Since we got a bus to convert into an RV, I have been doing quite a bit of research into them, and gathering information which I might find useful. One question which came up was how fast one can and should drive their bus. There is a lot of variation, and many factors which come into play, but given freely available resources on the internet today it's actually not that difficult to figure out.
The short, short answer is 60 MPH. This may not actually be true, however. The factors involved include terrain, aerodynamics, legal restrictions, rear end (differential) gear ratio, transmission gear ratios, wheel size, and the engine's "sweet spot" where it produces the best power output for the fuel consumed. I'm mostly interested in the issue of flat ground, or at least mostly flat ground, which is what I intend to address.
Assuming a bus that has more power than you need to go 60 MPH, it is still often the correct answer if you care about mileage, because of aerodynamics. 55 MPH is where drag tends to become significant, so somewhere from 55-65 MPH is likely to be your most efficient speed in a vehicle with a big front. The shape of the nose of the bus is relevant, but the total frontal area actually makes a much bigger difference than the shape — the frontal area is the two-dimensional front silhouette that would result if your bus could go Wile E. Coyote style through a canvas and leave a perfect cutout. That's not to say that a dog nosed ("conventional") bus can't have less drag than a flat-faced ("transit style") one, but the height and width are still more important overall. In addition, smooth shapes have less drag than ones which are broken up, because the wind will tend to impact each projection separately. However, subsequent projections which are in line with one another will tend to affect drag less, because each one tends to be located in the turbulent wake of the one preceding it. This is the same effect that makes "drafting" efficient, where one vehicle follows another closely.
Under the speed at which drag dominates, there are still other factors which come into play. The main goal in selecting a cruising speed is to get into the engine's "sweet spot", where combustion is efficient, and where internal losses due to friction and vibration are minimized. The location and size of the spot varies from engine to engine, and is generally at higher RPM for gasoline than for diesel. My interest is primarily in diesels, as they tend to be much more efficient than gasoline engines. Direct injected gasoline engines have narrowed this gap, but diesels still tend to be much more efficient under load. When pushing against drag, and moving a large vehicle, the engine is always under load.
The sweet spot is generally broader in diesels with fully electronically controlled injection systems, typically known as "common rail". Direct-injected gasoline engines also generally use high pressure common-rail designs, in which solenoid-based fuel injectors are connected to one or more fuel rails which are fed from a high-pressure injection pump driven from a gear on the engine. Electronic systems are more capable and adaptive than mechanical fuel injection systems, with the ability to alter both the amount of fuel delivered per combustion cycle, and the time at which it is injected. In general, non-electronic diesels do not use common-rail systems, with the partial exception of two-stroke engines made by Detroit Diesel, like the Series 71. However, those engines only build final injection pressure in the injector itself, which is driven by a cam lobe. Electronic injection permits fine and immediate control over injection, while mechanical systems have limited adjustment.
The engine's sweet spot defines the ideal engine cruising RPM, and vehicle gearing determines the speed of travel in that RPM range. In large vehicles like semi-trucks and buses, this is usually intended to be about 60 MPH, because drag is such a significant factor. In cars it is usually set up to be faster, for performance during high-speed freeway travel, because they have much less drag. Gearing is defined by the transmission gear ratios, the driven axle's differential gear ratio, and the size of the tires. This is not immensely complicated, but rather than working it out yourself, it's simpler to use a RPM calculator to tell you what speed is going to be most efficient. Pick an engine speed around the middle of the sweet spot for the purpose of doing these calculations.
Next measure, calculate or research the outer diameter of your tires, and determine the ratio of your transmission's top gear. This is usually either 1:1 or lower, with lower ratios (With the first number less than 1) referred to as an "overdrive". Finally, you will need to determine the axle gear ratio. Substantially overdriven transmission gear ratios can be inefficient due to gear loss, but so can very low-geared rear axle ratios — that is, those with a higher numerical value. Differential gear ratios tend to range from about 2.5:1 (a "high" ratio, meaning relatively many wheel revolutions to each driveshaft revolution) to around 8:1 (a "low" ratio, where the driveshaft has to turn more times for the wheels to turn), though there are even lower ratios in use, primarily for off-road vehicles. Gear ratios for buses in particular tend to lie between 3.55:1 and 6.15:1; our bus has a 5.38:1 rear, because it is a former Yosemite park shuttle, and it had to be able to handle hilly terrain. Selecting a transmission and an axle ratio is a careful balancing act. A transmission without overdrive is more efficient, but a higher-geared rear end has more driveshaft stress than a lower one, because the driveshaft turns less time per wheel revolution.
