2 years of driving, for some of us). And if your car starts with lower MPG and/or you do better then 5% better, than the "payback time" would be even shorter. And once the device is "paid for", all savings are money in your pocket.
FWIW:
While it's not exactly the same thing, I spent a little under $150 for a whole house electric meter. I knew it was a "gamble", but I really felt I needed to try to find were our "hidden electrical loads" (which I had been unable to locate in several years of trying) where in our house (and yes, I had already done "obvious things" like using energy efficient lighting before getting the meter, and it wasn't helping enough). Guess what? In under a month after getting the meter, I was able to use that meter, along with some experiments with turning the breakers on and off (which wouldn't have yielded the info it did, without the real-time electrical meter I purchased to tell me what the effect on power usage was), and was therefore able to find the top several "hidden" electrical loads (including both my sump pump and the blower for our central air, among others). With that info, I've been able to save around $80/month on my electrical bill (roughly cutting our electric bill in 1/2), ever since. So that meter actually "paid for itself" in around 2 months time, just in savings to my electric bill!
And while I realize that my $80/month home electrical savings (due to getting our whole house electric meter) may be higher than many would get, it still does a good job of illustrating my point. The simple truth is, you really don't have any idea how inefficiently you are doing things, until you have some instrumentation to give you that feedback (and the more "real time" the instrument, the better). Without that info you are just making "educated guesses". They may be close guesses, or they may be far off the mark. But without the testing equipment, you are really "flying blind"...