Ask Away! with Jeff Smith: Optimizing Gear Ratio for Overdrive at Cruising Speeds

I have an early 70s Chevelle and I am planning for either a 700R4 or maybe a 4L60E. The car right now has a mild big-block 454 and I’m looking to put together a nice running street and highway car. I don’t drag race this car but I do want to be able to cruise down the freeway at a decent speed—somewhere around 2,200 rpm. Is there a way to figure out what gear I should run? Right now, the car has a 3.08:1 rear gear but I’m guessing that won’t work well when the trans shifts into overdrive. Thanks.

S.C.

Jeff Smith: We can certainly help you with this. It sounds like you have a great combination, and with a little bit of simple math, we can offer several options. There are multiple variables to deal with, and while you didn’t tell us how fast you intend to cruise, or the height of the rear tires (which affects gear ratio), we can still get pretty close based on some estimates. So let’s get started.

The variables that we have to deal with include:

• Vehicle speed
• Rear gear ratio
• The overdrive ratio
• Rear tire diameter

All of these affect the selection of the rear gear ratio.

First, let’s cover some basics. In order to determine the overall gear ratio in overdrive, the math calls for multiplying the rear gear ratio by the overdrive ratio.

So in the case of a 700R4 or a 4L60E (they are the same with regard to the overdrive), the fourth gear ratio is 0.70:1.

Stated another way, this overdrive will reduce the engine rpm by 30 percent. So if the engine was spinning 3,000 rpm at 70 mph—in overdrive it will now spin at 3,000 x 0.7 = 2,100 rpm.

That’s a simplified version of what’s really going on, so let’s look a little deeper.

Speed has an impact on the formula that we’ll use to determine the correct gear ratio as does rear tire diameter. Let’s use 70 mph as our speed and a rear tire diameter of 28 inches. That may be a bit taller than most. We’re basing that on a 275/60R-15 rear tire since you’re probably running big rear tires to handle that Rat motor. A shorter tire will add rpm at a given vehicle speed.

First, let’s determine your current combination running with a 3.08:1 rear gear and our estimate of the 28-inch tall rear tire. The formulas we’re using are from John Lawlor’s Auto Match Handbook, an HP Books publication that offers a wealth of basic hot rod math formulas.

We will plug in inputs of 70 mph, a 3.08:1 rear gear and a 28-inch tall rear tire:

rpm = mph x gear ratio x 336 / tire diameter

rpm = 70 x 3.08 x 336 / 28

rpm = 72,441 / 28

rpm = 2,587

This number is probably low because what we’re not taking into consideration in this formula is torque converter slippage. We’ll assume this Chevelle is running a TH400 with a mild performance non-lockup torque converter. Since there is only a minor amount of load on the engine at cruise, the slippage is probably 200 rpm so we’d have to add 200 rpm to 2,587 rpm—making the actual cruise rpm very close to 2,900 rpm.

Now we have some baseline information from which to make decisions. Your cruise rpm goal is 2,200 rpm and we’ll stick with our 70 mph speed estimate. Now we’ll use the gear ratio formula to determine what gear we need.

The formula is:

gear ratio = rpm x tire diameter / mph x 336

gear ratio = 2,200 x 28 / 70 x 336

gear ratio = 61,600 / 23,520

gear ratio = 2.62:1

This is a pretty tall gear. Keep in mind that we’ve chosen the 700-R4/4L60E, this is the effective rear gear in overdrive. In order to determine the actual rear gear ratio, we need to divide the 2.62:1 ratio by the overdrive or 2.62 / 0.70 = 3.74:1. To double check our work merely multiply 3.74 x 0.7 = 2.61999.

The math has shown us that a 3.73:1 rear gear would give us exactly what we’re looking for, especially if you choose a lockup converter to go along with this gear ratio combination. A lockup converter eliminates the slippage and as a side benefit would reduce heat buildup in the transmission.

The only downside to this combination is that a 700-R4/4L60E uses a really deep First gear ratio of 3.06:1. Multiply 3.06:1 x 3.73:1 = 11.41:1 which is the effective First gear ratio. This is very steep. As a point of comparison, let’s compare that to the same effective First gear ratio with a TH400 trans that uses a 2.48:1 First gear. If we divide that 11.41 by 2.48:1, we can see that the 3.73:1 rear gear with a 4L60E is equivalent to a TH400 with a 4.60:1 rear gear. If that doesn’t bother you, then this package would work well.

Another way to go would be to cut the rear gear to closer to a 3.42:1 instead of the 3.73:1. This would reduce the overall cruise rpm a bit more down to 2,007 rpm at 70 mph—or a reduction of effectively 200 rpm. This would also soften the overall First gear slightly, making it feel much more streetable.

There is one more formula that we often use in these calculations to find mph and it looks like this:

mph = rpm x tire diameter / gear ratio x 336

mph = 2100 x 28 / 2.39 x 336

mph = 58,800 / 803.04

mph = 73.2

You now are armed with enough information to make an intelligent decision on rear ratios for both cruising and street driving, along with some direction for torque converter selection. One thing that we’ve learned from playing with lockup converters is that they pull the rpm down much more than you might expect. Essentially, the looser the converter, the more rpm the engine will drop when the converter locks up. We’ve seen a greater rpm drop from locking the converter than from shifting into overdrive.