Improving Fuel Mileage
Everyone wants to know what a big RV gets on fuel mileage. This is a good question. Fuel is a major part of using an RV. It pays big dividends to improve your fuel mileage. even an improvement of .1 mpg will save you money. Everything you do to improve your fuel mileage will big benefits.
My first trip to Dayton, OH showed about 9 mpg. I wasn't surprised, I was driving 60 to 65 mph. This was my first look at what it cost to run a larger RV. My small Toyota based Dolphin would usually get up to 12 mpg and the speed was usually 58 mph. The big RV had no trouble going 70 mph or better. The big 454 cu inch motor chugged along just great under those conditions. You just had to pony up at the gas pump. I slowed down to 50 mph to see what the mileage would be. It was about 12 mpg. 3 miles per gallon improvement meant I saves a lot on fuel costs. The only drawback was it took longer and 50 mph seemed so slow.
During my trip to Florida I got 9 mpg. I checked my fuel mileage after the first fill up. I lost 3 mpg towing the car. That's OK though, since I expected that. Easy on the gas while accelerating and a slow 50-55 mph would keep the fuel mileage at that level. My expectations changed when I got my MPGuino fuel mileage meter. You must briskly accelerate to 50 mph!
Our car usually gets 27 mpg and if you figure eating out for every meal and staying at motels at $60.00 per night or more I think that the cost would have been more for the 3 weeks we were on the road. The fuel cost was $1100.00 Having the car along made the stay at Lakeland nice, since we could come and go easily.
I installed a vacuum gauge to monitor the efficiency of he engine. The vacuum gauge measures the engine vacuum created by the suction of the pistons as they draw air. The higher the vacuum the more efficient the engine is. The highest reading occur at idle reaching 25 inches of mercury. (hg)
Read about the MPGuino Here
Finding out why the mileage was low.
I didn't check my engine computer for best operation then, but
this winter I went to my favorite auto mechanic whom I have known for many
John plugged in his expensive tester and quickly narrowed the
problems down to one. The oxygen sensor was bad.
Once installed I noticed that the idle went down to the 650 rpm
instead of 950 rpm that was there.
Using a Vacuum Gauge
A vacuum gauge shows how hard the engine is working. The engine vacuum is determined by engine RPM, throttle position, transmission gear and friction loads.
An RV has a large wind load since the frontal area is very large. A small auto will have only several square feet of frontal area. My RV has about 70 square feet. It is like driving your garage door down the road.
This means that the speed when air friction kicks in and you consume more fuel trying to go fast.
I took readings while running steady on the expressway at increasing speeds. Here is the chart.
Engine vacuum is read in inches of mercury (hg). The larger the engine load requires you to press harder on the gas pedal. That opens the throttle blade and reduces the vacuum. You guessed it, fuel is being used faster and fuel mileage goes down.
My vacuum chart for my RV.
50 mph = 15 inches hg
55 mph = 12.5 inches hg
60 mph = 11 inches hg
65 mph = 10 inches hg
70 mph = 7 inches hg
You see how the value of inches of mercury diminishes as the speed increases. This shows how much throttle you are giving the engine. More throttle means the throttle plate is open more and the vacuum goes down. It also means that the engine is getting more air. That increases the rate of fuel flow as the Electronic Fuel Injection adds more fuel so it burns at the correct ratio. Meaning you are burning more gas and getting more horsepower. That means your fuel mileage goes down.
To use the vacuum gauge all you have to do is watch it once the transmission has up-shifted to the top gear and the torque converter has locked up. Add throttle and the value goes down. lift up on the throttle and the value goes up. The best fuel economy is just after torque converter lockup and the vacuum gauge shows the highest value it can . You slowly increase your speed, noting the reading. Sometimes engine efficiency will increase as the cam timing and other factors kick in to increase horsepower. The torque curve of the motor and the differential gearing and diameter of the rear wheels determine the actual speed when that occurs. Sometimes those conditions never show up, the size of the RV can mask this effect. Automobiles with high performance engines always do, and you notice it when accelerating and as you near cruising speed you have to back off the throttle a lot to settle down at the speed you want. That's good, I hope that your RV does the same.
Look here for Chevy, Ford and Dodge horsepower curves. http://www.gearvendors.com/hrgmhptorque.html
By driving near the peak torque RPM you maximize your fuel mileage. I found out that the torque curve from the engine is very flat, which is good, making for easy driving.
You will need to know the type of engine in the RV to get the correct graph. Look the graph over to see the RPM that will give you the most MPG. Look for peak torque and note the rpm it occurs. Then drive that RPM and note your speed. Obviously you need a tachometer in your RV. This is probably the most efficient speed. Other factors such as aerodynamics, tire pressure, even the fuel quality and air barometric pressure can have an effect.
I found out the engine RPM is at 1600 rpm, showing about 350 ft/lbs torque available at full throttle. Lots of torque to power the RV up mountain hills. 1600 RPM is 50 mph in high gear and the torque converter locked up, right where we want to be for the best fuel mileage.
The hardest part of saving fuel is to SLOW DOWN! I have found that driving at 50 mph is so slow. I usually drive at 55 mph. The 5 miles faster seems much better. I do feel jealous when a big pusher RV flies by me with a toad locked in behind him with no care about fuel mileage. Actually 55 is still too fast I'll have to settle on 50 to 53 MPH to get 15 to 20 mpg.
A head wind makes he driving for mileage harder. On a 4 lane expressway with traffic gives you more mileage. The stream of vehicles causes a column of air going down the road and acts like a tail wind. You can use that as a way to gain mileage.
Drafting behind a semi truck and trailer sounds like a good idea. It really isn't. There are inherent dangers when following a semi. Visibility is reduced. Other traffic may interfere. Generally I don't follow any closer than four car lengths behind the truck.
Semis have big motors and will usually out pull you down the road. Because you are in cruise mode the engine is loafing. When you get a hill or overpass the semi will maintain speed and your RV will slow down. The effort to keep up causes you to lose mileage trying to keep up. If you have a diesel pusher RV that effect may be less for you.
As you can see this can be complex as the variables mount up when looking at ways to evaluate and improve your fuel mileage.