Rear End Gears FAQ
Lately there have been quite a few threads asking about gears, what they are, what the numbers mean, what do they do and frankly there is way too much information to give just a simple answer. We’ll start at the beginning.
NOTE: The following information is based upon the ‘99-‘04 “New Edge” Mustang body style. However most of the information can be applied to the ’94-2010 Mustangs (SN95-S197). This information is also good for the first generation Mustangs as well as the ’86-’93 “Foxbody” Mustangs and in fact can be applied to many other vehicles. Some of the included information (e.g. GT and V6 rear axles may be model year specific). All other types of information can be considered “universal”.
What Are Rear End Gears?
Rear End Gears, commonly referred to as gears and hence will be referred to in this article, is a common modification for most cars. They are actually gears which are placed in the rear differential to change the driveshaft to rear tire ratio.
What Do They Do?
While they do not provide any actual horsepower, they increase the amount of torque at the rear wheels.
What Do The Numbers Mean?
The concept behind the numbers is not hard at all and can be described so anyone can understand. The numbers you’ll hear is usually shorthand to display the ratios. For instance when you see 3.73 this is supposed to be the ratio, which is always to one (:1) Example: 3.73:1. Same goes for 4.10 and 4.60 which really means 4.10:1 and 4.60:1 respectively. The ratio indicates how many times the driveshaft will turn as opposed to the rear wheels (driveshaft : rear wheel).
We’ll put this idea into play. Let’s say there is a 3:75:1 gear (or the actual closer one of 3.73) and it just happens to be installed on your car and you’re looking at both the drive shaft and rear wheels as both slowly spin. They don’t spin at the same speed; in fact you would notice the driveshaft spins a bit faster than the rear wheels. Every time the drive shaft turns 3.75 (3 and ¾) times your wheels will spin once. That’s how we get the numbers. If we had a 5.00:1 gear ratio the axle would spin 5 times for every time the wheel makes one complete turn. You can actually turn the driveshaft with your bare hands and watch the tires spin with.
Here are some problem questions if you want to test your true understanding. All questions involve 3.73 gears and yes some math is required.
1) How many times will the drive shaft turn if you rotate the tire once?
2) You spin your driveshaft once, how many revolutions does the tire make?
3) You rotate your wheels 3.73 times, how many times will the driveshaft turn?
Answers:
1) 3.73 rotations for the driveshaft. We are just turning the wheel instead of the driveshaft, whereas instead of turning the driveshaft to see the wheel turn, as was mentioned in the above paragraph.
2) This is a little tricky since it will involve a fraction, which would be 1/3.73 or 0.268 revolutions. Basically the tire would get just past a quarter of a revolution. It wouldn’t even make a complete turn!
3) 13.913. We are just multiplying the :1 factor, so we have to multiply the 3.73 factor. 3.73 x 3.73 = 13.913.
Pros/Cons of Gears
The only thing gears really do is increase the power to the wheels which means faster acceleration at a stop. They can also help when passing slower vehicles.
The downside to gears is that they can affect a vehicle’s mileage for better or for worse. Chances are you will see a small decrease (1-3mpg) in fuel mileage if you go by the previously mentioned gears for Manuals and Autos. Increasing the gear ratio will generally decrease the mileage. Some owners have stated that they have not seen any change in gas mileage and some have seen an increase as opposed to the community’s average. The biggest factor to gas mileage is what it has always been and that is the driver’s habits. A driver whose actions would decrease their fuel economy will suffer. Those who are easy on the gas and easy on the breaks may see that increase in mileage with or without gears. Of course that doesn’t mean having some fun once in a while will hurt.
Who Can Use Them?
Any vehicle can use them. It does not matter if you drive a V6, V8 or any special Mustang (MACH 1, Cobra/Shelby) or even one of the Foxbody with an I4 engine. The 4.6L and 3.8L Modular engines can benefit from gears more than the 5.0L Pushrods mainly because the 5.0’s power band is low in the RPM range giving it plenty of torque. While the Modular engines power range is a bit higher on the power band and thus the extra gears help in the low RPM’s to increase torque.
Which Ones Do I Get?
The biggest question we hear is what Gear Ratio should the driver get. The quickest simplest answer is “Whichever one you want to get”. Now there are general guidelines members suggest. Often you will hear about two of the ratios which are 3.73 and 4.10. Mustang owners will suggest that if you have a Manual transmission you install the 3.73 ratio. For those who drive an Automatic at least a 4.10 ratio is suggested. Stock gearing for the newer cars is 3.27, although some came with 3.55. Any number above this would be considered an improvement. Now for manual transmissions it is not suggested to get 4.10 gears or higher because, basically, you will have no use for a first gear. Some do recommend getting the 4.10 gears while others will say to install the 3.90 ratio for a compromise.
The only other difference comes to which rear end axle you have. V6 Mustangs have a 7.5” rear end, unless somebody swapped in the GT axle. GT, Mach and Cobras have the 8.8” rear end. If you are still unsure you can measure the differential across diagonally. Most can differentiate between the two sizes simply by looking at the differential cover which are notably different. Other than being bigger the 8.8” is a bit beefier. The IRS has a noticeable notch comes out of the casing to form over the interior gear.
