Fuel economy is a significant concern when diesel is over $3.50 per gallon, and it is even more so in California, where diesel is over $5.00 per gallon. Virtually every bus builder claims to have an advantage, so it might be worth trying to give the elephant a physical.

Three blind guys are examining an elephant. The first grabs the tail and decides the elephant is a snake. The second grabs the trunk, convinced it's more like a fire hose. Number three wraps his arms around a leg and says, “You dunderheads... clearly an elephant is like a tree trunk.” Which one was right?  They all were, and yet none grasped the whole picture.

That being said, in a recent article, I said that bus manufacturers' advertising claims were essentially honest, and they are. However, it is worth noting that none of them necessarily presents the whole picture. 

Various factors affect fuel consumption in a bus, or any converted vehicle, including weight, wind resistance, parasitic losses, gearing, the drivetrain, and any appendages, such as roof raises, solar panels, air conditioners, and roof decks. Almost all appendages degrade your bus's performance, so to achieve the best fuel mileage, you should limit the number of appendages when converting a bus, lest you will be spending more money on fuel.  

The point is that out of the factory, each bus manufacturer has a pretty good idea of what they do best and selects a test protocol that stresses that trait. 

A bus with reasonable aerodynamics fares well in tests above 40 MPH. A light bus thrives on stop-and-go traffic as there is less mass to accelerate from a stop. As per Newton's Second Law F=ma, where the Force needed to start a bus rolling from a stop equals the mass of the bus times the acceleration component. The faster you accelerate and the heavier the mass, the more fuel you will be burning each time you leave a traffic light. The mass can be limited by building less stuff into your bus, and the acceleration can be controlled by your right foot. A highway bus with a remarkably efficient accessory drive consumes less fuel at idle. 

Engines have a sweet spot in a small RPM range where they use the least amount of fuel. Manufacturers try to pair the gearing that keeps the bus running in this sweet spot during most of its duty cycle, at the speed and location where they expect it to travel the most. The builders' challenge is determining the ratios that best serve the majority of their customers. Your problem is deciding if your operation fits their assumptions. 

When looking for a new or used bus, it may be worthwhile to determine which builder's configuration most closely aligns with your operating profile. Several manufacturers offer optional gear ratios, and since small fuel savings can amount to thousands of dollars a year.

Some engines and transmissions are more economical than others, but they won't do their best if they're mismatched to the bus or each other.  When different bus brands are compared, they should be equipped as nearly identically as possible: engine, transmission, payload, and final drive ratio. If you swap out an engine for one with higher horsepower, you may also need to change the transmission or rear end to match the engine's speed and torque.  If an advertised comparison is a significant factor in your buying decision, request detailed design specifications.

A friend who sells small rear-engine buses taught me an important lesson. He had to hammer a chassis builder to install a larger engine. They insisted that the smaller diesel had a better power-to-weight ratio than the big, 45-foot buses and that it was adequate. 

My buddy finally prevailed because he convinced them that the targeted market operated buses at highway speeds, where power/weight was less relevant. The frontal area of a 35' bus is nearly the same as that of a 45', so accelerating above 50 MPH demanded more horsepower (and fuel) than the bus's weight suggested. 

A bus going 65 MPH burns much less fuel than the same bus at 75 MPH, and Jack Rabbit starts guzzling more diesel. Even if one brand idles more efficiently than another, idling any engine wastes fuel; therefore, driver training may be the most critical component in determining how much fuel their bus consumes.

If you rely on a manufacturer's claims when spec'ing and buying a bus, consider a couple of questions. What does the builder advertise? If one touts aerodynamics and another lightweight, the former will perform better at speed, while the latter excels in stop-and-go conditions. Which best fits your planned usage profile? 

You'd be amazed at the horsepower and fuel consumed by components such as HVAC, cooling fans, and electrical systems. If your operation demands a lot of idling, consider how efficiently other buses you are considering power their accessories. If most of your trips are long with few stops, aerodynamics matters.

As usual... this topic is way beyond my intellect. It's both complicated and ever-changing, and everyone has different driving habits. Ask yourself which manufacturer's tests most closely fit your modus operandi (a little Latin to impress you). I'm old, so humor me.

On one of my trips in the past, we had put about 600 miles on the last fill-up in a Flxible bus and were topping off before a long stretch with little fuel available. At 30 gallons, it burped and stopped drinking diesel, which was an impressive 20 MPG. About 400 miles later, the Detroit Diesel 671 sighed and then died. We were never sure why the bus burped at 30 gallons, but clearly, we hadn't filled the tank. If 20 MPG sounded too good to be true, it was. LOL!

You can learn from builders' advertising. They are honest but understandably put their best foot forward. Like the elephant parts, no single one gives you the whole picture.