Differential Gear Ratio determines the number of times the drive shaft (or pinion) will rotate for each turn of the wheels (or ring gear). So if you have a 3.73:1 gear ratio the drive shaft turns 3.73 times for every turn of the wheel.
Gear ratio is calculated by dividing the number of teeth on the ring gear by the number of teeth on the pinion gear. The higher the number, the lower the ratio: a 5.29 gear has a lower ratio than a 4.10 gear. With a lower gear ratio the drive shaft (and thus the engine) turns more for each revolution of the wheel, delivering more power and torque to the wheel for any given speed. Lower ratios are generally desirable when going off-road. Higher ratios are better for freeway driving since they run at lower RPM?s and offer better fuel economy.
Changing tire size affects the final drive ratio. Switching from a 30" tire to a 35" tire changes the final drive ratio by about 17%. This may drop the engine out of its' "power band" and result in poor performance and fuel economy. To restore performance you must change the gear ratio to compensate for the change in tire size. If you originally had 3.07 gears you need a ratio that is approximately 17% lower, such as 3.55. If you want to increase off road performance you might want a 4.10 or lower ratio.
Generally you can expect one day down time. We schedule two vehicles a day, and stay about a week to a two weeks booked. Occasionally we have a vehicle hung in a rack from the previous day with an unexpected issue. Most time this can be worked around. The actual job takes the better part of one day. When we schecule your vehicle we will check to make sure all parts are in stock. If it is a vehicle that we do not stock parts for, or a special order part, we do require a deposit on those parts. We carry all the rearend parts for most of the common makes and models.
What gear ratio do I have?
Jack up one tire if you have an open diff, or both tires if you have a working posi or locking differential. Rotate the tire one full revolution for posis and lockers and 2 full revolutions for open diffs. Carefully count the number of full revolutions the driveshaft makes. This is your gear ratio. In other words, if the drive shaft turns 3 ¾ turns, you probably have a 3.73 gear ratio. Turning the tire for twice the number of full revolutions and dividing the drive shaft revolutions by two will give you a more accurate reading. This should be accurate in most cases. Fords have tags on the diff. that will give you the ratio and tell you if it is a Limited slip or not. Mast Dana's have a tag also.
Do I have a posi or a locker?
Put the transmission in neutral and jack up both rear tires. Turn one tire. If the other tire spins the opposite direction you have an open differential, and if it spins the same direction you have a posi or a locker. Again, in most cases this will work.
Before You Call For Rearend Parts
It may sound silly, but there are many things to consider and lots of data to collect prior to picking up the phone to find rearend parts. Unlike engine or suspension parts, rearend parts are not easy to look up, and can vary a lot for any vehicle. Just because it has a specific engine size or transmission type does not mean that the vehicle uses one specific gear ratio. Vehicle manufacturers will usually offer at least two gear ratios for any given vehicle model. The gear ratio can vary with little or no correlation to its intended use or market area. I have seen many models released that offer four different ratios with little or no other variations in the vehicle. Another thing that complicates matters is there are really no good application books that a salesperson can use to look up the right parts for a vehicle by year, make, and model. And, for whatever reason, the VIN number is not helpful, as on most vehicles it does not give any information on the differential gear ratio or limited slip options.
One way to get the right parts to the end user is by process of elimination. In this scenario, the vendor sells parts that they think may be right until they find the right combination. This is often referred to as the "we got your credit card number and that's all that is important" method.
Don't count on your friends to identifiy the differential for you, as they may not be accurate. If the diff is not the model they say it is, there may be restock charges for sending back parts. The only sure-fire way to determine the right parts for a rearend is to find someone who really knows rearends and be prepared to answer a lot of questions. Some of the questions to answer BEFORE calling are:
How do you use or plan on using the vehicle?
Year, make (Chevy, Jeep, Ford, Dodge, etc.), model (1/2 ton, Cherokee, etc.)
Old gear ratio
New ratio you think you want
Axle spline count
Number of cover bolts, or is it a drop out
Number of ring gear bolts
Bill of material number or tag numbers
Engine size (I hope you know this one, surprisingly a lot of people do not)
Engine rpm that you are comfortable cruising down the highway at. This will only work if you have a tach, and it may require driving in a lower gear to experience higher rpms.
Number of wheel lugs
Trailer Weight (For those of you who pull trailers or other vehicles)
Standard or reverse rotation front ring & pinion. This can be determined by whether the pinion enters above or below the axle housing centerline. Reverse rotation gears have the pinion shaft entering above the axle centerline and standard rotation gears enter the housing below the axle centerline.
Independent Front Suspension or solid straight axle in front.
Outer diameter axle bearings
It may seem ridiculous to have to answer so many questions, but in the end, it is far easier to order parts once correctly, than to order and return the wrong parts several times. If you take the time to find out the answers to a lot of questions, it will save a whole lot of frustration for everyone involved.