I have been asked to address the general issue of motorcoach
performance. Specifically, how do you get good fuel mileage and what
contributes to poor fuel mileage? And a small piece on tires.
I have written on this subject before but it has been a
while and probably needs to be restated.
Fuel mileage is determined by several factors:
1. The driver (you!)
2. Speed
3. Weight
4. Tires
5. Frontal area (that barn door you are driving)
6. Weather (specifically the wind)
There are additional factors but in my experience these are
the most significant and the first four are owner controllable. Let's start
with numbers one and two - You and speed.
For reference, the most efficient drivers get 30% better fuel mileage than the
least efficient drivers. Where are you on that curve? Want to get better fuel
mileage, talk to that gent in the mirror.
While underway the driver has control over two main areas;
speed and inertia. Obviously the driver can control the speed, but what is the
best speed? Well, the faster you go the more fuel you use just because of the
increase in wind resistance (drag). It takes X horsepower to move Y weight at Z
speed. Let's look at an example:
For a 34,000 lb coach it takes 100 hp to travel at 55mph on
level ground.
It takes 150 hp to move that same coach at 65 mph also on
level ground.
As you increase your speed the hp required is increased,
duh! Air resistance increases by the square of speed, so there is 8 times as
much at 60 as at 30. Drag is so important to vehicle manufacturers that they
spend thousands of hours in wind tunnels testing designs. However, not so much,
if any, for motor coaches.
In the first example it takes 60 hp of that 100 hp just
because you are driving a barn door. Increase the speed to 65 and you now need
102 hp for the same barn door. Go faster and it just gets worse. When that
Prevost motors past you at 70 he is using over 200 hp. I hope by now that you
have realized that more horsepower required means more fuel required. As a rule of thumb, it takes 0.1 more fuel for each 1
mph over 55 mph. Going from 55 to 65 will cost you 1.0 mpg not counting any
other factors.
Let's introduce one other factor, terrain. I know, Kansas is
billiard table flat. Humor me. Think about climbing the Donner Pass eastbound;
zero to 8,000 ft in 50 miles. I have done that more than a few times. You can
kiss your fuel mileage goodbye. If you want to climb a 6% grade at 55 with your
34,000 lb coach you will need more than 400 of those horsepower ponies. That is
why I was usually at 45 mph climbing Donner. Now you know why those engines are
so big.
At any given speed there is only one way for your coach to
get better fuel mileage than the next guy, lie about it. When I go to Florida I
am at max gross weight and am pulling a 3,900 lb car. Do the math. I am not
going to get my "normal" everyday fuel mileage.
Now that I have you motoring along at a reasonable speed
what about stopping. Obviously your RV will not stop in as short a distance as
a car. What is your stopping distance? A car can be stopped from 60 mph in
250-300 ft depending on several variables one of which is reaction time. A semi
takes about 400-500 ft. Since we weigh about one-half of a semi, but several
times more than a car, your stopping distance is going to be closer to 400 than
300. Just stop and think about how far 400 ft is. Oh, how about your reaction
time. Still catch flies with your bare hands? Yeah, right.
What about inertia? You always need to maintain
momentum. When approaching a red traffic signal, try to time it so that you
keep rolling when it turns green. Let others trip the sensor, while you slow well in
advance. Even a creeping start uses less fuel than starting from a complete
stop. Also, from a complete stop, the only thing you will out accelerate is a fully
loaded semi. Easy on the throttle big boy! There are other examples but I think
you can get the idea.
So now we have addressed the first two items. On to weight. Again refer to the first
example concerning Y weight. How much of what you are hauling around do you
have to always have with you. No, you do not!! I am a minimalist. When we get
home from Florida I empty the coach and start over. That way I clean out all of
the nooks and crannys (look what I found!) and I put back in what we need for
the summer. No, I do not need to carry that 50lb screen room all summer. Also,
do you haul around a full tank of fresh water? Why? How much could you use
before you get to the next faucet? You say to yourself, "I am already a
heavy vehicle why worry about another 100 lbs.' Well, good luck with that.
Tires. One of the
contributors to hp requirement is rolling resistance. Remember that in the
first example it took 60 hp to overcome the drag at 55. Well, the rest of the
100 hp required (40) is from rolling resistance. How is the air pressure in
your tires? Do you know what it is supposed to be? No, it is not stamped on the
tire. That is the maximum pressure you can use and that is only necessary at
your maximum weight. If the pressure is higher than required it leads to more
rapid tire wear and a harsher ride. Conversely, if it is too low the tire will
get hotter and there will be more
rolling resistance. Just like you, tires do not like to be hotter than necessary.
It shortens their life and lowers your fuel mileage. For every tire there is an
ideal air pressure for any weight. It can be found on the tire manufacturer's
web site or in a brochure you can get at the tire store. Therefore, you will
need to know the weight of your coach. Have not weighed your coach? Shame on
you.
Frontal area. You
have no control over this factor. It is what it is, large! Remember, wind
resistance rises at a greater rate than your speed because of simple physics.
No, physics is not simple.
Weather. The
biggest weather factor is the wind. I know, it is always a head wind. Well, not
really, it just seems that way. For me wind is the biggest factor affecting my
fuel mileage. I drive 60 mph. A 15 mph head wind is just like driving 75.
Driving that 34,000 lb coach into a 15 mph head wind requires 63% more hp than
when there is no wind. Unfortunately, a 15 mph tailwind only reduces required
hp by 38%.
Let's go back to the
subject of tires. Are you aware that all tires have a maximum speed? That
is one reason you cannot put just any old tire on that neat Corvette. Therefore,
your motor coach tires also have a maximum speed. For example, all Michelin RV
tires except one have a maximum speed of 75mph. The one that does not has a max
speed of 65 mph. All Goodyear RV tires have a maximum speed of 75. So the next
time you are doing 70 and that Prevost roars by you he is not doing his tires
any favors. And neither are you.
Another point about
tires. When fitted to an RV they will usually age out before they wear out.
How do you know how long they will last? How can you tell if they need
replacing? The answer to both questions is - it depends. The general consensus
on when you should replace RV tires is 5 - 7 years. I know, you know a guy that
has 10 year old tires and they look just fine. Well, good for him. He probably
keeps his unit inside and drives 1,000 miles a year, if at all. Plus, you
cannot tell the condition of a tire by looking at the exterior. You can have a
tire tech dismount your tires and give you an estimate of the condition but you
will never get him to put it in writing. Here is a good way to look at
replacement time. Your coach is now six years old. How long are you going to
keep it? If you say, at least another four years why not just get new tires
now? Are you going to leave them on for another four years? If not, what is the
point of waiting. If you change now you will probably be good on tires for
another six years.
Hopefully, this is helpful information. For anyone who would
like more information let me know and I will forward documents on this subject
written by CAT and Cummins. They have both done quite a bit of research in
these areas.
