Even newer vehicles can benefit
On my 2008 GMC Canyon extended cab pickup with a 2.9l 4 cylinder/auto transmission - EPA sticker claimed 23-29 MPG, real life was barely 20 MPG (governed to 99MPH) when it was new and totally stock. This is where vehicle fan pages come in, they help with the individual idiosyncrasies of whatever vehicle you might have. Problems, performance, and general upkeep procedures can be found there. With that said, what I did with my truck to increase mpg/performance (in my mind, performance/efficiency equals MPG - better performance, less foot needed, better mileage).
K&N cold air intake
gutted 1st catalytic converter
ported & polished intake/exhaust manifolds
ported/modified throttle body
Flowmaster exhaust w/sierra flexible SS braided expansion joints
NGK iridium sparkplugs
Beryllium copper spark plug->coil pack connectors
Corvette shift servos
All synthetic oils for less friction
Belltech antisway bars
Hypertuned for 89 octane, better shift points, delete traction control, de-GM'ed extra clutter from ECM/BCM
Low rolling resistance Michelins
Now, even with 155000 miles on her, she gets 28.8 MPG average & tops out around 137MPH.
Really only concern I had with the Jag rebuild was the final gear ratio - seems to me it would be a dog to get rolling, but once there would just cruise at max mpg - seems it might cancel out each other as far as benefits of the waaaay-beyond-highway gearing.