As for brand name the highest quality gears we have come across come from both Ford Racing (FRPP) and Motive. Many of the other brands tend to whine.
[IMAGE] [IMAGE]
4.10:1 FRPP gears on the left, 3.73:1 Motive Performance gears are on the right.
NOTE: The 2003/2004 Cobras have an Independent Rear Suspension (IRS) and while the gears in do not matter the differential uses a different cover as opposed to the other models which have the solid rear axle (SRA), also known as a live axle.
As for the gear ratio there is a pretty extensive list to choose from. You can get from Motive some of the following ratios: 3.55, 3.73, 3.90, 4.10, 4.30, and 4.56 for the 8.8”. The 7.5” is really limited to the 3.73 and 4.10 gears.
What Else Do I Need?
Usually an install kit is recommended which includes a new seal, oil and new bearings and some friction modifier especially when the car is after it’s “break-in period”. The friction modifier is used for vehicles with a Limited Slip Differential (a.k.a. T-LOK). A speed calibrator or a new or upgraded tune is also usually a need. New gears will offset the vehicle’s speedometer. One account was 7mph for a New Edge (’99-04) GT. Older cars (those with Mechanical Speedometers) can also accept new speedo gears inside the transmission. The new speedo gear is said to be not as accurate as the speed calibrator. The calibrator is also much cheaper and easier to install. Speedo gears involve rebuilding the transmission. A new tune will be a good decision as well if a person decides to do more modifications to the car, although even a stock Mustang will heavily benefit on a new Tune. Some shops which do tunes, as well as reputable online sites (such as American Muscle) offer a free custom tune for life (*of the vehicle). The best option for a new tune however, always involves bringing the car in where they can tune it in person. Cars which do not have ECUs (Computers or that are Fuel Injected) cannot be re-tuned.
Can I Install Them Myself? How Much Do They Cost?
The job to install them is very time consuming and hard physical labor. The actual process of installing them isn’t a difficult concept to understand however. It is best to get the job done at a shop, which will be discussed in just a bit. There are a few articles which are floating around on the Web on how to install them, however more than likely these installers know what they are doing and have the proper equipment to perform the install.
If you would like to see the process involved you can take a look at American Muscle’s article, New York Mustang’s Article and a personal experience on another forum.
All information is regarded as for New Edge Mustangs, unless otherwise stated.
Mustang Differential Gears - Installation Instructions at AmericanMuscle.com http://newyorkmustangs.com/forums/ho...ears-t667.html http://www.stangnet.com/mustang-foru...-own-gears.htm
D.I.Y’ers: Special tools and measuring equipment is needed, as well as some heavy duty presses, breaker bars, and torque wrenches. The install must be done carefully and caution and accuracy must be taken into account with each step of the install process. Along with learning some new vocabulary like lash, heel, and toe. The concept is easy, you are just taken out a set of gears and putting in new ones, along with many new or refurbished parts (bearings, shims, crush sleeve(which will be new)). The rear axles will have to be removed, and the process will go back forth from front of the differential to the rear throughout the install. However the driveshaft must also be disconnected and the differential set up just right and then the shims must be inserted correctly for the right “lash”. Improper lash will cause a whining sound from the install as well as adverse affects to the rear end. Of course this isn’t the step by step process, but an oversimplified view of what it involves. This will basically be the last you hear of a DIY gear install in this article, and perhaps on many forums. Those curious can always ask. There are residential mechanics and gear heads that reside on these forums, as well as many who just have a wealth of information.
It is always highly recommended that owners go with a reliable, experienced shop. Not doing so can cause problems. The shop should also offer a warranty on their installation, just in case the mechanic is off on a measurement you can go back in and have it corrected and not worry about paying for all the labor again. Often an improper crush sleeve has cause problems (noises) and you will much rather want someone else to do that labor then you personally want to do.
There are some other upgrades you can plan on which will be discussed in just a bit.
Shops: It is always hard to quote prices especially at a time when an article is written that may exist for years. Currently in the United States during the 2009-2010 year, the gear install has been quoted to be anywhere from $200-$1000. That’s a pretty big gap. However you can expect the higher prices to include parts, labor and maybe even an included warranty. Now for those who are looking at the $200 price range; the price takes (or doesn’t take) into account a few things. For instance, the gears may have been purchased separately from the shop, or the gears may even be second hand (i.e. used). The $200 price tag is much likely just the cost of the labor. Now before you go through the yellow pages looking up these places, these installs are usually so cheap because the car owner is best friends with a mechanic, and the mechanic being a good friend offers a great deal.
How Do I Tell Which Gears I Have?
So you bought a used GT, perhaps with some simple modifications and you aren’t used to the V8 engine. Could you have gears already installed on your car? Or you want to know if you should go big from here or just a slight improvement, but how do you tell? Or you have used gears out of your friend’s wrecked Mustang (knock on wood) but you don’t know what ratio they are, so how do you know to use them or sell them? Well there are actually some pretty easy methods to find this out.
