General Motors’ June sales favored light trucks as gasoline prices declined and pent-up demand kicked in, the auto maker’s top sales analyst says.

GM says its May mix was 51:49 in favor of cars. “That essentially flipped in June,” Don Johnson says today in a conference call.

The auto maker sold 119,919 trucks last month, compared with 95,416 cars, en route to a 6.3% jump in year-over-year sales, according to Ward’s data. Through June, GM sales were tracking 17.1% ahead of like-2010.

Fullsize pickup demand primarily drove last month’s mix shift, GM says. June deliveries of the Chevrolet Silverado, the auto maker’s volume flagship, ticked up 1.3%, while the GMC Sierra recorded a 4.4% gain on like-2010.

“As people got over the fear of fuel-price spikes, those who actually need a pickup have come back into the market,” Johnson says.

Today’s national average per-gallon price of regular-grade gasoline is $3.56, down from nearly $4 in May, according to the American Automobile Assn.

But GM stops short of predicting a sustained mix shift in favor of light trucks.

“We think, for the rest of the year, cars will be an increasingly important part of the mix,” Johnson says, citing consumer sensitivity to operating cost.

Underlining Johnson’s position is the June sales performance of the fuel-efficient Cruze Eco. Boasting a highway rating of 42 mpg (5.6 L/100 km), it accounted for 17% of Cruze deliveries in June.

Still, Johnson defends GM’s 122-days’ supply of fullsize pickups, saying the buildup is necessary to maintain the pipeline through scheduled summer shutdowns and because the auto maker expects steady demand in second-half 2011.

Against this backdrop, three GM cross/utility vehicles achieved historic sales milestones last month, according to Ward’s data:

  • The Chevrolet Equinox, a Middle CUV according to Ward’s segmentation, which launched in 2004, crossed the 800,000-unit threshold.
  • The GMC Acadia Large CUV, launched in 2006, passed the 300,000-unit mark.
  • The Buick Enclave Large CUV, launched in 2007, eclipsed the 200,000-unit barrier.

Despite the light-truck bias in GM’s June results, only GMC’s sales were in positive territory, compared with prior-year, with an 11.6% gain.

Buick, Cadillac and Chevrolet truck sales slipped 15.5%, 2.8% and 8.0%, respectively.

On the car side, two of GM’s three brands were in the black: Chevrolet, with a 27.1% increase, and Buick, with a 21.8% jump.

June marked the 21st consecutive month of combined car-and-truck sales gains, year-over-year, for the tri-shield brand. Some 30% of Buick Regal sedan buyers, GM notes, opted for the car’s new 225-hp 2.0L turbocharged GDI 4-cyl. engine.

Cadillac car deliveries plunged 19.1% as demand dipped 3.7%, according to Ward’s, for its volume leader, the CTS. Ward’s data is adjusted for selling days; there were 26 last month, compared with 25 in like-2010.

Low inventories at Japan-based competitors, such as Toyota, contributed to a surge in GM conquest sales in June. “We’re getting close to 45%-50% of our sales are coming from non-GM owners,” Johnson says.

Japan-based auto makers still are feeling the effects of a March 11 earthquake and tsunami in their home market. The disaster temporarily paralyzed production.

emayne@wardsauto.com