Tech Tip Tuesday: Rear Gear Ratios

February 5, 2018 3:38 pm

Spending a lot of time in and around the automotive world means constantly learning. There’s always new technology, new methods, and a vast lexicon. One of those terms is “gears,” or more specifically, rear gears. You’ve likely heard the term thrown around by mechanics and savvy car enthusiasts, but don’t fret if you haven’t grasped a full understanding of it – this article explains what they are, why they matter, and how you can manipulate them to get more out of your vehicle!

The rear gears are the ring and pinion gears that are found in the rear differential. The rear diff connects the driveshaft to the rear axle, and, via the gears, transmits power to the rear wheels. As you can imagine, it’s an important component since it directly affects the amount of power that is transferred from the engine to the wheels. Usually, when people are talking about the rear gears, they’re doing so in the context of the gear ratio. This simply refers to the number of times the driveshaft needs to rotate for the wheels to turn once. Thus, if a car has a 3.15 gear ratio – as did our silver 2018 Mustang GT – it means the driveshaft needs to turn 3.15 times in order for the wheels to complete one full rotation.

Owners tend to upgrade to a higher gear ratio, as it gives the engine increased leverage which translates to quicker acceleration. Ultimately, altering the ratio simply means manipulating your available torque. Higher ratios sacrifice a bit of top-end speed and gas mileage, but the car is quicker to react when you put your pedal to the floor.

In terms of getting better performance from your vehicle, this is a modification with a very high return. You can always go spend $6000 on a supercharger and get a couple hundred more horsepower, but that’s an entirely different level both in terms of performance and cost. Swapping the gears, on the other hand, will run you just a few hundred bucks, and it can significantly impact your race times and the quickness of your car.

Some common gear ratios for the Mustang are 3.15, 3.31, 3.55, 3.73, 4.10, and, on the very high end, 4.30. The gear ratio that you want depends on what you’ll be using the car for. If it’s a daily driver, going to high will negatively impact fuel efficiency and have the car running at higher RPMs. If you want more aggressive performance without sacrificing those aspects, a 3.55 to 3.90 ratio will do you just fine. 3.73 is a popular ratio for autocross and road racing competitions, but there are more factors that go into choosing the best gear ratio for these types of events. You won’t want to sacrifice top end speed if the course or track has a lot of good straightaways for you to get speed, but other things like the sharpness of the turns and the distance of the track are important to keep in mind. For drag racing and for automatics, 4.10s are a popular match. This ratio gives you plenty of acceleration, without sacrificing as much top end speed as 4.30s.

Whatever ratio you do end up going with, it’s important to note that you will need to calibrate the speedometer, and for an automatic, the shift points as well. If you don’t do this, the speedometer will give you incorrect readings, which can land you a ticket, and can affect the operation of your automatic transmission.