Despite its sagging sales, the Yaris B-car is vital to Toyota Motor Sales U.S.A. Inc.’s lineup, the auto maker says.

Yaris deliveries plummeted 46.1% in July compared with like-2009, according to Ward’s data, which is adjusted for selling days. There were 27 last month, compared with 16 in July 2009.

Toyota attributes the discrepancy to a “huge surge” in demand prompted by last year’s government-funded “Cash for Clunkers” trade-in program, which began in July, says TMS Group Vice President and General Manager Bob Carter.

But how does Toyota explain the 40.1% dropoff when the first seven months of 2010 are compared with like-2010?

Carter blames a market shift that favored larger vehicles over small ones, such as the Yaris. But that trend is moderating, he says, as improving economic conditions lure buyers – particularly young, entry-level consumers – back to the market.

“It’s a great youth vehicle for us,” Carter says, promising Toyota has plans to improve the Yaris. However, he is mum on details.

Carter says TMS is “generally pleased with the progress we’re making.”

Total light-vehicle sales were 169,224 in July, which made for a dip of 6.8%.

Car deliveries dropped 13.4% to 96,014 units. Light trucks, sparked by the ’10 Toyota Sienna minivan, gave the auto maker a boost with a 3.6% sales uptick on 73,210 deliveries.

The redesigned Sienna chalked up 10,381 unit sales for a 41.5% jump.

Carter says light-truck numbers likely suffered because availability of the Toyota Tacoma was limited as output of the small pickup transitioned to the auto maker’s plant in San Antonio.

Production there now is in “full swing” and the Tacoma’s arrival in Texas will be marked with a line-side ceremony Friday.

The truck had been assembled in Fremont, CA, at New United Motor Mfg. Inc., Toyota’s discontinued joint-venture with the former General Motors Corp. The NUMMI site has been sold to electric-car maker, Tesla Motors Inc.

Through July, Toyota’s total light-vehicle sales were tracking 7.5% ahead of like-2009, according to Ward’s.