What Is a Rear Gear Ratio and How Does It Work?
When you get to the point in your Mustang build where you’ve already hit the basics like an intake, tune, and exhaust, it may be a good idea to examine your rear gear ratio and see if it’s worth swapping gears to something more or less aggressive. In simple terms, the rear gear ratio dictates how quickly your car accelerates and its top speed from a mechanical standpoint.
With that said, there are various factors that go into choosing which gear ratio is best for your Mustang build, so keep reading for a brief overview of how and why rear gears are important to any Mustang build.
Shorter vs Taller Rear Gear Ratios
The higher (or shorter) the gear ratio is – 4.10, 4.30, etc. – the quicker your car will accelerate through the gears but will sacrifice some top speed. The lower (or taller) the gear ratio is – 2.73, 3.15, etc. – the slower your car will accelerate but will have a higher top speed and lower cruising RPM for better gas mileage.
Where do these numbers come from? Well, if you’re in your transmission’s 1:1 gear, such as 5th gear in a manual 2011 Mustang GT with 3.31 gears, it means that for every revolution of the rear wheels at 60 MPH, the driveshaft will rotate 3.31 times. So, this means that the engine does not have to work as hard to spin the rear wheels and at cruising speed you will be at a lower RPM to obtain better gas mileage.
Low vs High Rear Gears For Your Mustang
On the other hand, if you have that same Mustang and transmission combination but with aftermarket Ford Racing 4.10 Gears, your driveshaft will have to spin 4.10 times to get the same exact revolution out of your rear wheels. Meaning that your engine is putting 23.8% more effort over 3.31 gears into one wheel revolution equating to noticeably quicker acceleration. However, RPM will be higher on the highway, thus sacrificing a little top speed and gas mileage for quicker acceleration.
We know it can be a lot to grasp, so here are a couple of bullet points to make things easier to understand:
- The higher the number or shorter the gear, like 4.10 or 4.30, the quicker your car will accelerate, but sacrifice some top speed and cruise at a higher RPM.
- The lower the number or taller the gear, like 2.73 or 3.31, the slower your car will accelerate, but will obtain better MPG on the highway due to a lower cruising RPM.
- Keep in mind, going with too short of a rear gear ratio can result in reduced traction, bad gas mileage, and unnecessary wear and tear on your engine at highway speeds.
Which Rear Gear Ratio Is Right For Me?
Road course Mustangs tend to have longer rear gear ratios, which means less time shifting.
That is a loaded question. It all depends on what you plan on doing with your Mustang and what other modifications you plan on adding. Yes, that means there are different ideal rear gears for a car that is strictly a street car versus one that runs solely on a 1/4 mile dragstrip. Another factor can include whether or not you plan on adding a forced induction setup (supercharger or turbo) in the future. On top of that, the engine, transmission and tire combination can also dictate what type of rear gears you should put in your car. All of these various factors go into choosing the correct rear axle gear ratio on your Mustang.
Street Car With Occasional Dragstrip Use - Most of the Mustang enthusiasts out there probably fall into this category. With this particular combination, you’re going to want to find a set of rear gears that will take into account not only acceleration at the dragstrip, but also RPM when you’re cruising down the highway. There are outliers, but more times than not the ideal gear ratio for this combination is 3.73 on most naturally aspirated Mustangs.
Dragstrip Only Use - There are probably a few of you out there who plan to build up your Mustang as a drag-only car. That’s awesome – but it’s important that you find the right gear ratio for getting your pony down the quarter-mile the quickest. With a drag setup, there are many other factors that go into figuring out what will make your car the fastest at the strip including the size of your rear tires and transmission specifically.
Road Course Only Use - Some of you may not be into the straight-line racing scene and prefer to carve corners. That's great - but that means there is a different ideal rear gear ratio for your Mustang as well. Most road course cars tend to have longer rear gears due to the fact that it means less shifting time. And less time spent shifting means quicker lap times. A track car may use something like a 3.55 rear gear ratio, whereas a drag car may have 4.10 or 4.30 rear gears. It all depends on the build.
A Note About Your Speedometer
After replacing the rear gears in your Mustang, you will more than likely notice a difference in your speedometer reading. This is because your Mustang’s computer thinks that you still have your factory gears installed and will output the speed using that gear ratio.
There are a couple of ways you can go about having this corrected:
- If you have a tuner such as the SCT X4, COBB Accessport or nGauge Tuner, have your tuner fix the gear ratio in your tune so your speedometer reads correctly.
- If you do not have a tune on your Mustang, you can have the Ford dealer reflash your ECU and correct it that way.
- For those who have Mustangs older than 1999, you can pick up a speedometer gear to correct the issue.
Will Swapping Rear Gears Affect My Mustang’s Fuel Economy?
The simple answer is yes. By how much all depends on how drastic of a change you’re making to your Mustang’s rear gear ratio. The taller a rear gear you put in your Mustang, the lower your RPM will be at 60 MPH, equating to better gas mileage. However, the opposite goes for shorter gearing. If your Mustang comes stock with 3.55 rear gears and you install 4.10s, then your cruising RPM will be higher at 60 MPH equating to decreased gas mileage and more wear and tear on your car’s engine.
Point is, if you do a lot of street driving, you want to definitely consider the ramifications of putting a steep set of rear gears in your Mustang. A typical middle-of-the-road approach for most Mustangs would be picking up a set of 3.73 gears if your car doesn’t have them standard already.
Mustang Rear Gear Installation
If your car isn’t new or near new, it is probably in your best interest to pick up an installation kit on top of the gears themselves. Consider this cheap insurance in case the other components of your rear may be worn and used.
Unless you’re a mechanic or have a background in working on cars, CJ Pony Parts highly suggests that you have your new rear gears installed by a professional. When re-assembling your rear, there are many precise measurements that are required. To put it in perspective, if backlash is off by just a couple thousandths of an inch, you could experience annoying rear end gear whine or possibly even failure.
Other Things To Consider
CJ's Gotta Have It Race At The Dragstrip
Drag Racers - If you’re concerned about getting down the 1/4 mile faster, then another factor that goes into obtaining the perfect rear gear combination is finding the right tire height. If your rear tires are too short, you may have to upshift before crossing the 1/4 mile traps. If your rear tires are too tall, you could have more left on the table to getting that perfect ET/MPH.
Proper Sizing - Be sure you’re getting the right diameter gears for your Mustang as well. Most late model Mustangs are 8.8” live axle cars. Some V6 models have 7.5” rears, whereas 2015-2019 Mustangs have 8.8” IRS-specific rear gears. Be sure to double-check what rear axle you have before picking up your new rear gears.
Automatic Transmission Owners - If you drive a Mustang with an automatic transmission, another thing to keep in mind is your transmission’s shift points. If you were to swap your rear gears, your ECU will want to shift gears at the wrong time. Be sure to have a proper tune loaded onto your Mustang to support these changes before you do any kind of serious driving.
With that said, you can solve a lot of these issues, such as transmission shift points and speedometer calibration, by adding a tune that supports these changes on your Mustang.