I read that more carbon is actually put into the atmosphere in the manufacture of SUVs than from their fuel consumption over an average on road life span, by like a factor of two.
So, keeping an old vehicle on the road longer is way better than continually buying new ones.
That said, my 1980 Jetta with a 1.6 liter engine was getting 40mpg and had over 300,000 miles on it when I finally retired it. And my previous vehicle, a 1977 Chevy Blazer with a 350 V8 only got 12 mpg, but I still squeezed 330,000 miles out of it and it is still running as a piece of mining equipment in Colorado.