@DangerousDan1090 The axle ratio (also known as final drive ratio) refers to how many times the drive shaft has to rotate in order to make the wheel spin all the way around 1 time. It is usually shortened to just the number of drive shaft rotations for simplicity (so rather than 3.23:1 or 3.73:1, it's just written as 3.23 or 3.73).
Once you post the door jamb sticker, the fellas here will be able to tell you what your Explorer's factory axle ratio is, and that will basically tell you whether or not you NEED to change it. But basically, we're going to tell you to change it, because we here are torque geeks!
The reason it matters is that when the tire is replaced with a taller tire with a larger circumference, the engine has to apply more rotational leverage (torque) to the drive shaft in order to make the wheel rotate because (1) the vehicle is now traveling a greater distance per one wheel rotation, and (2) larger tires tend to be much heavier, so they have more inertia (and thus more rotating mass, which will affect braking, and more unsprung weight, which will affect suspension performance, but those are a separate discussion).
So, what can you do about it? Starting with some simple math helps.
Let's use made-up numbers to make it simple, then you can plug in your own Explorer's numbers to figure out what you want to do.
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Tire size in inches / Gear ratio = "Strain Factor"
I must advise you firstly that this "Strain Factor" is a made-up metric that nobody in the world uses besides me. It's just a helpful figure in understanding the design parameters of the machine, and calculating torque loss and fuel consumption (which we'll discuss later).
So with 29" tires on a 3.23 axle ratio, that looks like
29 / 3.23 = 8.97
When Ford designed the vehicle, they decided that this was a good idea. But they also made Explorers with 3.73 axle ratios, so let's look at that next.
29 / 3.73 = 7.77
Why do we want to know this? It helps us to grasp what Ford considers to be an acceptable amount of strain on the transmission, based exclusively on the metrics we want to change: gear ratio and tire size!
Acceptable SF = anywhere between 7.77 (lowest-strain factory configuration) and 8.97 (highest-strain factory configuration).
So if you want to run 31" tires and your Explorer has an axle ratio of 3.73,
31 / 3.73 = 8.31
You'd be within the acceptable SF range, but pretty close to the top end of it.
Now let's see what we get if we put 31" tires on a 3.23 axle.
31 / 3.23 = 9.60
Definitely more strain than Ford intended for the transmission. In fact, we're looking at 7% more strain than Ford intended. And that doesn't even factor in all the extra weight from the larger tires. A few dozen guys on this forum can tell you what this setup will do to the transmission, and it's not pretty.
In order to reduce strain, you can either choose less tire, or more gear. We all know what we're here for, so let's ditch that puny diff and swap in some 4.10 gears, for, let's call it science.
31 / 4.10 = 7.56
Less strain than the original design? Yes, please! But remember that this doesn't account for all the extra weight from those fat rubbers, so your practical strain is going to be higher than that. It puts you in a much better spot.
Now, what does 33" tires on a 3.73 axle look like?
33 / 3.73 = 8.85
That's hazardously close to the high SF limit. It's been done before, but the guys on this forum can also tell you that the extra weight from all that tire kinda takes the life out of the truck.
33's on 3.23 gears, let's break stuff!
33 / 3.23 = 10.22
Waaaay outside the ideal range. Has it been done? Probably. Is there a reason that it's not a popular choice? Absolutely!
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So, now that we have established this abstract "Strain Factor", we can use it to calculate the percentage of change in RPM at any given speed between the old configuration and the new configuration, not accounting for change in tire weight.
(Old Tire Diameter / Old Gear Ratio) / (New Tire Diameter / New Gear Ratio) - 1 = Expected % Change in RPM
(Old Strain Factor) / (New Strain Factor) -1 = Expected % Change in RPM
So, let's say you're changing from 29" tires on 3.23 axles to 33" tires on 4.10 axles.
(29/3.23) / (33/4.10) - 1 = 0.12 = 12%
Since this is a positive number, we're looking at a 12% increase in RPM at any given speed. She'll be thirsty, but she'll be happy!
However, if you changed from 29" tires to 33" tires without changing from the 3.23 gears, like we suggested earlier...
(29/3.23) / (33/3.23) -1 = (8.98) / (10.22) - 1 = -0.12
That represents a 12% reduction in RPM at any given speed. It will be hard to get the engine into the ideal RPM range to deliver a good amount of torque, and the transmission won't want to shift until the vehicle reaches a really high speed. This makes the vehicle feel sluggish. The engine will be trying to spin, but the tires won't want to, and that is going to chew up the transmission really fast.
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So, I hope this helps you to grasp all the ins and outs of the matter. Basically what you need to remember about all of this is that the axle ratio is the number of times the drive shaft must spin all the way around in order to rotate the wheel all the way around once.
3.23 ratio = Drive shaft spins 3.23 times for each full rotation of the tire
4.10 ratio = Drive shaft spins 4.10 times for each full rotation of the tire
5.13 ratio = Drive shaft spins 5.13 times for each full rotation of the tire
However, against all that should be logical...
"Lower" or "shorter" gear ratio refers to a higher number of drive shaft rotations per one wheel rotation.
"Higher" or "taller" gear ratio refers to a lower number of drive shaft rotations per one wheel rotation.
If you want better torque control and less strain on the engine and transmission, it must always come with an increase in fuel consuption and a reduction in top speed.
), but I keep reading about gear ratios and I need someone to explain them to me like I'm a baby. Thanks in advance!
