Why are tractor-trailer rig so much more fuel efficient

[trikkonceptz]

Your analysis aside, I think big rigs get better mpg per pound because they have to. Meaning their purpose is to haul in order to make money, passenger cars are simple transportation for individuals.

Therefore the industry put forth much more effort to make those monsters more fuel efficient in an effort to reduce operation costs. Same would hold true for trains. There is less pressure to make improvements in those fields because it serves the greater good, product and services. Whereas commuter cars are expendable and simply a profit center from which to reap benefits.

This also translates to replacement parts. More passenger cars are repaired at shorter intervals than tractor trailers. My guess is for the same reason.

[greenitup]

well the frontal area is bigger but isn't 10 X bigger, also when you have 8 gears (think that is the amount) you don't get high rpm's

[MazdaMatt]

gearing varries... up to 14, i think.

[Clev]

Originally Posted by Gregte

The fact is, cars are already geared as they need to be to most efficiently do their job, as are trucks.

Trucks I agree with, but passenger vehicles are seldom geared to efficiently do their job. They're often geared to make them seem peppier or smoother. Only those marketed as fuel efficient are actually geared properly for efficiency.

[mavinwy]

The inline engines are actually still used because of reliability. The 7 main bearings in an inline 6 supports the abuse better than the 5 in a V-8. Of course an inline 8 would have 9, but the length of the engine becomes an issue and they do not produce that much more torque for the weight added. The higher compression of a diesel does also add a higher amount of force on most all of the rotating parts

Jim

[99metro]

They are more aerodynamic.

[SuperTrooper]

Originally Posted by MazdaMatt

gearing varries... up to 14, i think.

Some Road Ranger trannys have 18!

[Gregte]

Originally Posted by Clev

Trucks I agree with, but passenger vehicles are seldom geared to efficiently do their job. They're often geared to make them seem peppier or smoother. Only those marketed as fuel efficient are actually geared properly for efficiency.

Well then why don't we ecomodders raise, or lower, our final drive gear ratios by changing differential gears?

Trucks have more gears because diesels have a much more limited RPM range. And once you are up to hiway speed the only gearing that matters is your final drive gear. More gears has nothing to do with hiway driving for a car unless your terrain is so hilly that you would actually be downshifting on the hiway.

[Clev]

Originally Posted by Gregte

Well then why don't we ecomodders raise, or lower, our final drive gear ratios by changing differential gears?

Trucks have more gears because diesels have a much more limited RPM range. And once you are up to hiway speed the only gearing that matters is your final drive gear. More gears has nothing to do with hiway driving for a car unless your terrain is so hilly that you would actually be downshifting on the hiway.

If I had RWD, I would, but I don't know enough about FWD transmissions to comfortably replace the gear.

[jamesqf]

IIRC, the ones I used to drive (decades ago) had about 20 speeds: 10 in the gearbox, plus a 2-spd rear end.

This is another reason why trucks get better fuel economy. The large number of gears means the engine can be run closer to its optimum efficiency. In addition, long-haul trucks just get out there on the highway and cruise at a steady speed for hours, which is a lot more efficient than the typical passenger car pattern.