It's likely a waste of money, and it could damage the car.
Here’s something to make all of us obsessive-compulsive motorists a bit uneasy. When you roll up to a gas station you really, really shouldn’t keep clicking away at the pump to land on a nice, round number. Yes, it’s satisfying to see zeroes in the final cost, and perhaps that bit of extra fuel can add a few more miles before the next stop. But the above video from Roadshow is chock full of good reasons why you should call it a day when the pump clicks off, and it’s not just for cost savings.
How could a few more clicks of the pump possibly hurt? For starters, you may not even get extra fuel. Pumps are designed to automatically stop when air pressure changes are detected as the tank fills up. Should you continue to add fuel, it could simply dump it onto the ground through the overflow tube. Furthermore, since the pump relies on pressure and vacuum to operate, giving the handle a few extra clicks could be adding money to your final bill without actually giving you more gas. The fuel could go through the hose into the filler neck, then be drawn right back in.
Those examples can cost you money, but there could be more dire consequences as well. Modern vehicles have an evaporative fuel system designed to prevent vapors from exiting the tank. The video gives a good overview of how it all works, but the takeaway is that overfilling could damage the system by dumping raw fuel into something meant for vapors. The result could be hundreds or even thousands of dollars in repairs.
There are a few other points worth pondering in the video, but these factors are reason enough to leave well enough alone. If you really must round-up to the nearest dollar, go inside and by a donut to make up the difference.
Source: Roadshow via YouTube