As we’ve been discovering, every part of your car needs to work in unison with all the other parts of your car. If there isn’t a mesh between your components, your car is gonna suck going down the track. As usual, you need to know what you want your car to do. This rule is no different for the gearing in your rear differential.

So, do you want it for the track? For the street? For both?

The gear ratio is perhaps one of the most overlooked aspects of modifying a car. Newbies tend to think, “okay, all I need is a huge motor, a manual transmission, some decent tires and a way to stop and I’ll be good to go.” Wrong answer.

Photo Credit: Mecum

What the gears do:

The gear ratio you have out in that pumpkin under the backside of your car decides how your car will run in a few different ways – some of which you may not even be aware of. For example, you could have the biggest motor you could stuff up front, but if you have a set of gears with a really low ratio number in the back, it’ll move like a golf cart off the line. But, at the same time, you may get a really high top speed from it.

Something else that your rear end decides is how good your fuel economy will be. I know most gear heads don’t think about fuel economy, but I know I feel the pain at the pump (mostly because my baby gets 7 mpg). If it’s your daily driver, and you are geared to get up and go off the line, your gas consumption is up there with a dragster (well, maybe not that high, but you get the point).

How to pick a gear ratio:

A lot of thought goes into picking the rear end gears. A lot of it depends on the rest of the car, and how it is set up. To illustrate my point, let’s look at my 1967 Chevy C20 pickup:

The truck has a 375 horsepower 350 with a decent cam and a full exhaust. Power is transferred to the super tall tires via the 4 speed Saginaw manual; and her big ol’ butt tips the scale at right around 4,800 pounds. With me in the driver’s seat, the weight goes over 5k. Yeah, I’m a big boy…

Anyway, the gearing in the rear differential is 4.10:1 which is pretty high (though definitely not the highest out there). What this means is that my truck hauls arse up until about 35-40 miles per hour. Unfortunately, about halfway through 3^rd gear, it flattens out and has nothing left (because 4^th gear acts as an overdrive with no oomph).

Now, this is where knowing what I need my truck to do comes into play. My truck very rarely sees track time. In fact, I’ve never brought this vehicle to the track. So really, those gears are almost useless. Keyword there is “almost”…

I keep the gears because I haul a lot of junk in the back (scrap metal, helping people move etc.), and the steep gears help me out a ton in that regard. However, if you have a car, you won’t have that problem.

(So, the point is that I know what I need my truck to do. It is a dual purpose work truck/rat rod. I use her to do stuff, but I ain’t afraid of driving her to the local car show, either.)

Photo Credit: Mecum

Now, that’s not to say that Ol’ Bess (as she’s lovingly called) won’t ever make a trip to the drag strip. And, this also doesn’t mean that I don’t demolish some Corvette booty up to 35-40 miles per hour while driving around town.

So, I keep the gears…

Bad news for the engine:

My issue, as would be yours in a similar situation, is that I run out of gears in my transmission because my rear end forces me to get up to speed super-fast. This means that I launch off the line faster than most other vehicles out there.

It also means that I’m on the highway going 48 miles per hour and the tach reads 2800 rpm (as you can see in this picture of my dashboard). My engine literally sounds like it will blow up if I ever hit 65 on the highway.

This is bad for the entire vehicle, and puts a lot of needless stress on the engine and other components.

The fix:

So, what do I (or you if you are having a similar issue) do about this? Well, you could do a couple different things:

First, and the option I’m going with at least at this point, is to swap out the transmission for something with more gears. My first pick, though I don’t have it yet, is a T56 out of a Corvette or 4^th generation f-body. These babies have 6 gears and are proven to hold up to some serious power output.

The second option is to opt for a lower number gear ratio. This would be the easiest and cheapest option for you. It doesn’t work for me, because I’d have to swap out my entire axle, and that’s ridiculous.

But, as for now, I’m saving up for a T56-6 speed transmission. That way, I’ll be able to keep my hauling gears for moving my friends and beating down on other “fast” cars, while adding to my top end. In the process, I’ll be able to increase my fuel mileage. Because let’s face it, watching the needle on my fuel gage drop as I drive sucks…

So again, figure out what you need your car to do. If you have 5 or more gears in your transmission, a higher number gear ratio in the back could work just as long as you’re not driving this beast to work every day. But, then again, keeping it in the 3.70:1 to 3.50:1 range is great for a dual purpose car.

If you have no desire to ever race your car, a set of gears with a ratio of 3.20:1 or less is great. Bottom line, know what you need out of your car, how you need her to perform, and what will agree with your other components. What’s your take on the gearing situation? What gear ratio do you have in your car?