A Chevy Cruze Eco arrived at the office in late June, which got me thinking: Can GM's new small car really rack up 42 mpg (5.6 L/100 km) on the highway?

One way to find out. I was heading to my sister's cottage in northern Michigan and knew this would be an ideal road test. The Cruze accommodated me, two teen-agers and a full trunk. Load a car with that much stuff and mileage will suffer. The Cruze Eco's turbocharged 138-hp 1.4L 4-cyl. performed quite well on flat terrain.

But north of West Branch, where Michigan becomes hilly, this engine was taxed to the point that downshifting a gear or three with the 6-speed manual transmission was required. Low-end torque is not this engine's strong suit, although on paper it's a respectable 148 lb.-ft. (200 Nm).

When I got the car, the trip computer read 33 mpg (7.1 L/100 km). By the time I reached my destination, it never reached 39 mpg (6 L/100 km).

Trip computers are not the best mpg calculators because the average often is compiled over hundreds of miles until the driver resets to zero. I reset when I left to come home, and the tabulated fuel economy shot up. I arrived back in Detroit with an average fuel economy of 44.6 mpg (5.3 L/100 km), based on the trip computer.

But I have my doubts after calculating our fuel economy the old-fashioned way. From the time we got the car with a full tank of gas to the point that I fueled up near empty the first time, other Ward's editors and I logged 360 miles (579 km).

Divide that by a tank with a capacity of 12.6 gallons (47.7 L), and the average fuel efficiency is an unremarkable 28.6 mpg (8.2 L/100 km).

After fueling up, making a day trip to Traverse City and driving home, I logged 458 miles (737 km) — virtually all of it on the highway — before the tank returned to empty. Divide that by 12.6 gallons, and the fuel-economy rating is only 36 mpg (6.5 L/100 km).

Would that deter me from buying a Cruze Eco? With a sticker price of $19,615, it's hard to quibble with the car's value. But if 42 mpg were a top priority, it could be a deal-breaker.

Tom Murphy is executive editor of Ward's AutoWorld magazine, with an emphasis on technology and suppliers.