There are many factors that affect fuel mileage. We often talk
about a few of them, but mileage is more than just a few big items
(tire pressure, tread pattern, etc). Overall mileage is a function of
the inputs of many, many details.
This post will examine the factors that affect mileage (steady-state
hwy mileage, in particular) in detail, and how we can consider this
when seeking mileage improvement. We’ll start with the road and worm
back to the engine.
Below 50mph, the rolling resistance of your tires is the biggest source
of resistance the engine must overcome. This resistance means engine
power (and therefore, fuel) that is consumed before it has a chance to
be applied to the load. So we want a tire with low rolling resistance.
What creates rolling resistance? Primarily—tire flexibility. The round
tire must conform partially to a flat road, and it must be flexible
enough to distort to do so. The more flexible the tire’s tread area is,
the larger the contact patch with the road will be. This means more
traction. This leads us to an important point: the bigger the contact
patch, the more rolling resistance and thus, lower MPG.
Consider a steel railroad wheel. Designed for traction? Nope. Low
rolling resistance? Absolutely!
This is why tire pressure plays such a big role in fuel economy. But
the carcass design of the tire ALSO plays a big role in economy,
because the carcass determines how the tire pressure will affect the
rigidity of the tire and where.
Related to carcass design is tread pattern. Most people know the more
aggressive tread has more rolling resistance, but can’t tell you why.
Ever wonder why commercial tires (designed primarily for durability and
fuel economy (at a given load), not traction or performance) are
usually a simple parallel-ribbed design? Keep these questions in mind.
But for now, consider a simple wire-spoked wheel—only this wheel has
only 4 spokes! How much load does each spoke carry? How does this
compare to a wheel with 50 spokes? Much less, right? You can see where
we are going—LOAD DISTRIBUTION!
Now let’s go back to our aggressive tire tread. The fewer and farther
apart the tread lugs are, the more load each individual lug must carry.
This means that the load is concentrated at a point on the tire’s
carcass. This concentration causes the carcass to deflect more than it
otherwise would have to. It takes energy to do this!
An interesting point about tire noise is that it relates to
inefficiency. Consider the difference between a runner that glides
quietly over the ground compared to one that stamps his feet loudly.
Which is making better use of the energy they are expending? Remember,
sound waves are energy. This energy must come from somewhere! If you
stamp your foot into the ground, how much does it push up on you
compared to if you squatted down and pushed into the ground with the
same amount of force?
There are many others examples, but the point is clear. An efficient
tire is a rigid, harsh-riding, low-performing nightmare—but at least
Tire size (both height AND width) affect the size of the contact patch,
the amount of tire deflection necessary, and thus rolling resistance.
All other being equal, a large tire has more rolling resistance. This
is related to our next point-gearing.
The main effect of gearing is upon engine rpm, so I will cover that
when we get to the engine. But there are other elements to
consider. First, is differential lube. A higher viscosity lube
increases the work necessary to turn the differential gears.
Second is the effect that ratio has upon gear design. Namely, the more
torque multiplication a gearset must do, the less efficient it is at
transferring energy. A 3.54 axle ratio has less of a difference in
tooth count between ring and pinion than does a 4.10 ratio gearset. As
you go higher numerically, the efficiency gets less as the teeth must
be machined in numbers and at angles that make them less efficient
conduits of energy.
I use this to refer to driveshafts, U-joint and the power conduit
between transmission/transfer case and axles. The driveline adds little
resistance overall in the MPG picture, but it can become a factor if
not set up right. As long as the angles are proper and the U-joints are
functioning properly, then you will have a smooth power delivery. Lift
kits and other things that change the angles can cause decreases in
efficiency if they exceed the design range of the components.
Obviously, a 4x4 has more resistance in this area, especially if
there’s no way to disconnect the axles or hubs.
Ignoring the effect of ratios upon RPM, there are all of the factors at
work in a differential (lube viscosity, etc), and then some.
Here, the 4wd will be less efficient as is simply has more components
turning, and thus more resistance. So transfer cases add more
resistance, just like having front axles turning, etc.
But the gearing INSIDE the transmission is important as well, from a
design standpoint. The helical gearing inside the CTD manual
transmission is typically less efficient than the planetary gearing of
a typical automatic transmission. Moreover, a constant-mesh manual
trans is turning more components than the planetary setup in the auto
trans. There are MANY design variables that go into a torque
converter, so we control for these variables by assuming an engaged
Here we are—the big enchilada. Since there are so many variables, I’ll
try to hit the key ones.
- Operating temperature: the less heat energy is lost to the coolant,
the more is available to turn the crankshaft
- Cam timing: the breathing of the engine is optimized for only a
certain rpm—everything else is compromise. You can have a very peaky
powerband of high efficiency, or you can broaden it but reduce
efficiency. This is usually done with lobe separation angle manipulation
- compression ratio: the higher, the better for efficiency. Not
necessarily better for keeping the head from popping a gasket.
- pumping losses: the more the engine has to work to move air in and
out, the less efficient it will be
- Burn quality: this determines how many of the fuel’s potential BTUs
are actually converted to heat energy. This accounts for fuel quality
and injector efficiency.
RPM is a huge component of engine efficiency. Higher rpm multiplies
things like piston ring friction and pumping losses. But rpm also
affect BSFC. Brake specific fuel consumption (BSFC) is simply how
well an engine converts fuel to HP in terms of lbs of fuel per hp per
hour. So an engine with a BSFC of .300 would burn .300 pounds of fuel
per hp for every hour it operates. Thus, if it’s a 100hp engine, it
burns 30 pounds in an hour. Given the weight of diesel (7-7.3 lb/gal?),
that’s about 4.2 gallons.
The relationship between RPM and BSFC is critical in a diesel. If one
diesel can produce 100hp at 1300rpm, and the other needs 2500rpm to
produce that same 100hp, which is more efficient? YOU CAN’T TELL!