General Motors rides a good month for U.S. industry sales in March with daily deliveries increasing 7.8%, but the auto maker says it will remain conservative in its production schedule because certain elements of the improving economy remain risky.

“A good month in (a) steadily improving economy,” Don Johnson, GM’s chief sales analyst sums up in conference call earlier today with journalists and Wall Street analysts to discuss last month’s performance.

GM delivered 231,052 units in March, compared with 206,620 year-ago, according toWardsAuto data. There were 28 selling days in the month this year and 27 in 2011.

GM sold more than 100,000 vehicles in March achieving more than 30 mpg (7.8 L/100 km) on the highway, Johnson points out, a record for the auto maker.

GM’s mix of 4-cyl. engines amid escalating pump prices reached 42% of sales, also a record for the company.

“Inside the numbers, you can see GM’s balanced product portfolio at work,” Johnson says.

The auto maker’s Chevrolet brand led the charge in an industry expected to turn in a seasonally adjusted annual sales rate of up to 14.5 million units.

Deliveries of the stalwart Chevy Silverado fullsize pickup hit 36,491 units, or 8.4% ahead of the same period last year. Strong incentives helped, as GM rolled out its annual “Truck Month” promotion in March and traditional pickup buyers continued to return to the market after holding off purchases during the recession.

However, GM’s performance on the car side of the business arguably was more impressive. Every car nameplate at Chevy posted a year-over-year sales gain.

The all-new Chevy Sonic compact car recorded its best month since launching late last year with sales of 8,251 units.

The slightly larger Chevy Cruze compact car accounted for 21,607 deliveries, or 15.6% more than year-ago. March marked the 2-year-old car’s seventh straight month of year-over-year sales gains and volume exceeding 20,000 units.

The $40,000 Chevy Volt extended-range electric vehicle appeared to shake off scrutiny arising from last year’s safety inspection, with 2,289 deliveries. That’s twice what it sold year-ago and its best month since launching in late 2010.

The Chevy Equinox midsize cross/utility vehicle remained popular, too, with sales of 20,064 units, up 29.4% from like-2011. Later this year, GM will add production of the Equinox to its Spring Hill, TN, assembly plant to complement output at the Ingersoll, ON, Canada, facility.

Total Chevrolet sales were up 12.6% in March.

GMC enjoyed a strong month, up 8.2%, led by a 15.2% gain by the Sierra fullsize pickup to 13,849 units. Sales of the GMC Terrain midsize CUV jumped 29.5% to 8,461 units.

However Cadillac volume fell 16.5% and Buick deliveries tumbled 19.3%. GM blames the declines on its reluctance to match aggressive incentive spending by competitors and fewer lessees returning to the market.

“Luxury (competition) is tough,” says Kurt McNeil, who heads Cadillac U.S. sales. “It is not for the faint of heart.”

McNeil estimates the segment has shrunk from 12.9% of industry sales last year to 11.1% so far in 2012. Cadillac also continues to struggle with its 3-vehicle showroom, he says, noting the situation will brighten later this year with the addition of the all-new XTS large sedan and ATS small sports sedan.

Despite an overall positive start to the first quarter, with industry light-vehicle sales rising to an estimated 14.5 million SAAR, Johnson says GM will remain conservative on the production front.

Economic positives include rising consumer sentiment and spending, a shrinking unemployment rate and an improving credit market.

But March also marks the end of the fiscal year for some auto makers, which often leads to higher-than-normal fleet sales in the month. With inventories at Japanese auto makers rebuilding from last year’s earthquake and tsunami, competition could get tougher.

Oil prices also remain volatile and the mild winter in the Northeast and Midwest may have pulled some car sales into the first quarter.

“We are going to continue to be a little bit cautious in our outlook,” Johnson says.

GM’s first-quarter production grew 9% compared with year-ago against an industry that was up 15.3%, according to WardsAuto data.

WardsAuto forecasts GM output to rise 0.5% in the second quarter and 3.3% in the third quarter, a modest increase that reflects in part the closure of its Shreveport, LA, assembly plant and downtime at truck plants changing over for next-generation pickups and SUVs.