IN A WORLD of heart stopping fuel prices, where big American pickup guys are looking enviously at Suzuki Samurais, it’s time to talk once again about fuel economy. This time, we’ll talk about it in terms of gearing.
What Happens When You Build Up
A 4x4 buildup always involves three things; bigger tires, more height and more weight. The height increases aerodynamic drag and the big tires increase rolling friction, as well as increasing aerodynamic drag. Yep, a wide tire adds quite a bit to the frontal area. The big tires also change the effective gearing... taller tires giving you a higher effective ratio. Accessories and modifications make your rig heavier. All these things combine to change the performance and economy equation for the worse. It’s like having to pull a trailer that you can’t unhook.
There is no way to counter the mpg loss that comes with a buildup. All you can do is minimize it. Correct gearing is one way.
To a degree, you have to choose between gearing for performance or economy. But here is a spot at the center of the “curve,” between these two goals, that offers acceptable levels of each. If you need one more than the other, you’ll have to give something up in the other area. Often, you can edge slightly towards performance and compensate with efficient driving and always traveling as light as possible.
The Speed Factor
Because of the atrocious aerodynamics of most built 4x4s, speed is the “Great Satan” of fuel economy. One of the first decisions you must make is to determine your cruising speed. It takes gobs more torque to push your “brick” through the air at 70 mph than it does at 55. The faster you go, the more fuel you will burn.
Engine Factors
It’s just as inefficient to run an engine too slow under load as it is to run it too fast. The low rpms that many modern engines run in factory trim are as much for low emissions as for fuel economy, and stock rigs can get away with it more because they are lean, low and have small tires.
The gearing goal is to put the engine into an rpm range where large throttle openings are not necessary to maintain speed. An engine’s economy “sweet spot” will vary according to displacement, with small displacement engines running faster than bigger ones. Many fours deliver the best combo of all-round performance and freeway mpg when geared to cruise at 2500-3000 rpm at 70 mph in O.D. A small V6 might be slightly lower, say 2200-2800 revs. A big V or inline six likes 2000-2600. A small V8; 1800-2400. A big V8 is happy as low as 1700-2200. A V8 diesel can run happily at 1500-1800. These are very general numbers.
The key to gearing and mileage bliss is finding the engine’s torque band and making sure the engine is always working there. That’s best seen on an engine performance graph (see the nearby illustrations). You can get a hint from the advertised peak torque number, but that’s not always useful because many modern engines have a long, broad torque line and the peak is at a relatively high rpm. Older engines had a much narrower band of useful torque and made it at a lower rpm. Without extensive tests to determine the exact sweet spot, a cruising rpm, one third to one half the way into the torque band will supply the best economy. That’s where the above generalized rpms came from. Your particular setup could be different.
Combining trail performance and street economy is most important to the dual purpose rig owner. To a degree, you can balance those two elements to have, if not the best of both worlds, at least the “real good” of both worlds.
The Master Equation: Tire Diameter + Axle Ratio
The relationship between tire diameter and axle ratio is critical to mpg. Torque multiplication is the key element to performance. It takes a certain amount of torque to move a vehicle and to keep it moving. Torque comes from the engine, but it is multiplied by gearing... both the transmission gears and the axle ratios. The overall gear ratios are designed to keep the engine operating in its most efficient rpm range at various speeds and loads.
Tire size and axle ratio combine to form an “effective” ratio. The manufacturer figured out the right gearing combo for the stock rig... we hope... that combines good performance with economy and low emissions. It’s an equation that involves the weight of the vehicle, aerodynamics, intended use, engine power and torque and certain minimum performance requirements.
The point is this: We can often use that OE balance point as a baseline to choose a new ratio that maintains the same effective ratio after a tire diameter upgrade. By using the equivalent ratio formula (see MPG Math in the sidebar), you can calculate a new ratio to compliment those big tires. What happens if you don’t? Let’s say you install 33s on a ’92 Wrangler 4.0L that originally came with a 3.07:1 axle ratio and 215/75R-15 tires. You end up with the equivalent of a 2.61:1 ratio with the stock tires. At 70 mph in fifth, that Jeep is turning 1,725 rpm and is well below its ideal torque range. It’s grunting in agony and the pedal is to the floor at the slightest bump in the road. It probably can’t pull fifth gear at all. Fuel economy will suck and the engine will suffer. For comparison, the stock rig at that speed was turning 2,033 rpm. For a stocker, that’s about right, but for the increased effort needed to push a lifted, heavier, big-tire Jeep, you need a few hundred more rpm.
OK, using the equivalent ratio below and plugging in 33- inch tires, we get a 3.61:1 ratio. There’s no such animal, so you have to average up or down to the next nearest available ratio for Jeep axles, which are 3.54:1 and 3.73:1. Making this choice comes down to your driving routine. In general, because of the aforementioned rolling resistance, extra weight and loss of aerodynamics, the better performance and mileage results will come from the lower ratio choice. With a 3.73:1 ratio, our Jeep is now turning 2,100 rpm at 70 mph, a smidge over stock.
Especially when you are working with the small axles, gear strength should be part of the decision in a gear swap. As you go lower, the pinion gets smaller. As a result, there is less tooth contact and more torque load is placed on less area. When you combine this kind of weakness with larger tires and rough ‘wheeling, you produce a grenade with its pin out and its safety lever secured with old masking tape! For the small axles like Dana 35s, Ford or GM 7.5-inch, Mopar 7.25, etc. the generally acknowledged magic number is 4.56:1 Lower than that sacrifices too much gear strength and you are better off with an axle upgrade. For Dana 44s, GM 10- bolts, Ford 8.8s, etc, 4.88:1 is about it. Bigger axles can go lower. Whether you need to worry depends on how you use the vehicle and how much input torque the pinion is getting.
