Driving Cross Country In A FWD? Don’t Use A Donut If You Get A Flat Tire Or You Could Damage Your Differential & Transmission

If you are planning a cross country trip and only have a space saving spare tire (donut) for your front wheel drive (FWD) vehicle, you may want to consider getting a regular-sized spare instead . . . just in case you get a flat tire somewhere along the way. Driving on a donut can damage your differential and transmission. Here's what happens to the drivetrain when you drive on a donut and how to avoid unsafe driving conditions if you get a flat tire on your long trip. 

What Happens When You Drive On a Donut?

  • The Differential Spins the Wheels at Different Speeds

The differential is the part of the drivetrain that spins each wheel. In a FWD vehicle there is only one differential, which is located on the front axle. Through a turn, the differential rotates the wheels separately, so that the wheel on the inside of the turn spins slower than the wheel on the outside of the turn. When going straight, the differential spins each wheel at the same speed when the tires are the same size. But when the tires are different sizes, the differential spins each wheel at different rates to keep the vehicle moving straight.

This constant spinning faster on one side and slower on the other side can break down the fluid inside the differential, which can cause the differential to have more wear and tear. In turn, more wear and tear on the differential can cause it to fail.

  • Bad Differential and/or Fluid Can Damage the Transmission

When the fluid loses the ability to keep the moving parts lubricated, it can cause the differential to lock up or blow out, which can also cause expensive damage to the transmission. This is because the various parts of the drivetrain are all connected to each other. When one component fails, other components in the drivetrain can also fail.

More importantly, most FWD vehicles use fluid from the transmission for the differentials. Obviously, if the fluid is being broken down as it goes through the differential, it will not be as viable as it cycles through the transmission. This can damage the transmission because the moving parts can lock up from lack of lubrication.

What Can Be Done to Avoid Damage to the Drivetrain?

Obviously, you want your spare tire to be the same size as your other tires. If possible, it's a good idea to go to a tire center to trade in your donut for a spare tire that is the same size as your regular tires. Unfortunately, it isn't always possible to carry a regular-sized spare tire, especially in vehicles that have holding compartments that are sized for smaller tires.

If you must use a donut, avoid placing it on the front. If one of your front tires gets a flat and there are no auto shops or tire centers nearby, move a good tire from the back axle to the front in place of the flat tire and then place the donut on the back. The differential won't be affected if a donut is used on the back, since the differential handles only the front axle in a FWD vehicle. But to do this, you would need to have two jacks to lift the vehicle, one for the front and the other for the back.

Another thing you can do is to have the transmission fluid changed before you leave for your trip, especially if this has been neglected. By having new fluid in the transmission and differential, you'll be less likely to break down the fluid in the differential through excessive wear and tear if you need to drive on two different-sized tires.

If you find yourself in need of transmission repair after driving on a donut, contact a company like Interstate Transmission & Automotive.