The burger wars are heating up as McDonald's loses ground to an old rival

Some diners are turning away from McDonald's and toward rivals.

Mike Kemp/In Pictures via Getty Images McDonald's reported slowing sales growth for its second quarter.

Some customers are looking elsewhere for cheap meals, from Burger King to gas stations.

It's the latest evidence that the burger wars are heating up.

Peter Lauwers used to swing by McDonald's to buy dinner for his family on busy weeknights.

A few years ago, though, the Michigan-based father, who runs a mobile app development company, noticed rising prices for Big Macs and Quarter Pounders.

The chain also offered fewer deals he liked, such as McDonald's former buy one, get one for $1 deal on burgers.

He even started keeping a spreadsheet to track price changes at his local McDonald's.

These days, Lauwers said, he and his family don't go as often.

"It's got to be a special thing," Lauwers said, adding that his family's McDonald's visits are "maybe a quarter of what we used to do." Lauwers is among the diners looking for other places other than McDonald's to get their burger fix.

The chain's most recent quarterly sales growth slowed, while rivals like Restaurant Brands International's Burger King are gaining momentum.

The shifting burger landscape comes as the economy has turned K-shaped in recent years, a situation where higher earners are spending consistently while lower earners are cutting back.

That scenario has sent diners looking for value, and fast food chains that provide it are seeing the benefits.

McDonald's is still the largest fast-food chain by far, with systemwide sales of $139 billion last year.