PREVOST, The Ultimate Class

As we drive our magnificent Prevost motorhomes down the highways, especially the Interstate, it seems that the faster we drive, the better our buses run. Just last fall I was crossing Texas in mine. Pulling a car four-down, with an Overbilt bike rack loaded with a big cruiser, 65 certainly didn’t seem to be a problem, then 70 and, well, much more.

Once we arrived in Florida, we stopped at a small campground overnight. A very nice fellow in a nearby Class C came over and we talked. "So what kind of mileage do you get with that?" I hadn’t really thought about it before, so I, somewhat embarrassed, said, "Well, it depends on how you drive it." Very quickly I did some calculations in my head and said, "Probably between 7 to 10 miles per gallon." Our guy then noted that he got the same mileage with his Class C, we spoke of other things and that was that.

Later, I began to wonder just what kind of mileage I was getting.

When we were underway with our load, over-the-road air cranking, towing, it seemed that sometimes we could go through half a tank a day. Other days, with more uphill pulls, heavier foot, maybe faster, we would go through more. There seemed to be quite a discrepancy in the apparent mileage that we were getting and I became more curious about what factors, other than simply plopping my foot on the accelerator off the line, might be.

With all the talk in recent years about fuel mileage, it is clear that quick accelerations and panic stopping don’t help fuel mileage. Further, given the braking ability on our 42,000 pound bus, the Jake Brake use and the old maxim, "Drive slow and turn wide," lend themselves to more leisurely driving.

But the difference between, what I was now measuring, 4 mpg and 10 mpg began to make me think: What factor might be causing such a wide variance in mileage?

Enter a little research, and stay with me on this one.

Here’s an oversimplified and partially accurate statement: Wind resistance increases x3 with each mile per hour over the effective torque of the bus. That means, if your bus has a peak fuel efficiency of say, 65 mph and you drive 70, you are increasing the wind resistance on the front of your bus at least 30 times!

Most Prevost conversions are quite different than one another, Xls and H3's are different from one another in shape, weight distribution and certainly different in convertors, so it is not possible to accurately determine a universal peak fuel efficiency for all Prevost motorhomes. Not only that, over the course of the last, say, 15 years, there is a wide variety of other variables, such as engines, torque, transmissions, use of OTR air conditioning and the like, which would also be big variables in determining, "peak efficiency."

The rule of thumb here is that going a little faster does not result in a little more fuel being used. Instead, after a certain speed, the wind resistance of a bus increases dramatically with each and every mile per hour, at a very high rate. Once you go over the peak efficiency speed, you will be consuming a vastly higher amount of fuel.

It is my contention (not tested) that Prevost buses seem to ‘squat’ down a little at higher speeds. I am certainly no engineer and have no way of testing that assumption, so it is possible that the peak efficiency of a Prevost may be a higher speed than one may realize. Yet, with a headwind, the OTR on max, significant differences in fuel consumption may result.

Here’s another way of looking at peak efficiency. If your bus has a differential temperature gauge, look carefully (with eyes primarily on the road). On a hot day at highway speed, it is likely that the gauge will climb rather rapidly to the 225-250F degree range. If it stays there, it just might be that speed is about at peak efficiency (this is just a guide, not a direct correlation). Should it climb higher, as some buses do, you may be running the bus beyond peak efficiency. Now there may be good reason to be running your particular bus above that number, but the point of this little note is to give you some ideas about fuel efficiency and the variations you may note if you even care to keep track of such things. (But note that the gauge will not help on a rainy day because the rain water will keep the rear end so cool that it won’t register at all on the temperature gauge).

If you too are retired and like to have something to do with a calculator and like to keep track of things, keep a little diary for awhile. Try to use your average speed (from your Diesel minder?) And then average fuel consumption and after some number of days of driving you will probably find something close to the peak fuel efficiency of your bus. Once you have that speed, then you can decide whether you are driving for just for pleasure or perhaps gulping far more fuel than you had expected.

The bottom line here is that the faster you go, the more fuel you will use...not just a little, but it could be a great deal more.