We’ll start with those used gears since they are already out of the car and we will be very easy to tell. The bad news to this method is that you have to do some counting and division. That’s the hardest part. So here’s what you do: There are two parts to a gear set – the ring and pinion and if you haven’t already guessed the gear which looks like a ring is the ring, and the pinion is the gear which looks like a pin. Now perhaps you can’t read the Manufacturer and Part# on the gears (you could also start there in identifying them) so you need your Math skills. First count all the teeth (those peak-like things) on both the pinion and ring. Divide the number of teeth on the Ring by the number of teeth on the Pinion (i.e. Ring/Pinion). So if you have 41 Ring teeth and 10 Pinion teeth; 41/10 = 4.10 and that is your gear ratio. You can even try this on the FRPP gears pictured above (the Motive image may not be clear enough). You will probably see this set ratio between the gears, meaning 4.10s will normally have 41 Ring teeth and 10 Pinion teeth.
What if you think the gears are already installed? What can you do? It may be possible to open the differential cover and read the engravings on the gears, but then you will probably make a mess and then have to add new fluid after since the old fluid will have to be drained out. There are other ways though.
For the first method you will need a jack and a friend (NOTE: a friend named Jack will not “kill two birds with one stone” you will actually need these two things and a friend who is a jackass should also be avoided) and a Sharpie marker or some type of instrument that will make a mark. Using all safety precautions, chock the front wheel and carefully raise the rear end enough so that a person can slide under it. Release the parking brake and leave the car in neutral. One person will slide under and make a mark on the driveshaft, then slowly rotate the driveshaft counting how many times it rotates by using the marked reference on the driveshaft. Do this all while the second person watches the wheel and allows it only to spin once. When he stops the wheel, the number of rotations of the driveshaft is the gear ratio. Be forewarned that the number may be uneven and the exact point it stops may be obstructed from view, so try your best to get an approximate reading. You can also do it backwards and have your friend rotate the tire once (extremely slow: remember the driveshaft spins faster than the tire) and count the rotations of the driveshaft this way. You can do it both ways and compare readings and repeat until you feel you have an accurate reading. You could also do the math as mentioned above in the What Do The Numbers Mean section of this FAQ to double check any numbers or be thorough on this test.
What if I told you there was an easier method? You may be saying “Joe, what is easier than what you’ve told us? You are already so smart and handsome that there is no way you can make my life easier.” Well my completely correct and accurate reader, it is so. Now I do advise CAUTION on the next test since you will actually have to drive the car! Well that’s not the cautionary part, the cautionary part is that you will have to get up to between 70-75 MPH on a designated highway. I don’t condone anything illegal here, but if you do anything illegal I recommend cocaine and hookers. Speeding tickets aren’t any fun.
So as you are driving on the salt flats or a federally funded road that has an 80MPH speed limit you will want to cruise anywhere from 70-75 MPH in 4th gear (4th gear in the transmission is a 1:1 gear ratio, you don’t have to currently know this). I would say start at 70 and gaining speed up to 75 MPH read your RPMs. Whatever the RPM your car is spinning at, well you guessed it that’s your gear. Depending on the speed of the vehicle the RPM may be a little higher or lower but a general guess can be very accurate here. This test is so simple you can do it on your way picking hookers on your way to your Coke dealer. This is also why it’s important to obey speed laws, you don’t want to get pulled over for speeding with a hooker doing lines in your car. I’m not a lawyer either but I’m pretty sure there’s a felony in there somewhere.
Further Advancements to the Rear End.
While you are basically dismantling your whole rear end, it becomes a great time to improve on even better concept.
This will mostly apply to those who are driving a V6 but all owners can build of the basic ideas. Previously we mentioned how the V6 rear end is a 7.5” differential, but there is always a seen improvement here by upgrading to an 8.8” rear end. Now any V6 owners who have done a burnout may notice how only one wheel will actual spin. Believe it or not it’s because only that one wheel is getting any power. The other rear wheel? Well it just rolls along when the car moves. So what to do? Get a T-LOK (Limited Slip Differential; LSD). Get the 8.8” T-LOK one as well (can be aftermarket or from a GT). You will have a beefier rear end as well as those two wheel burnouts. The install price shouldn’t be any more for labor since the new gears will have to be installed anyway, the mechanic can install it off the car and install the differential altogether with gears included. You probably just made his job easier (I wouldn’t actually know, I’m not a mechanic, they may like cramped spaces).
Now it’s also a good idea to replace any wearable parts at this time. New bearings, U-Joints for starters. If you want you can even get an aluminum driveshaft and safety loop and maybe even new axles, perhaps those 31-spindle ones. Or you could try to find an IRS for the car and install those gears and then install it onto the car. The install is a near direct swap.
Bearings and U-Joints are definitely possible, drive shaft is easy enough for another day. New spindles aren’t necessary, and that Cobra IRS is an expensive dream (but possible). The purpose here was to try and show you just what you can do.
Some Legal Mumbo-Jumbo I Use To Cover Myself Original article written by Joe O’Rourke (OsirisGuy) with the help of all ME regulars and some non-regulars. Dated 09/22/2009
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Roxie
Rear End Gear FAQ 
Lynn Marie
2003 Mustang GT
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2003 Ford Mustang Cobra Convertible
