Differential ratios  3.73, 4.10, 4.88…

What’s your differential numbers?   3.55, 3.73, 4.10, 4.88.   Often times these numbers are overlooked when buying a truck.  However, depending on how you’re going to be using your rig you may want to take a closer look. These number reflect your towing capacity and will affect your fuel economy.

What do these numbers mean?   It’s the relationship between how many times the drive shaft rotates in correlation to one revolution of the axle/wheel.  A 3.73 gear ratio means the drive shaft will rotate 3 . 73 times for every one revolution of the wheel.  The step up or down in gear ratios may seem incremental but the difference is noticeable when driving and at the fuel pump.

An article I wrote about oversized tires, gearing and MPG.

In basic terms, a lower gear ratio coincides with a lower RPM while driving at speed.  Lower RPM correlates to better fuel economy.  Great for the truck you drive to get groceries.  However, when you need a work truck you’re compromising  your towing capacity.  In this case a lower gear ratio lowers the amount of weight you can tow.  At the other end,  higher number such as 4.88 will significantly increase your RPM’s.  Which will cost you more at the pump but will give you more towing capacity.

Here’s a rpm/speed/gear ratio calculator you can use to see some actual numbers Randy’s Ring & Pinion

There are other factors such as engine size.  Type of engine, gas or diesel.  Engine configuration V8 or I6.  Transmission ratios and number of selectable gears.  Automatic vs manual.  However for this article I am going to focus on the 6,001 through 19,500 GVRW class range trucks that most of us drive.  Plan on writing an article later on trucks and weight configurations.

I.g, my 2000 F450 with a GVW (Gross Vehicle Weight) of 15,000 lbs is a class four truck.

Truck weight class

Truck weight class

Visit Equipment page for a short video on replacing a cam shaft position sensor on the old 7.3

You then need to asses how you’re going to be using your truck.  A daily driver that you may pull a boat with on the weekend should do fine with a 3.73.  However if your planning on pulling a 5th wheeler a long distance you best opt for a 4.10.  Terrain is another consideration.  If you use your truck for local work on mostly flat terrain you may be fine with a lower gear ratio.  Here in the hills of TN, and being in the landscape business, I always opt for the high gear ratios.

When you buy a truck, new or used, you should  verify the numbers yourself.   Believe me, most people don’t know anything about trucks!  There are two ways to check.  The VIN tag or crawl under the truck and locate the actual differential tag.  Below is the VIN tag on my little ranger.  The axle code is R7.  With a  simple Google search you can find a chart of axle codes.  I.g.  R7 is a limited slip with a 4.10 gear ratio.

Differential ratios 3.73, 4.10, 4.88...

Differential ratios 3.73, 4.10, 4.88…

Can you change your gear ratios?  Sure, but it’s usually not worth it.  Prices will vary.  Yet I would plan on paying in the neighborhood of a $1,000 + per differential.  It’s also a job I’d advise not doing yourself.  There’s more work involved then what you may think.  The gearing must be properly calibrated and shimmed otherwise you may damage your investment. With newer trucks you may have to consider adjusting some of the engine and transmission programming.  That’s why I wrote this article.  It’s best to know up front what your getting vs. trying to swap out gears later.  The other thing to consider is resell along with correct documentation.  Unless it’s a truck your really attached to I’d recommend trading up or down to find what your looking for.  For the most part a truck will do what you ask of it in the first place.  Just because it doesn’t pull your 5ft wheeler up a hill as fast as you like doesn’t mean its not a good truck.

In closing, most trucks will accomplish what you need it to.  However, if you’re asking more from your truck then most I would recommend looking into these numbers.  Either check the ratio yourself or have a trusted mechanic look it up.  The individual your buying it from may not have any knowledge of the gear ratio.