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Rear End Gears: What's Best For You?
by Chris “Shambles” Chamblee
A more common upgrade for many Mustangs is to upgrade to a higher ratio of rear end gears. However, this upgrade can have a fairly costly install price, so many times, picking a set of rear end gears is a one and done shot. So what is the best set of rear end gears to get for your car? Look no further, help has arrived.
The function of a ring and pinion gear is to multiply your engine's torque to get the car up to speed easier. All gears are numbered as a ratio, for example 3.27:1. This means that the drive gear (pinion) must rotate 3.27 times for the driven gear (ring) to rotate once. This is important in order to get your Mustang actually moving forward. A stock Mustang GT weighs in at about 3500lbs, its engine has a horsepower rating of 225hp and a torque rating of say 300 lb/ft of torque. Assuming the rear end gear has a 1:1 gear ratio, the engine, only making 225hp and 300lb/ft of torque, will have to try to move a 3500lb mass without any assistance throughout the driveline. The weight of the car, not to mention the lack of torque, won’t be able to get the car moving very quickly at all.
Ring and pinion gears are designed to solve this problem. A typical Mustang has a first gear ratio of 3.3:1, so by taking the engines torque (at the crank) of 300 ft/lb and multiplying it by 3.3, you now have 990 ft/lb of torque to help get the car moving. That's still not enough to make for trouble free take-off. This is where a ring and pinion gear comes in handy. A ring and pinion gear with a ratio of 3.27:1 will multiply that 990ft/lb of torque and transfer that power to the wheels. 990 ft/lb of torque multiplied by 3.27 gives a hefty number of 3237 ft/lb of torque to get you moving. Although much larger than a 990 ft/lb of torque, this amount of torque is still less than what the car actually weighs! When launching off the line, the car will take longer to speed up and longer to get into the cars power band. With a higher ratio of 4.10:1, your cars torque to the wheels jumps to an astounding 4059 ft/lb of torque! This gain of 822 ft/lb of torque means your car will bog less off of the line, and make a car that will be much more fun to drive (if you enjoy shifting gears a lot for that sporty feel).
However, please note: Gears do not give you extra horsepower or torque on a dyno. They have been known to take 1-5hp, but give back much more in the quarter mile as far as a lower ET goes. The 4059 ft/lb of torque is more of a measurement of force on the wheels rather than the torque that is actually being put through the driveline of the car. If you have 340 ft/lb of torque at the wheels, then you have 340 ft/lb of torque at the wheels, not 4000+ fl/lb of torque.
Mathematically, rear end gears make perfect sense, but these days, gas mileage plays a big role in everyone’s pocketbook. Simply put, rear end gears will raise your rpm’s when traveling at faster highway speeds. This can shed a few miles per gallon depending on how fast you are traveling. City driving, however, will see a raise in miles per gallon, as you can get into higher gears faster at lower speeds. For example, with 4.10 rear end gears at 35 miles per hour, your car can already be in fourth gear. Ten miles per hour more, and, with a 5 speed transmission, you can actually be into the overdrive (5th) gear. You will also have more pull in both 4th and 5th gears, making it easier to pass those slow Camaro’s in the left lane.
At the track, however, rear end gears can play a very different role. Most Mustangs are very dominant at the mid to top-end RPM range. Rear end gears will actually help keep your RPM’s higher when you go to shift into a higher gear, keeping you in your power band. Keep in mind, however, that when you finish a quarter mile run you might find yourself in 4th gear. Most 3 valve and 4 valve cars will benefit most from a higher gear ratio at the track, as they have a higher redline and tend to pull a bit harder through the upper RPM ranges than a 94-04 GT would. Too high of a rear end gear ratio, and you will move through your power band too fast, making you top out and actually lose time because you will be out of your power band before the quarter-mile is over. .5 second gains have been seen by just changing rear end gears on a stock 4 valve Cobra from 3.27:1 to 4.10:1 in the quarter mile.
Finding out which rear end gear for you is somewhat of a personal preference and somewhat of a financial decision. For 5-Speed vs. Automatic cars and for 2 valve vs. 3 valve vs. 4 valve cars, the recommended gear ratios can be more personal preference at best; so take the following into consideration when considering what gear is right for you:
The majority of Mustang owners will tell you to go with 3.73:1 gears if you have a V6 Mustang, 4.10:1 gears for a 2v Mustang, and 4.30:1 gears for most 3v/4v Mustangs. These numbers are recommendations that people have seen work quite well for Naturally Aspirated (N/A) engines, but don’t limit your choices to just this list. Many GT owners decide to go with 3.73 or even 3.90:1 rear end gears because they don’t want to shift as much than if they had a 4.10:1 rear end gear, but still wish to up the amount of torque that is being put to the wheels. In the end, the rule of thumb is “Don’t Fear the Gear.” Doing a 4.10:1 ring and pinion set in an automatic 2 valve Mustang won’t hurt anything at all, and will probably make your driving experience much more enjoyable.
Forced Induction engines tend to do better with a smaller gear ratio, as boost kicks in at a certain RPM, and you want to use as much boost in your power band as possible before having to shift. The best suited gears for this will probably be 3.73:1 and 3.55:1 ratios.
When installing gears you will need a few extra accessories. First of these is an installation kit. This kit contains everything you would need to install a set of ring and pinion gears, including shims, crush sleeve, gaskets, and so forth. The only thing it does not include is bearings. For the bearings you will need to purchase a bearing kit. Although more expensive, a bearing kit can be extra insurance for the future in taking care of your drive train. The normal rule of thumb here is if you have 30,000 miles or more on your car, install bearings. Otherwise, you should be able to use an install kit and reuse your factory ring and pinion bearings. If you have a Limited Slip Differential, you will need of friction modifier added to gear oil. Many high quality gear oils already include friction modifiers, such as Royal Purple 75w90 oil. Normally, 8.8” rear end gear differentials will need about 2.7 quarts of oil to be filled properly.
Installing rear end gears will also throw off your speedometer. There are a few ways to correct this, ranging from a simple speedometer gear, to a flash programmer to reprogram your computer to read your speed sensors differently. 1993 and earlier Mustangs require a speedometer gear in order to recalibrate the speedometer. For 1994-1998 Mustangs, a speedometer gear or a Speed-Cal can be used to reprogram your car for the change in ratios. Speedometer gears will normally run $10 to $20, while a Speed-Cal runs about $100. Many times, going with a Speed-Cal will give you a more accurate speedometer reading, as the speedometer gears can wear away over time. For any 1999+ Mustangs, you will need to either get a Speed-Cal or a programmer such as a Diablosport or an SCT X-Calibrator 2. The Speed-Cal can only be used for 5-Speed manual transmissions up to 2004, while the programmers can be used for both automatic and manual transmissions. These cars differ from their earlier counterparts because their transmissions have an electronic sending unit and a digital dashboard cluster. Because of this, there are no mechanical parts that can be replaced in order to modify your speedometer and odometer. A hand-held programmer can change the setting in your computer to accept the new rear end gears, giving you an accurate speed reading as well as a few more ponies if programmed for a higher octane gasoline.
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