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Lyft's "Taco Mode," which allows riders to go through a Taco Bell drive-thru, is being tested in select markets.

Many drivers are complaining that Taco Bell is making their cars smelly and messy.

The drivers also say they make less money if they opt into the service.

Lyft's service that allows customers to push a button and go through a Taco Bell drive-thru is annoying many drivers.

The drivers have started airing their grievances with the so-called "Taco Mode" in a driver message board called Uberpeople.net.

The messages were uncovered by Nicholas Upton at food delivery blog Food on Demand.

"If you willingly sign up for the beta of Taco Hell Mode you are asking for a cheesy, beefy, and greasy backseat," one San Diego driver wrote.

"Vomit bags & plastic seat covers for all seats in vehicle. The drunk-circuit shift suddenly became a three-ring circus," said a San Francisco driver.

In addition to the complaints of cars smelling bad, drivers also made less money while sitting in a drive-thru.

"Sitting in the drive-thru means longer times being paid the per-minute rate," Upton writes. "Drivers earn 9 to 10 cents per minute, and 85 cents per mile in the region."

The backlash comes after drivers predicted that the service, which is being tested in select markets, would be a headache.

"That Lyft might go ahead and do this — encourage riders to do something most drivers dislike doing — without offering drivers an incentive or otherwise communicating to us what the plan is is pretty bold," one Lyft driver told Business Insider's Kate Taylor in July.

"This is Uber-type behavior, and I don't think even Uber does stuff like this anymore," he continued. "I wonder if it occurs to Taco Bell that drivers don't like going through the drive-thru."

Lyft has stressed that opting in for "Taco Mode" is totally voluntary and drivers aren't required to do it. Lyft has plans to expand the service across the US in 2018.