What's The Diff? Why Using A Donut Spare On The Front Of A FWD Vehicle Could Damage The Transmission

Posted on: 7 March 2017


If you have a front wheel drive vehicle and you have a donut-sized tire as your spare tire, you need to understand what would happen if you were to drive your vehicle with the spare in the front. Here's what you need to know. 

How it all works... or doesn't

It's important to understand what happens to the mechanical parts of a front wheel drive vehicle when it's driven with a small spare tire on the front. 

Wheels—When one of the front tires on a front wheel drive vehicle is a small spare tire, it will not spin at the same rate as the normal tire on the opposite side. The smaller tire will have to spin a lot more than the normal tire in order to, essentially, keep up with the other tire so the vehicle will drive straight. The function of spinning the wheels together at the same rate of speed or separately at different rates of speeds is the role of the differential. During normal driving, the differential spins each wheel at different rates through turns and at the same rate while driving straight. By putting on a smaller spare tire, you basically confuse the differential. 

Differential—Confusing the differential in this way, for lack of a better phrase, will cause fatigue on the parts of the differential, particularly the gears. The differential consists largely of tongue and groove gears. The confusion essentially leads to a grinding down and wearing down of the metal gears inside the differential. This, in turn, causes metal shavings to accumulate in the fluid inside the differential. Another thing that happens is the differential tends to overheat when it is overworked. Overheating of the differential can cause the fluid inside of it to break down, which would make the fluid ineffective and unable to provide protection to the differential and to the rest of the drivetrain parts that use the same fluid. You see, the differential in a front wheel drive vehicle is typically located inside the transmission housing and, therefore, uses fluid directly from the transmission. 

Fluid—Since the fluid inside a differential can get ruined by using a small spare tire—and the same fluid is used by the transmission—it should go without saying that your transmission could be put at risk. Transmissions require fluid to keep the moving parts cool and lubricated. If the fluid has been overheated and/or contains metal shavings, then the fluid will not be able to do the job it was intended to do for the transmission. The fluid may get so filled with metal shavings in the differential that it will not easily flow between the various components within the transmission housing. Burnt and/or damaged transmission fluid can cause your transmission to slip gears, respond sluggishly, make grinding noises, or not respond at all. 

As you can see, you want to avoid driving your front wheel drive vehicle with a small, spare tire in the front or you could risk ruining your transmission. 

What to do if you get a flat... or not

If you do get a flat tire in one of the front tires, rotate one of the good tires from the back axle up to replace the flat tire. Then, place the small donut-sized tire in place of the back tire you used. However, if you do drive the vehicle with the spare tire on the front, be sure to have the spare replaced as soon as possible. You'll also want to drain the transmission fluid and have it replaced, especially if you drove more than the recommended few miles to the tire repair shop. 

Talk to a professional at a place like Huntington Beach Transmissions for more information.