A difference in fuel consumption is very rarely going to be a worthwhile factor for trading in a vehicle for a different one, especially if you are buying new as opposed to used. IIRC there was a discussion about this very early in this thread. A used Chevrolet Volt made sense for replacing an older gas-powered pig, but the numbers don't make sense for a new one.
Almost always the least-cost choice is to keep driving what you already own up to the point that it is no longer safe or trustworthy, or needs repairs that are not economically viable. I generally keep driving until the car needs to be taken off the road for an extended period to take care of a list of issues. (The down-time costs money, too.) At that point, what I get for selling the car is a rounding error.
On the buying side ... best bet is usually a lease-return vehicle. These are 3 years old give or take, and the original lessee ate basically half the original purchase price, yet the vehicle is still mostly as good as new. The dealer keeps the best lease-returns for themselves, but an off-brand can be a good deal; who buys a VW from a Honda dealer - but the dealer's other choices are to auction it or trade it to another dealer.
Bought a 3 year old VW Passat with 95,000 km ... sold it at 10 years old with 462,000 km. And I sold it, not scrapped it. The chap who bought it brought his mechanic friend, and the mechanic's words were "consider it sold, because if he <chap> doesn't buy it, I will". That was a good car.
