Automotive investors are slightly on edge after the industry's slower start to 2014, with sales dropping 3% in January and checking in flat for February. A rule of thumb for those who follow the automotive sector is to take sales in three-month chunks, which means March needs to be a strong month. Let's take a look at two automakers that stumbled in February, one that surged, and why I think March will indeed be strong.

Chrysler's Jeep Grand Cherokee is surging. Source: Chrysler Group.

Chrysler surges with 154,866 units sold
While winter weather has dampened dealership traffic and put a dent in most automakers' sales, Fiat Chrysler Automobiles' (NASDAQOTH: FIATY) Chrysler Group continued to shake off the snowflakes to post an 11% sales increase last month. It was the company's best February sales mark since 2007, and the severe weather injected Chrysler's Jeep brand with extra demand.

Jeep sales were up 47%, which was good enough to capture the brand's best February sales ever. The charge was led by the Jeep Compass, Patriot, and Wrangler, which each recorded their best February sales ever. The success wasn't limited to Chrysler's Jeep brand, as Ram truck sales surged 26% higher to earn the brand's best February since 2006. While the Ram truck still trails its Detroit rivals' F-Series and Silverado in terms of volume, it is closing the gap in 2014 with strong sales increases.

Investors considering Chrysler when the company goes public would be wise to keep a close eye on Ram truck sales later this summer. That is when Ford's all-new 2015 F-150 hits the showrooms and GM will decide whether to ramp up incentive spending to prevent more market share loss.

Meanwhile, Chrysler's two Detroit rivals couldn't shake off the harsh weather quite as well.

Ford and GM stumble
General Motors (GM -0.05%) delivered 222,104 vehicles in the U.S. in February, 1% lower compared to a year ago. The biggest disappointment for investors was the ground lost in the industry's most profitable vehicle segment: full-size pickups.

This was supposed to be GM's window of opportunity to stop the Ram truck from gaining market share, as General Motors' trucks are the freshest and newest, and to convince consumers to buy the Silverado now ahead of Ford's 2015 F-150 launch later this year. Unfortunately, sales of the Silverado plunged 12.1% in February and are down 15% for the year.

The upside to the bad sales performance is that so far GM has refused to boost incentives on its trucks, so while sales are down each one remains profitable. That's about to change, as GM is gearing up for its steepest and most aggressive incentive spree to date on its new trucks. It is offering employee and supplier pricing to buyers of its new Silverado in March -- an attempt to tap pent-up demand caused by the harsh January and February weather, as well as take advantage of a month with five weekends on the calendar. Investors absolutely need to watch how many sales GM can take back from the Ram and F-Series, as well as take note of how much these incentives will eat into first-quarter profits.

While GM definitely stumbled, there were some bright spots. GM's Buick lineup was the star in February with an 18.8% gain. In fact, Buick is the only brand under GM's umbrella to post sales increases for the first two months of 2014 compared to last year; it is up 9.6% year to date. The charge was led by Buick's Encore and Regal, up 92.7% and 49.3%, respectively., Another factor for GM investors to watch are sales of the luxury Cadillac brand. Cadillac is coming off an excellent 2013, but sales are down 8% year to date. As spring nears, and incentives rise on its full-size trucks, GM needs its luxury sales to pick up to help company profitability.

Ford (F 0.17%) also stumbled last month. While the company has truly been on a hot streak over the last few years, 2014 has started off a bit slower. Ford's February sales totaled 183,947 units, which was a 6% decline from last year. The weather clearly hurt demand for Ford's car lineup -- six of seven vehicles posted sales declines, which contributed to the segment's 16.8% decline compared to last year.

Ford's 2015 F-150 will launch later this year. Source: Ford.

While overall sales lagged, as well as passenger cars, there were two bright spots. Ford's F-Series sales nearly hit 56,000 in February, its best mark for that month in eight years and above the 50,000 threshold that Ford considers a great month. While sales of the F-Series only increased 2.6% for the month, it didn't lose any ground to its closest competitor, the Silverado, and Ford didn't need to increase incentives to save market share, either. Make no mistake, that's a win for the Blue Oval.

Another bright spot was the struggling Lincoln luxury brand that is in the early stages of its turnaround. Lincoln sales increased 36.4% over last year, led by strong sales of the MKZ and MKX. Lincoln's sales should remain strong with spring approaching; once the MKC hits dealerships in full force, it will take Lincoln sales to a consistently higher level.

Foolish takeaway
Ford also offered some insight to why investors can expect a better March sales performance. One reason for its lower overall sales mark last month was that weather caused delivery delays for roughly 10,000 fleet vehicles. Those will give March a small boost at the expense of February's sales totals. Also, demand for Ford vehicles in the West remained robust as the weather had less effect on the region, which leads me to believe the severe winter really is causing issues elsewhere. In addition, as weather cleared up in certain areas, demand returned in a strong way. That looks to remain the case in March.

March will be the last month in these automakers' first quarters, and it will need to be strong to offset the weakness of January and February. Investors would be wise to watch incentives, sales of full-size trucks, and luxury sales to get an idea which automaker could top or miss first-quarter-earnings estimates.