1. Vehicle design.
While the 2.8L CRD is of the recent breed of small and efficient diesel engines, the Jeep Liberty CRD is still a 4300 lb brick-on-wheels. The inherent limits posed on attainable mileage by its high weight and poor aerodynamics make it not so comparable to the sub-3000 lb Volkswagen TDI’s with 70% of the drag coefficient. Owner-reported mileage generally ranges right around the EPA estimate of 21/26, although the highway figure seems to often be exceeded by 2-4 mpg. Your mileage may vary, primarily according to the following.
2. Driving content.
Stop-and-go driving kills our mileage more than any other factor. While owners report 30+ mpg on the highway, city driving mileage is consistently very near the EPA estimate of 21 mpg. The main reason for this is the sheer weight of the vehicle---it simply takes energy to get mass moving. However, this effect is compounded by our automatic transmission’s torque converter, which loses energy as heat whenever its clutch is unlocked. While this inefficiency is increased by any acceleration, it is further aggravated by vehicle weight, and so the CRD suffers more to this effect than would a lighter vehicle. The practical result is that any acceleration will have a significant negative impact, but since the clutch locks at ~53 mph with overdrive selected or ~42 mph unselected, efficiency is particularly degraded by the amount of acceleration done accordingly under those speeds, or by acceleration/deceleration that prevents the torque converter from locking or remaining locked, such as that caused by very curvy or hilly roads.
Tire rolling resistance is probably the largest influence on mileage following vehicle design and the type of driving done. To improve efficiency, choose tires with a lower designed rolling resistance and inflate them to a higher pressure. Tire width, tread design, and rubber compound are the design characteristics affecting rolling resistance. A narrower width is always better; the other factors vary and are difficult to determine although tire category is a general guide. Tire reviews and tests, such as those found on TireRack.com, can be helpful. Increasing tire pressure degrades ride but improves rolling resistance, handling, and longevity. The tire pressure recommendations placarded on the vehicle are designed by the OEM to provide that plush, solid feel that most buyers want, but at the expense of mileage, handling, and longevity. They have nothing to do with safety. Disregard OEM recommendations and fill your tires to suit your preference, within the actual safety limits posted on the tires themselves. Most tires used on autos today like to be in the 38-40 psi range some even higher.
4. Engine Break-in.
Turbodiesels must be broken-in before optimum mileage is attained, which can take quite a while (some say upwards of 60,000 miles). You can hasten this by moderately loading the engine (towing, hard acceleration) whenever you can during the break-in period. Engine treatment can also permanently determine your engine's characteristics. Extended high-rpm operation can cause cylinder glazing while excessive idling can result in glaze-like cylinder deposits, both of which are permanent detriments to efficiency, power, and emissions. Extended high-rpm operation probably isn't an issue for us in the U.S. (given the 545RFE) but excessive idling may be for some. Don’t confuse the common idling of tractors and trailers for climate control or accessory needs and turbodiesels for turbo cool-down as examples for the harmlessness of idling. Diesel idling will be covered by another FAQ.
A higher cetane fuel will provide better mileage as well as more power. Diesel #2 in the U.S. typically ranges 40-45 cetane but higher rated, premium fuel can be found. Winterized diesel fuel has a lower energy content than Diesel #2, and therefore mileage and power suffers slightly with its use. This is why a seasonal drop in mileage is experienced by most throughout the winter. You can always boost your cetane by the use of an additive.
6. Driving style.
Minimize your braking by slowing-down and anticipating. All energy dissipated by the brakes is fuel L.O.S.T.. Decrease your highway speed. Optimum highway mileage will occur somewhere between 58-70 mph while in 5th gear, probably near the torque peak at 1800-2000 rpm. Mileage decreases rapidly above ~70 mph due to aerodynamic drag while the transmission shifts into 5th at ~64 mph but will remain in 5th until vehicle speed drops below ~58 mph. Use overdrive whenever possible. Although the torque converter locks-up at ~42 mph (~11 mph earlier) with overdrive deselected, it doesn’t compensate for the loss of efficiency that results from the lower gearing.
This is a good article on breaking-in a diesel: Diesel Stop
You can find good discussion on breaking-in a diesel like ours at TDI Forum
Here's a nicely comprehensive list of fuel economy tips: Improving Fuel Economy
Submited by forum member BioJeep.