Axle 101 Part III : All About Gear Ratios

[ExplorerDMB]

Gear Ratios

I decided to do another part, which dealt heavily on gear ratios and choosing the right one. It is a common problem that a lot of off-roaders have and even some street machines have. The wrong ratio can ruin your performance and sometimes isn't the cheapest upgrade (depending if you install it or have it done professionally).

Gear ratios are the relationship between pinion gear teeth and ring gear teeth. Some calculation will have to be done to find out which gear ratio will be perfect for your rig. The calculation is your new tire diameter, divided by your old tire diameter, multiplied by your old axle ratio, will equal your new axle ratio.

Example:

I had 3.55's in my axles stock and went from a 29" tall tire to a 33.4" tire. Now, when putting those numbers in the formula, you get 4.08 -- which is a 4.10 for the Dana35 and Ford 8.8"

If you would like to take the easy way out and have a computer generated formula figure it out - go here: Gear Calculator


Now, that you have figured out your ratio - why in the world do you need to change the stock ratio to the new ratio? Well, if you decide to keep your stock gears with say 33's, you will quickly realize that your "get-up-N-go" isn't exactly what it use to be. You'll also notice your engine working a little harder to get up to speed and you'll be wondering the whole time exactly how fast you are going. You will more than likely see premature failure of your transmission because of the heavier load of the tires.

Keep in mind my example above with my truck, the 4.10's would have put me back into stock range and the vehicle would perform as a stock Explorer would. Now, this is where a little "give-N-take" comes into play, you can either give up a few MPG for some more grunt and low-end power, or you can get the extra few MPG and be able to cruise on the highway all day long without hitting your top speed too early or running 3,500RPM at 70mph. I went with a 4.56:1 ratio (which is a little more aggressive in the low-end power department) and this allows me to get up to speed quicker but run a little higher RPM during normal driving. This is where you loose your gas mileage. Now if you went with the 4.10s, that is where you will have ok performance off the line, and have a nice drive down the highway. I recommend to always go to the higher ratio (numerically) to get a little more torque on the trails which is always needed!

So your going down the road and you see the ol' blue flashing lights behind you. You look down and your only going 45mph in a 45mph zone, but then you think "crap, my speedo must be off." Well, that is another draw back to upgrading tire size and not changing gear ratios. If you change ratios it should get you close to the right speed, but it always good to replace the gear on the end of the speedo cable (91-97 Explorers) to the right one or to reprogram your PCM (98+ Explorers). For the computer this will be calculated in revolutions per mile. To read more on reprogramming your PCM Read More HERE

There are other ways to correcting your speedo - you can get a computer chip (via Bama Chips) which will change your shift points and even your speedo. You could also go through Superlift with their Tru-Speed calibrater which works within 2%.

Speedometer Cable Gears:

Tru-Speed:

Gear ratios aren't too hard to pick when you start to understand why and what effects will happen with certain ratios. Now, for the ones that are still a little lost: the higher the number (4.88:1 or 4.56:1) the lower range/more low end torque. The lower the number (3.08:1, 3.27:1) the higher range/more top end torque. And when I say you will lose your top end if you gear higher (numerically) than you should is because when you start to get up to the 70-75mph you'll notice your running a higher RPM which will hopefully be enough to tell you to back off the throttle and go slower. Like I've said though - it's a game of give-N-take. Good luck on selecting your gear ratio.

Note: Not all ratios will work in Explorers, Rangers, and Mountaineers

Ratios for the Ford 8.8": 2.73:1, 3.08:1, 3.27:1, 3.31:1, 3.55:1, 3.73:1, 3.90:1, 4.10:1, 4.30:1, 4.56:1, 4.88:1, 5.13:1, 5.71:1, 6.14:1

Ratios for the Dana35: 3.07:1, 3.27:1, , 3.55:1, 3.73:1, 3.92:1, 4:10:1, 4:11:1, 4.56:1, 4.88:1, 5.13:1

BOLD: These are ratios that can work in 4x4 vehicles together.
Italics: These ratios possibly need modification to fit the slide pin in.

Important: If you are doing a Solid-Axle Swap (SAS) and the gear ratios are within 1-2% numerically, this is ok.


NOTE: This doesn't exactly go with Explorers, but some axles have a thing called Carrier breaks. This is when only a few ratios will fit on this carrier and another one will be needed if chaning to a different ratio. To read more on ratios click here!

-Drew


LINKS:

Axle 101 Part I
Axle 101 Part II
Axle Decoder - Find your Gear Ratio

[Hitchhikingmike]

Cool, very useful information. I enjoy learning about this stuff even if I never work directly with gear ratios.

[Hitchhikingmike]

What all work goes into changing the gearing ratio? Is it really that hard? I mean could a shade tree mechanic do it or is there a lot of calibration when putting it back together?

Thanks.

[matter2012]

how do i figure out what gear ratio i have on a 1998 ford explorer sport with the 4.0 sohc

EDIT: i found it haha sorry guess i should just look it up before i ask the question

Thanks

[johndi1962]

Hi
Do you not have to keep the RPM`s in a range where the engine is in its optimum RPM range so it is making enough power. So if you went to big on the tires then you would have no choice but to do a gear change.

Thanks

[draven822k]

Very useful!!! . Last sunday I was talking with a friend about this topic because he said he felt heavy his truck with his new big tires. I'm going to show him this post.