What happened

Shares of Winnebago Industries (WGO -2.15%) are up 14% as of 10:50 a.m. EST after the RV maker reported expectation-thumping results this morning.

Earnings for Winnebago's first fiscal quarter of 2019 came in at $0.70 per share. (Analysts were expecting $0.64). Sales for the quarter were $493.6 million, ahead of the $486.4 million consensus estimate.

RV parked on a lake promontory amid mountains.

This RV maker is sitting pretty at the start of fiscal 2019. Image source: Getty Images.

So what

Winnebago grew its sales only about 9.7% year over year in Q1, but its profits surged 22.8%, which might lead one to believe Winnebago enjoyed implied good growth in profit margins. In fact, gross margin grew 40 basis points to 14.4%, but operating profit margin actually declined 30 basis points, to 6.6%. What really helped Winnebago to outperform in Q1, it turns out, was a decline in interest expense (to 0.9% of revenue) and a smaller tax bite (just 1.4% of revenue).

After factoring in those effects, Winnebago ended up with a net profit margin of 4.5%, up from 4% a year ago.

Now what

CEO Michael Happe nonetheless emphasized Winnebago's "upward momentum within the North American RV business and the positive integration of our new marine division," giving rise to "robust" sales growth and greater "retail market share on the RV side."

Winnebago didn't provide earnings guidance for the rest of this year as part of its earnings report on Q1, saying only that the company aims to build a "larger, more diversified, and more profitable organization; one with a productive, healthy balance sheet" in 2019, and to expand market share as the year progresses. For what it's worth, analysts who follow the company predict earnings per share of $3.74 this year, on sales of about $2.18 billion.