We have moved away from untethered gas caps. Today, most cars have a small rubber lead that keeps us from losing our gas caps. There are even some new cars that come with fancy capless gas tanks. But what if that rubber lead breaks? What if you forget to put the cap back on and you drive away from the gas station? Is it ok to drive a car without a gas cap? 

Do you need a gas cap? 

According to How Stuff Works, “Sealing off the fuel tank with the gas cap helps your fuel system maintain proper pressure, which in turn helps your car operate smoothly. These are the primary purposes it serves.”

Despite this purpose, you can drive off from the gas pump without a gas cap. Given the way gas tanks are designed, it is unlikely that you will lose much gas from it sloshing out, but you will likely lose some due to evaporation. This effervescence may also lower your fuel economy. 

What’s the worst that could happen? 

Outside of maintaining the proper pressure, the gas cap also keeps debris out of your tank. Depending on the gas tank opening, small particles can enter the opening and clog your fuel filter over time, eventually leading to quite the mess. Even still, this is unlikely on most modern cars. 

Although modern car tech is really helpful, in the case of the gas tank, not properly securing your gas cap can force some modern cars into “limp mode.” This means the car will limit power output not to exceed 20-30 mph. So, with some cars, driving without a gas cap will not work. 

Don’t forget about car inspections 

Depending on which state you live in, you may have to have your car inspected every so often to ensure its compliance with your state’s laws on what makes a car roadworthy. Many things can trip an inspection and make you fail. Not having a gas cap will make your car fail inspection in some states. With some newer cars, leaving the gas cap off may trigger the check engine light, which can also affect your inspection. 

Many modern cars will light up the engine light if you drive off without securing your gas cap. Some new cars will light a “gas cap” light so that owners don’t panic over a check engine light. 

How Stuff Works points out that in some cases, the check engine or check gas cap lights might not shut off immediately after securing your gas cap. HSW says that it may take up to 100 miles of driving for your gas tank to pressurize enough to shut off the light. 

How to replace your gas cap if you lose it

Thankfully, most gas caps are universal. Of course, there will be exceptions to this by year and possibly make and model. Generally speaking, buying a new gas cap shouldn’t be too tricky or expensive. Typically, fuel caps have a diameter of 55-57 mm or 58-60 mm, including the threads.

Losing your gas cap isn’t the only reason to replace it. Like all threaded things, gas caps can get wore out. Signs of this happening are typically a gassy smell from outside the car or a jiggly gas cap. Again, this doesn’t need to worry you; most mass-market vehicles have replacement gas caps at the auto parts chains for somewhere around the $10–$20 range.