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Wagons Scion xB Scion Owners: MPG-Real World Numbers
494 messages, Last post on Nov 26, 2009 at 1:58 AM
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Replying to: jamesgang (Aug 18, 2005 5:38 pm)  I am running Nitrogen in the tires and Mobil One in the motor, best I get is 32/36 HWY  75 mpg on the Suncoast Parkway here in Florida, going to Tampa international using cruise. Got 36 mpg once using Mobil Premium. Straight regular gets me 32 Hwy. If you get 40 mpg it's on a trailer 
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Replying to: dbecker6 (Feb 20, 2008 8:06 am) The frontal area of the car is fixed, so lets call that "Y" The formula for wind resistance is "Y" times Mph squared. At 65 mph the result would look like "Y" times 4225. At 75 mph the result would look like "Y" times 5625 That is about 33% difference in wind resistance. Our 03- 4wd- 4 spd AT- CR-V gets 30-31mpg at 65. It gets 23-24 at 75mpg. Thats about 33% difference in mpg with a speed difference of 10 mph So, if you get 32mpg at 75, slowing down to 65 just might get you up there with jamesgang. You do the math! If you choose to not drive that slow, then you choose to get poorer mileage. Kip |
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Replying to: kipk (Feb 20, 2008 9:33 am) How about if I choose not to get run over driving so slow  ? Dennis
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Replying to: dwynne (Feb 20, 2008 9:41 am) No guts, no glory!  Kip |
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Replying to: kipk (Feb 20, 2008 9:33 am)
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Replying to: dwynne (Feb 20, 2008 9:41 am) The internal combustion engine (ICE) is actually more efficient as you increase the power output in steady operation up to about 80% of max power. The engine efficiency is expressed as a quantity called brake specific fuel consumption (BSFC) which is weight of fuel consumed per unit time per unit of engine power being developed. In "English" units BSFC is in lb/(hp*hr). An ICE has the lowest BSFC (and so highest efficiency) when it is developing 50 to 80 % of its maximum rated power. Since a Scion XB would require only about 25 to 30 hp to go a steady 70 mph, a 60 hp engine would be the most efficient engine which would still have some passing ability and freeway onramp acceleration. But you can see that a new Scion Xb which has a maybe 140 hp max engine is operating at say 30 hp/140hp = 21 % of rated max which is well below the optimum for best efficiency. Just to make clear what I am stating say that the engine in a Scion Xb has a BSFC of 0.48 lb/(hp*hr) when developing a steady 30 hp (required to go 70 mph on level ground). Suppose that the car requires 38 hp from the engine to go 80 mph and that at 38 hp the engine has a slightly lower BSFC of 0.46 lb/(hp*hr). So at 70 mph the engine will consume fuel at a rate of 0.48*30 = 14.4 lb/hr and the fuel consumption per mile will be 14.4/70 = 0.206 lb/mi. If we assume that the density of gasoline is 6.2 lb/gal, then the familiar mpg figure is 6.2/0.206 = 30.1 mpg And at 80 mph the engine will consume fuel at the rate of 0.46*40 = 18.4 lb/hr and the fuel consumption per mile will be 18.4/80 = 0.230 lb/mi. And the mpg figure is 6.2/0.230 = 27.0 mpg. So as speed is increased at around legal highway speed the engine is more efficient, but since it is being asked to deliver more hp, then this causes the mpg to decrease. NB My calculated mpg figures are lower than what is observed because I must have assumed too high a figure for the horsepower required to go at these constant speeds and/or I assumed values for BSFC that were too high. The manufacturers do not publish these figures for their vehicles and I made assumptions which however are in the range of what is the case for modern automobile ICEs. Note also that if you really did go for maximum efficiency and did power your vehicle with a say 60 hp engine, the engine would wear out sooner than desired unless you designed to operate at extgended periods at 50 % to 80 % of its rated maximum hp. Auto engines are not usually designed to do this, whereas aircraft piston engines are (e.g. Lycoming, Continental, and Rotax engines). Aircraft engines are designed to operate at say 70% to 80% of maximum rated power for extended periods of time. But these aircraft engines have a much lower max power rating per unit displacement than auto engines. A 360 cubic inch (5.9 L) displacement Lycoming O-360 is rated at only about 180 or maybe 200 hp whereas an automobile engine of that displacement would normally develop more like 300 to 350 hp. And the Lycoming O-360 as a crate engine costs maybe $25K, requires 100 octane leaded av gas (so called 100 LL, meaning "low lead"), and has no muffler or pollution controls. Compared to an auto engine the aircooled aircraft engines drink fuel, ?BSFC = 0.55 lb/hp*hr? Maybe higher.
