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Car Forum / Driving, Maintenance, Tuning / Maintenance and Repair / June 2007Auto v truck mileage
In his book "Common Carriers" John McPhee says that the "common weight limit for five-axle eighteen-wheelers is eighty thousand pounds" [page 22] and the the rig of the driver he rode with while writing part of the book "gets a smidgen over six miles per gallon" [page 23]. That's 40 tons and 6 miles per gallon or 240 ton-miles per gallon. Typical 2 ton autos get what 20 mpg? That's 40 ton-miles per gallon. Certainly there is more to the mileage that a vehicle gets than its weight coefficient of drag, diesel vs gasoline, number of gears, acceleration come to mind. What keeps our cars from getting so much less ton-mileage per gallon than our trucks? Are there any practical things that auto and engine makers can learn from trucks to substantially improve auto mileage? JKA > In his book "Common Carriers" John McPhee says > that the [quoted text clipped - 21 lines] > much less ton-mileage per gallon than > our trucks? lies, damn lies, and statistics. IIRC, Motor Trend had a sidebar where they created their own number PMPG or Passenger Miles Per Gallon. Basically, an 8 passenger Suburban getting 20mpg = 160 PPMG. A Honda Civic getting 35mpg but only carrying 4 passengers = 140 PPMG. It's all in how you look at it. Ray > In his book "Common Carriers" John McPhee says > that the [quoted text clipped - 6 lines] > > That's 40 tons and 6 miles per gallon or 240 ton-miles per gallon. Railroads are claiming well over 400 ton-miles per gallon. > What keeps our cars from getting so > much less ton-mileage per gallon than > our trucks? The fact that they can do zero-to-60 in a few seconds versus 10s of seconds is probably the most important difference. Truck and train engines operate at near 100% output all the time with very little reserve (that's why most trucks have to slow down to climb a hill, but most cars can climb the same hill without slowing). Operating an engine closer to its maximum output is always more efficient. Also truck tires are ninflated to well over 100 psi (and of course train wheels are steel rolling on a smooth steel track- the best possible world). Can't do either of those with a car. > Are there any practical things that > auto and engine makers can learn from trucks > to substantially improve auto mileage? Not and maintain most of the features that car drivers want. Other than to go to energy-storage drivetrains (hybrids) that allow the engine to run at something closer to constant output. > In his book "Common Carriers" John McPhee says > that the [quoted text clipped - 27 lines] > > JKA I have an idea - lets use semi's as super-size busses. The efficiency ratio of a semi with double trailers would go off the chart if we just stacked those passengers up like cordwood. What the others said. Also interesting to compare mpgs among big vs smaller cars. CR says the 108 hp 2500 lb Honda fit gets 32 mpg. And a Corvette at over 300 hp and about 3500 # gets 20 mpg. A *much* smaller differential than I would have expected! Intuitively, I would have expected something like 10 mpg vs. 40, a 4:1 ratio, instead of a 1.5:1 ratio.  Signature------ Mr. P.V.'d (formerly Droll Troll), Yonkers, NY Stop Corruption in Congress & Send the Ultimate Message: Absolutely Vote, but NOT for a Democrat or a Republican. Ending Corruption in Congress is the *Single Best Way* to Materially Improve Your Family's Life. The Solution is so simple--and inexpensive! entropic3.14decay at optonline2.718 dot net; remove pi and e to reply--ie, all d'numbuhs > In his book "Common Carriers" John McPhee says > that the [quoted text clipped - 27 lines] > > JKA > In his book "Common Carriers" John McPhee says > that the [quoted text clipped - 27 lines] > > JKA Think of it this way: for every 40 tons of truck you've got one engine, one transmission, eighteen wheels, one radiator, and one driver. For every 40 tons of car you've got 20 engines, 20 transmissions, eighty wheels, and 20 drivers. Plus you've also got 20 power steering pumps, 20 alternators, 20 AC compressors, etc. Then you need 20 radiators to shove through the wind to keep it all cool. All of that redundant redundancy comes at a price. Also, trucks carry more than their own weight, while cars typically carry a fraction of their own weight. Compare motorcycle ton-miles with truck ton-miles and it'll come out better. SignatureB.B. --I am not a goat! thegoat4 at airmail dot net
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