Why do trucks get bad gas mileage?

  1. ShawnD

    ShawnD 953
    Science Advisor

    A friend is getting a new Toyota Tacoma, the V6 model. I asked him if it was worth getting the V6 since, since they burn more gas, and he said it burns the same amount as the 4-cylinder. I checked toyota's website, and he's right.

    Toyota Tacoma stats
    4-cyl (159hp, 180ft-lb torque) gets 21mpg in the city
    V6 (236hp, 266ft-lb torque) gets 21mpg in the city

    The V6 has way more power, way more torque, and burns the same amount of gas.

    It's not a weight issue.
    4-cyl is 3455lb
    V6 is 4129lb
    The V6 is much heavier, and it still burns the same amount of gas. How does this compare to vans and SUVs from the same company?

    The Toyota Sienna (van) has 266hp, 245ft-lb torque, weighs 4140lb, and still gets 24mpg in the city. This van has more power and torque than the 4-cylinder Tacoma, is close to 600lb heavier, and it gets about 20% better gas mileage?

    Maybe Toyota just doesn't know what they're doing. Look at GM instead.
    Chevy Silverado
    4.3L engine gets 20mpg city
    4.8L engine gets 20mpg city
    5.3L engine gets 19mpg city
    5.3L hybrid gets 21mpg city
    6.0L engine gets 17mpg city

    Basically everything has been isolated and shown to have no difference. Does weight affect a truck's gas mileage? No. Does engine size affect gas mileage? No (until you get up to 6L in size, then yes). Does having a hybrid help? No.
    Why do trucks get such bad mileage?
  2. jcsd
  3. russ_watters

    Staff: Mentor

    It is mostly wind resistance for higher speeds, weight for lower speeds. Engine size only matters if you actually use the extra torque and I suspect the guys that actually do the tests practice hard to not use the extra power available to them.

    Also, thermodynamic efficiency is mostly a matter of compression ratio and at least in the case of the Toyota, the 6cyl has a higher compression ratio than the 4cyl. The 6cyl also has an optional 6 speed transmission that the 4cyl doesn't have. Better (better matched...) transmissions make a big difference too.
  4. Also those reported gas mileage figures can be misleading. EPA ratings have been under a system of self-enforcement where companies run their testing in their own manner. 2008 models will be subject to more uniform testing, as of now some companies such as volkswagon with their TDIs put more stringent requirments than other companies such as toyota with their prius. You can't judge a vechiles true efficientcy till you own one or ask people who have one.
  5. Integral

    Integral 7,288
    Staff Emeritus
    Science Advisor
    Gold Member

    A very good possibility would be a difference in rear ends. Frequently a smaller engine needs the higher RPM for power when hauling, towing etc. So they will trade power for gas mileage and top speed.
  6. Stingray

    Stingray 674
    Science Advisor

    I think the Canadian Toyota site (which is the one I'm assuming you used) just has an error. If you look at their fuel consumptions numbers in L/100 km, they don't match up to the mpg numbers given there. The models you were talking about get 11.1 and 13.4 L/100 km city. That's 21.2 and 17.5 mpg. The heavier one burns more fuel, as you'd expect.

    I'm not surprised that engine displacement doesn't have much of an effect. The 6 L example you gave probably only has a lower rating because it's been tuned more for performance. Modern engine controls allow the other engines to maintain similar efficiencies under similar loads.
  7. ShawnD

    ShawnD 953
    Science Advisor

    Interesting stuff here. Compression, rear end, tuning, etc. It all makes sense.

    Those EPA estimates are very accurate. My Civic gets exactly what it was rated for, my parents Altima and Xterras match correctly, and even my Ford Tempo made 15 years ago was very close to the EPA rating.
    Last edited: Jul 28, 2007
  8. My jeep started off getting 20-22 mpg and I just put a performance exhaust on it and that has bumped it up to 24-25 mpg, I even got 26 with it once or twice. If you are lucky a few mods can bump your mileage quite a bit, I was quite fortunate and my exhaust was cheap. Another thing I am looking into is a water/methanol injection system that is supposed to gain fuel mileage and power quite significantly. I haven't really had the time to research that yet so I don't know the ins and outs of it.

    Wind resistance is a big thing in determining gas mileage. And of course trucks and SUVS are just tuned differently than a car so that makes a difference as well.
  9. russ_watters

    Staff: Mentor

    My Mazda 6 doesn't get it's rated MPG - my last car, an Eagle Talon, did. Hybrids are notorious for getting nowhere near their rated fuel economy.
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