Here's a simple lesson in physics. The bigger and heavier the vehicle, the more fuel it takes to move it even if you choose a smaller, more fuel-efficient engine. I'm not sure how a physicist might write the formula. Maybe something like: mass plus wind resistance times the weight of your right foot equals an empty wallet at the speed of light squared. My recent experiences with a Jeep Grand Cherokee Laredo illustrated this lesson. Last year, I drove the revamped Grand Cherokee powered by a 4.7-liter V-8, the smaller of two V-8s offered. Gas prices already had reached $3, so I was sensitive to what this midsize SUV was guzzling. According to the EPA, the mileage rating for this vehicle is 15 mpg city and 20 highway. My results, during a week of fairly conservative driving mostly on the freeway, was a paltry 15 mpg overall, according to the on-board computer. So I wanted to compare a similar Grand Cherokee with the less powerful V-6, rated by EPA at 17 city and 21 highway. My results, according to the computer: 16 mpg. Which tells me that when you're driving a truck that weighs quite a bit more than two tons (4,665 curb weight for the V-8 Limited and 4,441 for the V-6 Laredo), you pay through the nose for gas. Simple physics. Don't blame the Jeep; the formula holds true for any vehicle. The Laredo proved to be a super-nice SUV in most respects, from its refined restyling to its comfortable maneuverability. The interior is really nice and everybody who rode in it liked it. I liked it, too, until it came time for another stop at the gas pump.

PERFORMANCE: The newly minted 3.7-liter V-6 is a nice unit, quiet and vibration free, but it works hard to motivate this 4,400-pound SUV. Around town, it pulls well enough, but it feels sluggish at highway speeds. As noted, gas mileage is not much better than with the 4.7-liter V-8, which is considerably stronger and will tow 6,500 pounds. That's 3,000 more than the V-6's maximum. There's also a 5.7-liter Hemi V-8, with loads more power and worse mileage. For 2007, Grand Cherokee will be offered with a diesel engine, which typically gets improved fuel mileage. There also will be a high-performance SRT8 version, which should be fun, but don't even ask about gas mileage.

DRIVABILITY: Grand Cherokee drives and handles well, with a slightly rubbery feel to the suspension to accommodate its superior off-road abilities. The Laredo came standard with antilock disc brakes and brake assist, electronic stability program, enhanced accident response system, tire-pressure monitor and four-wheel traction control. The four-wheel drive was upgraded to Quadra-Trac II active full-time four-wheel drive. Off-road performance was impressive.

STYLING: Carrying over from 2005, Grand Cherokee shows greater styling refinement, especially in the front-end treatment.

INTERIOR: Much nicer than past Grand Cherokees, with a luxury look to the dashboard and gauges and every system easy to operate and understand. The seats are comfortable and supportive, though back-seat riders are cheated on legroom. The test Jeep came with optional side airbags and curtains, definitely worth the extra cost.

BOTTOM LINE: Drivers of truck-based SUVs, not just those with Grand Cherokees, have to put up with the gas-mileage compromises inherent in these capable vehicles. Grand Cherokee has been greatly improved, but like all of them, still struggles with the mileage issue.

Jeep Grand Cherokee Laredo

Vehicle type: Five-passenger, four-door SUV, four-wheel drive. Engine: 3.7-liter V-6, 210 horsepower at 5,200 rpm, 235 pound-feet torque at 4,000 rpm. Transmission: Five-speed automatic. Wheelbase: 109.5 inches. Overall length: 186.6 inches. Curb weight: 4,441 pounds. Towing capacity: 3,500 pounds. EPA rating: 17 city, 21 highway.

Highs: Nice interior, good drivability, off-road ability. Lows: Sluggish, mediocre gas mileage, pricey with options.