I've been trying to figure out how to market a training course that effectively tells people they're bad drivers. In the same car, on the same roads, and with me usually being considerably faster I always maintain about 10-15mpg more (based on the dash reading) than the wife. She isn't an awful driver, but tends to be impetuous and reactionary instead of calm and prepared. I have taught a few mates about observation, planning, anticipation, and the tricks to ensure your car uses the minimum fuel, and always teach my clients the same. Some get it and adopt it into their normal behaviour, but most return to their natural way, even though it costs them several hundreds of pounds a year in fuel, and possibly more in wear'n'tear vehicle repairs.
Based on 10k/yr, my wife would spend £470 more than me in fuel than if she would listen and then apply the teachings. A mate with a Clio Cup Trophy who was getting an average of 24mpg now gets 33, saving him a potential £630, and he has stuck to it he says.
If you're due a new car anyway then go ahead, but often a change to a more economical car can be unnecessary if you can adjust your driving to make your current car work more efficiently under your control. If you consider the outlay on buying the new car, and the fact that you will drive it in the same way, then you may not benefit much as most cars of similar sizes and spec have similar consumption. Save on the finance, loans and cash outlay and modify your driving, and save thousands maybe.
I'm not implying that anyone here is a bad driver.