Confused yet? Let the calculator do the work. I'll use as example our bus with Cummins ISC 250 engine, B300R transmission, and 5.38:1 axle ratio. The bus has 10R22.5 tires, which have a 40.5" outer diameter. The "sweet spot" is around 2100 RPM. The B300R is a 3000-series Allison transmission; they have six gears, but sixth (or even fifth) can be locked out to prevent its use; our bus has five gears unlocked, which is typical. The ratio of the fifth gear is 0.75:1. If I punch all of this information into the calculator, I get a cruising speed in fifth gear at 2100 RPM at 62.73 MPH — right about where you want to be when taking drag into account.
If you're interested in your top cruising speed, do the math again with a larger number. From research I've found that it's common for ISCs to be set up to cruise at 2500 RPM. With my setup, the calculator returns 74.68 MPH. I've had the bus up to 70 on the I-5, and can confirm that it ticks along nicely, but you have to keep in mind that mileage will plummet at speeds too far above 60. It's not uncommon to get half the mileage at 75 that you'd get at 65. Naturally, you can also use the calculator to determine your maximum speed, if you can find your engine's maximum RPM. The ISC 250's maximum is said to be 2900 RPM, which in my bus works out to 86.63 MPH — far faster than I'd have any business going. This calculation does not take drag into account; some vehicles' speeds are "drag-limited", meaning that the engine doesn't have the power to push them to that speed given their drag. Other vehicles are "gear-limited", meaning that they have enough power to overcome drag.
So, how fast do I go?
This brings us back to the question of how fast you should drive. The safest answer is therefore "what is safe for conditions". In poor conditions like rain or snow, this will obviously be less than your maximum speed. But even in dry conditions, it is not safe to max out the vehicle. The suspension and brakes of roadgoing vehicles are not designed for those speeds. They often have enough power to go quickly, because they need to have enough power to accelerate quickly for purposes like entering freeways, or climbing steep hills. Other vehicles do not; in buses and trucks in particular, they can generally be ordered with a very broad range of engines, transmissions, and differential gear ratios to suit different purposes. A school bus intended for use in a mostly flat area might not need as much power as one which will be operated in hill country, and so it might have a smaller engine. Mostly, however, they are designed to cruise around 60 MPH. This doesn't only decide which engine and transmission will be used, but also the type of brake and suspension system, and even the speed rating of the tires. Exceeding the capability of any of these systems is dangerous to yourself, your passengers, and to other people on the road.
So, 60, right?
The answer is probably 60 MPH or less, depending on your particular equipment, and the road conditions. The faster you go, the more energy has to be transmitted through the tires, and that means that you're using more of their potential to grip the road. If you hit a patch of water, snow, ice, oil, or roadkill, the amount of available traction can rapidly decrease, and you can experience wheel spin. Then there's the issue of going up and down grades. Assuming the up and down sides of a hill have the same grade, you generally want to come down a hill in a lower gear (and slower) than you can go up it. One rule of thumb is that you should use an even lower gear going down a grade than you would have to use to go up it, to take advantage of engine braking. This is also helpful if you have a Jacobs engine brake ("Jake Brake") or an exhaust brake, because they provide more braking force at higher RPM.
Commercial Driver's Handbook
For more information on safe speeds, how to drive and such, you should study a commercial driver's handbook. These should be available from your local DMV or equivalent; I live in California, so I naturally studied the California Commercial Driver's Handbook. It explains many of the details of inspecting and operating such vehicles. In California, drivers of motor homes over forty feet in overall (bumper-to-bumper) length are required to have a noncommercial class B license, but this information should be of interest to anyone with a class A motor home, or really any large vehicle.