When you shift to reverse you should

What would happen if I shifted from Drive to Reverse right now? Of course, we'd never recommend putting your car into Reverse while driving, but in pursuit of scientific truth we've gone ahead and researched it.

That mindless quandary you have while driving down the road might spur an immediate experiment. Only two clicks separate Drive from Reverse in most cars. Manual transmission cars require more creativity to try reverse while underway, but the results may be imagined as badly spectacular, no matter what kind of gearbox. Pondering the outcome might conjure mental/metal images of ball bearings spraying in all directions and a fusillade of gear fragments chewing up anything in their path. But what's the reality?

Will It Ruin My Car?

Transmissions are not meant to engage reverse while traveling forward. Sure, you can jam it in at the bottom of your driveway apron while you're still rolling a little, but even that's not the best of practices. "The safest bet is to just not do it," said Craig Renneker, Ford's Chief Engineer for transmission programs. As a way of protecting us from ourselves, automakers design a function called Reverse Inhibit into transmissions to prevent inadvertent selection of reverse. "Putting it into reverse [in modern cars] when going forward has no action at all, the car just ignores the request until you get down to a proper speed," said Renneker, using Ford's six-speed automatics as an example; "It'll just say 'hey, I know you want reverse pal, but I'm just not going to give it to you until the appropriate time.'"

Manual transmissions have physical locks in the shift mechanism to make selecting reverse an active exercise. Barring lockout rings or pushing down on the stick shift, deliberately trying to select reverse while driving forward at normal speeds is basically impossible with a manual. "The main problem you're going to be fighting is what you're trying to get the thing to do is something it really does not want to do," Renneker continued. The gearset would likely growl if you tried, and if the protest of the machinery doesn't instantly deter you it could be injurious to your transmission. Ford's transmission guru elaborated that the synchronizer mechanism in manuals is only designed to change the speed of transmission internals enough for smooth engagement. Attempting reverse at road speed would force the synchros to try matching shaft speeds, building up lots of heat and potentially causing damage. "It won't be effective, it won't do anything for you, and secondly, you're going to be putting a lot of extra stress on that synchronizer," he told Autoblog. In short, it's best not to try it.

Older Cars? Well That's A Different Story

Modern automotive electronics tie all the systems together, so the right hand always knows what the left hand is doing. This modern architecture in systems like Control Area Networks is what enables the vehicle's electronically controlled automatic transmission to ignore a request from a driver for something that may cause damage – or worse – injury. Older, less-sophisticated cars may not have the reverse-inhibit function, though Renneker explained that there were hydraulic reverse inhibit systems in the past. The march of technology has made it easier and less expensive to design a car that can protect itself.

When your transmission doesn't know any better, selecting reverse during forward driving is still less exciting than you might imagine. "If the transmission was not designed with a reverse inhibit feature, engaging reverse while driving forward will, most likely, stall the engine," Renneker said. "The car won't lock-up or skid, but it will slow down a bit and act like the transmission is in neutral."

Losing engine power while driving means you're bombing down the blacktop without the benefit of power-assisted brakes or steering. Emergency maneuvering will be much harder, and brakes without vacuum assist require significantly more pedal pressure. Adding significant difficulty to controlling the car is a big safety concern that should keep your hand off the shift knob.

If All Goes Wrong, Prepare For The Cost

Experimentation with a vehicle you either like or depend on might end expensively. John Paul, AAA's Car Doctor for Southern New England says as much. "If you are lucky the car stalls and nothing happens," Paul said. "For those less fortunate the transmission self-destructs, and in most cases will require a complete replacement." (use our Repair Estimator to find out how much any repair should cost you)

In case you needed a reminder, transmission work is expensive, too.

"As an example, a 2005 Buick transmission will cost approximately $2000 plus 7-9 hours on installation time," Paul said. "On a similar year Toyota Camry the price for a factory replacement transmission is $3274 plus just about twelve hours of labor. With the current labor rate of $100+ per hour this can be one very expensive mistake."

Rebuilt transmissions are more economical, but only slightly. "An OEM rebuilt [transmission] can be purchased for the unfortunate Buick owner online for $1300 plus shipping," Paul said. True misers might go with a salvaged unit, but the Car Doctor cautions that while recycled auto parts are a viable and safe option to consider, "you are taking a chance on the quality of the part," though most operations will stand behind their merchandise with some kind of warranty.

The Lesson: Don't Even Try It

So, there you have it. To inadvertently drop your mom's wagon into reverse while drag racing likely won't be instantly fatal to the hardware, provided she has a modern automobile. Automakers have realized that people do astonishingly dumb things behind the wheel and have implemented countermeasures to protect us from ourselves.

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