Auto sales continued to slide in June, as car buyers react to higher vehicle prices and Detroit backs away from dumping unwanted inventory into rental-car lots.

General Motors Co., Ford Motor Co. and Fiat Chrysler Automobiles NV reported steep monthly sales declines compared with the same period in 2016. While retail demand is losing steam, each of Detroit's players also reported significant reductions in deliveries to daily-rental companies, long the Motor City's biggest customers.

Sales to Enterprise Holdings Inc.'s Enterprise Rent-A-Car or Hertz Global Holdings Inc. traditionally was a way for auto makers to keep factories rolling even as dealership traffic slowed. But that business has traditionally dented profits and soiled brand reputation.

The move away from rental sales reinforces a newfound discipline for domestic players that have been riding a seven-year growth streak since GM and Chrysler sought bankruptcy protection in 2009. The Detroit 3 reported tens of billions in profits during that span, bolstered by tailwinds from falling gas prices and surging demand for profit-rich trucks and SUVs.

Overall industry demand softened over the first half of 2017, however, falling about 2% through six months, according to JD Power. The development ushers in an expected plateau for auto sales, an important driver for the broader U.S. economy.

The fleet-sales pullback is having a disproportionate impact on wider volumes. Sales to retail customers at dealerships is down less than 1%, but sales to non-retail customers such as government fleets, commercial buyers and rental-car companies is off 7.8%, according to JD Power.

A rental-car reduction "is not something normally seen" at the start of a cyclical sales downturn, R.W. Baird analyst David Leiker said in a research note. The reversal of that trend could ease concerns that profit margins will erode as auto makers chase less lucrative business or resort to price wars.

Even as auto makers ramp up incentive spending to improve dealer traffic, transaction prices are rising as cars are loaded with more safety gear and connectivity features. A consumer shift away from sedans and toward pricier sport-utility vehicles also aided the trend.

Edmunds.com reported that the average monthly payment on a car or truck has soared above $500, forcing buyers to stretch more than ever to obtain a new set of wheels. The firm estimates the average auto-loan length reached a record 69.3 months in June, with the average amount of financing reaching $30,945, up $631 from May.

GM's sales fell 5% to 243,155 vehicles in June, while Ford's sales totaled 227,979 vehicles, down 5.1%. Fiat Chrysler's sales slumped 7% to 187,348 vehicles.

Japan's top sellers fared better during the period.

Honda Motor Co. reported a 1% increase compared with the previous June with 139,793 vehicles sold, aided by gains at its Acura luxury division, while Nissan Motor Co. sold 143,328 vehicles, or 2% more than the prior year, as it ramps up its reliance on trucks. Toyota Motor Corp. notched a 2.1% gain, with 202,376 vehicles sold.

Certain Asian auto makers, including Korea's Hyundai Motor, have fueled sales with rental-car sales. At Nissan, rental sales surged 37% in 2016 and were up 9% this year through May, making Nissan the only major auto maker to boost rental deliveries, according to data from Bobit Business Media, a publisher of trade magazines.

Detroit's sales declines, meanwhile, are fueled by lower fleet sales.

GM sold about 31,000 fewer rental cars in the first half of the year, a decline of 21% from a year earlier. Rentals accounted for 8% of total sales, less than half the level from a few years ago.

Ford said rentals accounted for 13.2% of overall sales in June, down from 15.6% a year earlier.

"We try to keep our overall daily rental within a reasonable number," Ford's U.S. sales chief, Mark LaNeve, said Monday. Rental-car companies are "becoming a little more cautious given the uncertainty in residual values," he added. Rental-car companies keep a close eye on the resale value of cars and trucks; a glut in used cars can lead to unexpected losses

Fiat Chrysler's fleet sales sank 15% in June, with rental deliveries at its flagship Jeep brand down by almost half.

Enterprise, one of the leading car-rental firms, reduced its vehicle purchases in the first half of the year compared with 2016, said Kurt Kohler, a senior executive in charge of the rental company's fleet acquisition. He said signs of declining used-vehicle prices heading into the year prompted Enterprise to narrow its shopping list.

"The market started to move on us, so we pulled back a bit," Mr. Kohler said. "The car segment already had been declining, but we also saw pricing coming down on SUVs and trucks. That affected how much we wanted to buy."

Alan Batey, president of GM's North America region, said an unexpectedly severe downturn in consumer demand for sedans has made it more difficult to ease rental sales, because the rental business would typically help make up the shortfall. "It has tested our commitment" to the strategy, he said, forcing GM to make "tough decisions" to reduce passenger-car production this year, which led to thousands of layoffs at its factories.

Write to Mike Colias at Mike.Colias@wsj.com and Adrienne Roberts at Adrienne.Roberts@wsj.com