Technically, neither one of these axle ratios is “better” than the other. Either one of them can be a good axle ratio to use, depending on what you intend to use your vehicle for.
Axle Ratios Explained
Before we compare these two axle ratios, let’s go over what exactly an axle ratio is and how it works. An axle ratio is a measurement of how many times the driveshaft turns to the wheels. For example, an axle ratio of 3.31:1 means the driveshaft turns 3.31 times per 1 turn of the wheels.
In most trucks, the axle ratio is usually around 3-4 driveshaft rotations per 1 wheel rotation, and depending on whether that number is higher or lower, it can significantly alter your truck’s driving characteristics. What you need to remember is that a lower gear ratio will give you better fuel economy, while a higher gear ratio will give you a better torque figure.
A truck’s axle ratio is largely determined by the gears in its differential, and if you’ve ever heard the term “tall/short gearing” used before, it refers to axle ratios. Tall gearing refers to a lower axle ratio; the gears are “tall” because they move the truck further with each turn they make.
In most stock trucks, the default axle ratio provided by the manufacturer is intended to give you a good balance between power delivery and fuel economy, often with a slight bias towards fuel economy. If you’re planning on using your truck as a daily driver and occasionally for towing or carrying something not too demanding, the stock gear ratio will probably work fine for you.
However, most truck manufacturers offer optional gear ratios for sale, so if you want, you can easily have a truck that is more suited for normal city or highway driving or one that is more suited for hauling heavy loads regularly.
It’s also important to note that your differential isn’t the only thing that affects your final drive ratio. Your vehicle’s tire size and the number of gears in your transmission also come into play here.
Driving on especially large tires, for example, will make your final drive ratio lower, meaning that you can use a differential with shorter gears and have the same final drive ratio as a differential with taller gears on smaller tires. So, as you can see, the “best” axle ratio depends on both what your own needs/preferences are and the specific vehicle you own.
Comparing the 3.31 and 3.55 Axle Ratios
Let’s say you’re just trying to compare two versions of the same truck, one of which uses a 3.31 axle ratio and the other a 3.55 axle ratio. Is there a better axle ratio to use in this case?
Again, the right answer comes down to what you want to use the truck for. However, let’s take a closer look at each of these axle ratios and see exactly how they stack up against each other.
Fuel Economy
We already know that lower axle ratios mean better fuel economy because they don’t require the engine to turn as fast, but how significant is the difference? In general, trucks gain about a 0.5-1.0 mpg increase for every 0.25 decrease in the axle ratio (although this only applies to highway mpg figures; axle ratios don’t significantly affect city driving mpg figures).
So in the case of the 3.31 and 3.55 axle ratios, the loss of fuel economy that you’d get with the 3.55 ratio probably wouldn’t be all that bad. It’s worth mentioning, however, that there are axle ratios available that are even lower than 3.31; you can get a differential with a ratio of 3.08 for some trucks.
Performance
If torque is what you’re after, however, you should at least use a 3.55 axle ratio. There are differentials with even larger axle ratios as well, usually up to around 4.30.
To understand how the differential affects the axle ratio and the torque, let’s go over how a differential works. At the end of the driveshaft, there is a pinion gear that connects to a larger ring gear in the differential. The larger the ring gear itself is and the more teeth it has compared to the pinion gear, the higher the gear ratio will be.
With a larger ring gear, more force can be applied to turning the axle, which, of course, gives you more torque. There are some exceptions to this, however; if you have a modern truck with 8 or more speeds, you can get away with using a lower axle ratio because the transmission will be able to compensate for it.
Summary
The 3.31 axle ratio offers slightly better fuel economy, and the 3.55 axle ratio offers slightly better torque delivery, but overall these two axle ratios are pretty comparable in terms of their performance capabilities. If you want to pick the best axle ratio for your use, consider what you’ll mostly be using your truck for and what sort of equipment your truck comes with first.
Hi, I hope this article was helpful to you. If you've ever owned a car before, it's inevitable that you will deal with your fair share of car issues. I know I have, and that's why my passion is to research common car questions and answer them.
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