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Replying to: jim314 (Feb 20, 2008 12:47 pm) You gave a wonderful explanation! Although it went way over my head! I don't always understand exactly "WHY" something occurs. I do understand results. Example: I know that if I touch something too hot , my hand will jerk away. I don't totally understand the interaction of the nerve and brain. The result is the jerking away from the heat source. You say "The internal combustion engine (ICE) is actually more efficient as you increase the power output in steady operation up to about 80% of max power." and I take you word for it. However when an AT shifts into OD and the Torque converter locks, the RPM goes down the engine is less efficient, but MPG increases. There are many things effecting mileage at different speeds, as you pointed out. I know that if I drive my car in 3rd gear at 60 MPH, which would put the engine in a higher RPM mode and closer to the 80% efficiency zone, the mileage will drop significantly vs the same speed in OD. Results! Apparently the 80% thing has to do with HP vs fuel consumed. At the speeds we drive, a much lower HP is needed and less fuel is consumed . I know that when battling a head wind the mileage will drop. I've experienced a 3-4 mpg drop when dealing with a 10-15 mph head wind. That isn't nearly the drop that I would have gotten if I had been driving 75-80 in calm air vs 65 mph.. Seems to me it is because the car was in OD most of the time at a lower RPM than it would have been running at the higher speed. My next door neighbor sets his cruise control at 75 mph. His Ridgeline delivers 20+ mpg at that speed. One time on a 300+ mile trip he set the cruise at 65 and got 26+/- mpg. That is a 30+/-% increase by running slower with 33% less wind resistance and the engine at a lower efficiency. Result ! In an above post I gave the Results of our CR-V at the various speeds. Our Pilot delivered 18+ mpg on a trip at 80 MPH. Next day, on the return trip with the same (4 person) load the mileage was 26+ at 65MPH. That is a near 44% increase by slowing down 15 mph. In this case the wind resistance difference was near 50%. While neither of these 2 examples show the mileage increasing exactly the same as the wind resistance decreased, it is still significant. With our CR-V the correlation was very close. My 3 examples are heavier, different shape, and more frontal area than a Scion, which surely will have effect of moving through the air. So, as you say, there are more things at play than just the wind resistance. Our friend Mr. dbecker6 only needs to increase his mileage by 25% to achieve a number he feels can't be obtained, unless on a trailer. I have no problem believing that by lowering his speed by 10 MPH and wind resistance by 33%, it can be achieved. Any way we spin it, slower speed results in better mileage. Looking at it from a cash standpoint, the person getting 30% less mileage is paying 30% more to travel a given distance. In effect paying $3.90 per gallon of fuel vs the 30% better mileage driver at $3.00 per gallon.  Thanks, Kip
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Replying to: kipk (Feb 21, 2008 8:36 am) The most efficient gear for travel at any speed is the highest gear that will keep the engine from lugging. Lugging occurs at a given rpm when so much fuel is delivered to the engine that the amount of fuel in each cylinder firing is so great that it stresses the crankshaft and perhaps squeezes out the oil film between the crankshft journals and the connecting rod bearings, mybe also the main bearings. Suppose you are going 40 mph on level ground, with no headwind in 5th gear of a manual tranny. So the engine speed might be 1600 rpm. You are keeping the accelerator in a certain position to maintain this speed. Now suppose a headwind beings to gradually build up and over a period of time increases to say 30 mph. During this time you must steadily increase the accelerator pressure to maintain 40 mph. At some point so much fuel will be in each cylinder that the car would begin to shake, but as you approach level of accelerator pressure which causes this lugging you would downshift to 4th gear to increase the engine rpms up to a level where the amount of fuel per combustion event is decreased, but the fuel flow rate (wt or vol per unit time) stays constant because there is an increase in the number of combustion strokes per unit time. This lessens the strain on the pistons and rings, the connecting rods and the crankshaft, but causes more frictional losses in the engine. Engine power output remains the same when you down shift if you maintain the speed. There are more combustion events per unit time.
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Replying to: jim314 (Feb 21, 2008 3:08 pm) 
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This NASA compilation of formulas (http://space-power.grc.nasa.gov/ppo/projects/heva/appx_a.html#AERODRAGgives the power required to overcome various forces which resist the motion of a vehicle. You can see that the power required to overcome aerodynamic resistance is given by P,aero = 1/2 x Frontal area x coef. drag x vel^3 x air density So the power reqd to overcome aerodynamic drag at a given speed is proportional to the cube of the speed. However, power is the energy consumed per unit time and what we want is the energy consumed per unit distance. The latter would be proportional to the fuel use measured in gal/mile or (in the UK and Europe) in Liters/100 km. We get the energy/unit distance by dividing the power by the speed. So Energy/unit distance = (Constant of vehicle) x (vel^2) x (Density of air) From this formula we see that the aerodynamic component of fuel consumed per unit distance traveled is proportional to the square of the speed and to the first power of the density of the atmosphere. The EPA highway mileage estimate assumes that the vehicle is driven at a faily low speed even on the highway, something like less than 65 mph. Driving faster than the EPA assumes is a major contributor to using more fuel/unit distance, that is getting lower mpg than the EPA estimate. This is the basis for the often quoted "folk" statement that "air resistance at 70 mph is double that at 50 mph". (70/50)^2 = 2.0. What this exactly means is that the aerodynamic resistance consumes twice as much fuel to go a given distance at 70 mph than at 50 mph. But we know that mpg wouldn't double if we slowed from 70 to 50. The reason is that there are other power consumers besides aerodynamic resistance. Note that the constant of the vehicle is less for vehicles with low coefficient of drag and low frontal cross sectional area. A Scion Xb must have a relatively high coefficent of drag and a high frontal cross sectional area. So aerodynamic drag will be a more significant factor for it than for vehicles like the other Scions which have a smaller frontal area and CD. If a vehicle has a very low drag coefficent, like a Porsche sprts model, then the effect of increasing speed will be much less than for SUVs or the Scion Xb. |
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