For example, what kind of drive you want, is it four-wheel drive, front-wheel drive, rear-wheel drive or all-wheel drive? Can you get more fuel economy? These are some of the questions that can be answered by the axle ratio and it is therefore important for you to know how to determine the axle ratio on your car.
What is an axle ratio?
This is the number of revolutions needed for the ring gear/driveshaft/output to make in order to turn the pinion/input onceor in simpler terms, makeone complete revolution of the wheel turn.
That number is expressed in a ratio represented bythe number of revolutions for the ring gear/driveshaft/ output divided by the number of revolutions for the pinion/input or the number of teeth on the ring gear divided by the number of teeth on the pinion gear.
For example, if a vehicle has a ratio of 4.10:1 it means that for every one complete turn, the pinion will turn 4.10 times. Often you find that people express the axle ratio as 4.10 instead of the ratio, but they mean the exact same thing.
the ring gear has, say 35 teeth on the ring gear and 8 teeth on the pinion, then the axle ratio will be 4.375:1. For every one revolution made by the ring gear, the pinion makes 4.375 revolutions.
In order to determine the axle ratio on a car, there has to be at least two interlocking gears. This is known as a gear train. The first gear which is also known as the drive gear is attached to the motor shaft while the second gear, also known as the driven gear, is attached to the load shaft.
In four-wheel drive vehicles, the axle ratio is the same for the rear and front wheels. You will find that in most cars you cannot change the frontal axle ratio but the rear one is adjustable depending on the top speeds or fuel economy one wishes to achieve.
Why is axle ratio important?
Axle ratio is important because it determines the top speed of a car, how quickly the engine is able to get there and how hard it needs to work to get to that top speed.
While still managing the performance of a car, the axle ratio also affects the fuel mileage so a fine line has to be tread to ensure an optimal balance between the two. Towing capacity in trucks is another factor determined by the axle ratio.
Some of the effects of the axle ratio on the vehicle are dependent on the whether the ratio is high or low. A higher axle ratio will give one better acceleration/torque while a lower one affects the fuel mileage as the engine will have to run economically so that it can maintain the given speed. This is done by decreasing the RPM.
Axle ratios can be adjusted or changed in order to compensate tire size changes.
Axle ratio and tire size
These two aspects go hand in hand in determining how your vehicle will behave. A great demonstration is given of Jared’s Ford truck. This demonstration shows just how tire size could affect aspect ratio hence throwing off the balance and how to correct it, in oversized tires. We see how 37-inch tires when paired with a 3.73:1 axle ratio instead of the original manufacturer pairing of 32-inch tires with the same ratio could have significant effects.
While it may not seem like much with the 5-inch difference from the 32-inch to the 37-inchtires, the ratio is reduced to a 3.19 axle ratio from the original 3.73.During manufacturing, the manufacturers ensure that the engine and drivetrain work together to give you maximum power in the most efficient way possible.
When this is interfered with during incorrect tire size and axle ratio pairing, the revolutions per gallon of the engine and gearshifts do not coincide as they should. The prime power range is thrown off due to lower rpm and fuel efficiency will greatly decrease because the engine is required to work overtime trying to make the same power as it did before.
How to Determine the Axle Ratio
Knowing how to determine the axle ratio of a car is important because for one it saves you expenses and two, it can help in making wiser decisions in figuring out how much gear ratio should be added or if you need any change at all.
There are two procedures that one can use to find out what the axle ratio of the car is. But before you can start on any of them, you need to ensure your safety and that you have the all the equipment for job ready.
- Chalk/ paint pen/ marker/ masking tape
- Wheel Chocks
- Hydraulic floor jack
- Jack stands
- Differential cover gasket
- Differential oil
- Oil catch pan
- Ratchet and sockets
The steps to follow are:
You need to make sure that your car is well positioned to avoid any accidents. First things first, make sure that your car is parked on a hard and level surface. It needs to be flat without any debris that can cause any interference.
Next thing you want to do is place those wheel chocks around the front wheels and then use the floor jack to start lifting the car. Make sure to use the recommended manufacturer’s lift points to avoid damaging the car.
Once the car has achieved a good lift, place the jack stands at their points and lower the floor jack. You now need to put the car in neutral to allow free movement of the car wheels. Having ensured that all those steps are in place, it’s time to get comfortable under the car.
- Position yourself strategically under your car so that you are comfortable enough to grab the driveshaft at the same time make sure that you can see inside the rear tires.
- Get your marker or chalk and draw a visible mark on the driveshaft and another mark on either of the rear tires. If possible, make sure that they are aligned and be sure to watch these two markings during the whole procedure.
- Start spinning the driveshaft slowly by hand as you count the number of revolutions it makes by watching your marker. Concurrently, count the number of revolutions of the tire.
- Keep doing this until you are confident that you have a sure record of the revolutions being made both by the driveshaft and the rear tire.
- Once you have your figures, simply divide the number of revolutions made by the driveshaft, by those of the tire.
If for example the number of revolutions of the driveshaft are 18 and the number of revolutions made by the rare tire are 6, then
A.R = 3:1
This means that it takes three revolutions made by the driveshaft to complete one revolution of the tire.
With the procedure discussed above, while it will give you results, those results will be approximations of what the Gear ratio is but it won’t give you exact values.
In this procedure, the preparations remain the same but the steps change and are as follows:
- Get your oil catch plan and place it strategically under the rear differential cover.
- Notice that there are bolts on the differential cover. These need to come off so make sure to use the appropriate sockets and ratchet to remove them and be sure to place the bolts at a safe place so that none of them is misplaced.
- Remove the rear differential cover allowing the oil to drain in the oil catch pan.
- The ring gear will be easily seen after the oil has drained. Start spinning the ring gear slowly and if you don’t mind, feel the ring gear with your finger for numbers which have been stamped on it.Those are your actual gear ratio numbers, no math required here.
- Now, time to clean up. Make sure you thoroughly clean the cover mounting gasket on the cover and also on the housing.
- Replace the differential cover using a new gasket, tighten the bolts this time using the torque wrench for tightening.
- Refill the oil to the required fluid level.
Note – make sure you consult your service manual for tightening the differential cover bolts and see what specifications are recommended for the fluid level by the car manufacturer.
After checking the gear ratio/ axle ratio, you need to set back your car in its original state.
Start by placing your car in park mode and then apply the parking brakes. This is to ensure that once the car has been lowered, it doesn’t start taking off on its own will. Remember that we had the car on neutral to allow free movement of the wheel so that we could count the number of revolutions.
Once you’ve done that, take your floor jack and start using it raise the car higher than the jack stands so that you can easily remove them. After removing them you can now start lowering the car to the ground and remove those wheel chocks we had placed earlier.
Some people prefer to go through this whole process by themselves and if you can actually do it then the better. You’ll get a deeper understanding of how your car operates and feel more in charge. However, there is no harm in getting your mechanic to come and do the procedure for you. The most important thing is getting the actual figures and making decisions for your car based off of those numbers.
Steven Reilly is a qualified mechanic and his passion for cars goes beyond just the technical aspects. He is also an amateur racer and all round car enthusiast. When he is not driving them, he can often be found in his garage under the hood of a rare model. Steven Reilly has lost track of the number of hours he has spent setting up his fine collection of rebuilt models. He believes that cars can provide a constructive and fun opportunity to teach the youth important life skills. In line with this, he is developing a community outreach program, potentially dubbed ‘Cars for change’.