Do it yourself? If you are capable of doing internal engine work, you can set up a ring and pinion or overhaul an axle. There are a few necessary tools, but most of them are useful in other areas of 4x4 tinkerage.
Most ring and pinion sets, like these from Superior Axle, are made of heat treated 8620 alloy steel. Don’t buy so called “racing” gears because they are softer and will wear much more quickly. A set of quality aftermarket gears will offer the same long, quiet service as the factory gears when installed properly.
Detailed Choices
Continuing with our Wrangler example, even stock, the 3.07:1 ratio offered so-so performance. An equivalent ratio, or one as close as you can get, will still put you into that same so-so ballpark. Experience says that a stock Wrangler with 3.54 gears and stock tires was a better performer, and the EPA mileage was just the same as with taller gears. If we plug 3.54:1 into the equivalent ratio formula, we get 4.17:1... close enough to the real ratio, 4.10:1. Experience tells us that 4.10s, combined with 33s, makes for a good street/trail combo. The 70 mph rpm are only 2,224. Still low enough for good economy, but far enough into the torque band to have some reserve.
So far, we’ve only spoken about freeway driving. Gear ratios also affect city mpg. Truth be told, a good many of you do a citycommute at relatively low speeds involving lots of stop and go driving. With tall gearing, the engine has to work harder to accelerate your rig. Harder work equals lower mileage. You burn fewer calories pedaling a bike into motion from a dead stop in a lower gear than in a higher one, and the grunt is a lot less. Your engine uses less fuel and also grunts less with lower gears. The point here is that if the majority of your street driving is in the city and below 60 mph, you may be better off on the lower geared side of the equation.
Special Considerations
Some vehicles have special problems. You may have a very low powered rig. It may fall flat on its face when you add big tires and so it may need much lower gears to maintain adequate performance. If you gear low enough to get adequate torque multiplication, you wind up zinging along at redline rpm at freeway speeds, sucking fuel like a V8. So why not swap in a V8? Many owners report better mileage swapping in a bigger engine than by keeping a smaller one. A lightly loaded bigger engine can deliver better mileage than a flogged small one.
Compromises must also come to owners whose rigs are without overdrive transmissions. If you gear for adequate acceleration and low speed economy, freeway rpm is much higher and mileage sucks. Gear tall for freeway mpg and your off-the-lineperformance and in-town mileage sucks. In this case, youhave to look hard at your driving situation and pick your poison. Careful ratio selection can offer “OK” results in all areas but seldom stellar ones. Long term, an overdrive transmission swap can really pay for itself in this scenario.
FIG 1. Here’s a very late model 4.3L V6 Vortec. This EFI engine has a broad, flat torque curve. Though the sweet spot is only shown in about a 600 rpm range, economy would likely still be good to 3000 rpm.
FIG 2. This small displacement V8 has an almost flat toque curve that peaks at a high rpm. This shows the engine breathes well. The arrows mark a suggested sweet spot.
FIG 3. This is a late model, big-block GM gas stump puller. Talk about a flat torque curve! The red arrows suggest a sweet spot but this engine is strong enough to go a bit lower. Big gassers have characteristics much like diesels.
FIG 4. The mighty Duramax is typical of most diesels. They like to run slow for best economy. Peak torque is 1600 and that spot will deliver the best economy. A little faster shouldn’t hurt mileage much.
If you rig has a dropout style diff, as do many imports, gear swaps are a bit easier in several ways. You can do the work yourself on a work bench, instead of on the floor, under your rig, or you can exchange pumpkins. Genuine Gear offers these ready-built Toyota dropouts in many ratios and set up with a variety of lockers on an exchange basis.
Here’s a torque and power graph from a ’69 vintage Chevy 292 six. The red arrows show the approximate sweet spot for this long-stroke engine. It’s a pretty narrow band to gear for. Note also the difference between Gross and Net readings. Today’s numbers are always Net.
Gearing Change Nutz & Bolts
Many ‘wheelers break out in a cold sweat when considering a gear swap. They won’t hesitate a moment to tear down an engine, but there’s bad mojo in them gears, dude! As a result, the expensive labor of a gear swap adds to the buildup expense and some will avoid this aggro and just live with mediocre performance and mileage. Well, a gear swap ain’t brain surgery.
What it boils down to is a few special tools, (many of them useful in other areas of truck repair) the right parts and a good place to work. In terms of level of difficulty, it’s no more difficult than installing a camshaft in a V8. Like anything else, if one takes his time and follows instructions, a homewrencher can do the work.
Sources
4 Wheel Parts
www.4wheelparts.com
800-284-9840
4-Wheel Drive Hardware
www.4wd.com
800-555-3353
GM Powertrain
www.gm.com/automotive/gmp owertrain
Genuine Gear
www.4wheelparts.com
877-474-4821
Superior Axle & Gear
www.superioraxle.com
888-522-2953
Gearing Glossary
High Gears – Numerically low gears, such as 2.76:1, 3.07:1, etc. Also known as “tall” or “long” gears. “High” is a carryover from the old term “high speed gears.” Anything from 3.50 and numerically lower are generally known as “high” ratios in truck and 4x4 circles.
Low Gears – Numerically high ratios, such as 4.10:1, 4.56:1, etc. Also know as “deep” or “short” gears. “Low” is a carryover from the old term, “low speed gears.” Generally, gears from 3.73:1 and numerically higher are considered “low” ratios.
