First for the GM-h8rs, veer off now because there are GM stories below.
Also my lawyer team (Hans and Franz) advise me to stipulate for the record that I'd buy a VW diesel again in 2 seconds if VAG had the nadsnerhugen to make them.
The 1.6L "whisper diesel" is plenty for the terrain including for highway uphill passing.
("Whisper" is a 100% ironic aspect of the engine name, of course.)
The 1.6L diesel is better than the gas 4 cylinder in countless ways. Maybe the rare 6 cylinder motor can pull harder, not sure.
As for MPG at highspeed, be serious, the vehicle is as aerodynamic as a brick. Drive more slowly and in the right lane if you want better mpg. The vehicle is excellent at highspeed by the way but of course has no chance of handling like a Cayenne Turbo or Macan-clubsport-GT or X3-M-whatever.
The 1.6L is not as zippy as in the cruze with stickshift but in both vehicles you gotta drive using torque not horsepower. Terrain transmission seems tuned accordingly, or maybe it has learned accordingly from my right foot. The first gear for both vehicles is super super LOOOOOW intentionally due to torque-curve and as annoying to stoplight-tailgators same as any diesel. The cruze 1.6L stickshift ratio spacing is also very weird compared to gasser ratio spacing - this is to match the torque cruve. The cruze diesel first gear seems absurdly short but is needed for start-from-stop, 2nd is too tall for 2nd-gear-start.
Another advantage of the diesel Terrain over gas model is the older-school/less-troublesome slushbox 6 speed automatic instead of the wacky 9-speed electronic transmission.
Btw if you happen to have a diesel stickshift cruze, sell it just before the flywheel or secondary-cylinder fails. :| My cruze had secondary clutch cylinder fail around 68k miles. Some owners sad them fail under warranty, GM replaced some beyond the 12k new-car-warranty period. Cruze has transverse engine and clutch replacement was $3400, at dealership. Clutch failed a week before i was to drive the car from NH to FL, pedal sank to floor 1/10 of a mile from entrance to Chevy dealership, drove the car using starter & engine & 1st gear, driving in circles around the dealership until a dedicated employee ran next to the car (in snow!) and told me where to let it die.
In 1991 similar thing happened to My 1989 camaro IRAQ-Z, pedal sank to floor at/blocking the entrance-gate to sunnyvale chevrolet dealersh. Mr Goodwrench walked out from the service bays and showed me how to drive a stickshift vehicle a few hundred feet using battery alone, ignition never firing, pausing 30 seconds to let starter cool each time, without ever letting the ignition fire up.