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Both are single aisle, 3 by 3 seating planes. Both are vaguely similar in length (with appropriate models). But they have fairly different ranges.

For similar sized airplanes, why the big difference?

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check the specs on wikipedia: B737 and B757 –  ratchet freak Jul 20 at 15:14
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the 757's engines are far more powerful than the 737's. –  shortstheory Jul 20 at 15:42

2 Answers 2

up vote 3 down vote accepted

Although the 737 has grown over the years, the 737 and 757 remain in different size classes. Both models can be ETOPS certified, although the 757 is more often used in long flights.

At the time the 757 was introduced (in the 80's), the 737 classic series was current. For the 737-400:

Passengers: 146
Length: 119 feet
Wingspan: 95 feet
Wing area: 1135 sq feet
MTOW: 138,000 lb
Takeoff distance: 8000 feet
Range: 2160 nm
Thrust: 2x 23,500 lbf
Max fuel: 5,311 US gal

The 757 was significantly larger. For the 757-200:

Passengers: 200
Length: 155 feet
Wingspan: 125 feet
Wing area: 1951 sq feet
MTOW: 255,000 lb
Takeoff distance: 6500 feet
Range: 4100 nm
Thrust: 2x 43,500 lbf
Max fuel: 11,489 US gal

The 757 is about 35 feet longer, seats about 50 more passengers, and flies about 2,000 nm further (roughly twice as far). The extra range is due to the larger wingspan and longer fuselage, being able to carry more fuel. For this reason, the 757 is used on longer and busier routes, while the 737 is used on shorter, less busy routes.

In the 90's, both models were updated. The 737 was significantly upgraded, including new wings and a new engine, which increased its efficiency. The 737 NG series eventually included the 737-900ER model, currently the largest 737 model.

Passengers: 180
Length: 138 feet
Wingspan: 117 feet
Wing area: 1341 sq feet
MTOW: 187,700 lb
Takeoff distance: 9843 feet
Range: 3235 nm
Thrust: 2x 28,400 lbf
Max fuel: 7,837 US gal

The 757-300 was also introduced. This was a stretched version of the 757-200.

Passengers: 243
Length: 178 feet
Wingspan: 125 feet
Wing area: 1951 sq feet
MTOW: 272,500 lb
Takeoff distance: 7800 feet
Range: 3595 nm
Thrust: 2x 43,500 lb
Max fuel: 11,489 US gal

Although the more efficiency gains of the new 737 bring the range up to nearly that of the 757, the 757 remains significantly larger.

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There's a good point implicit in here: it's important not to compare the largest 737 with the smallest 757 but to compare the models that were available at a particular time. –  David Richerby Jul 22 at 19:03
    
Of particular importance here: compare the MTOW of the planes. While the 757-200 only carried 33% more passengers than the 737-400, its MTOW was 85% higher, allowing it to carry more than twice the fuel of the 737-400. –  reirab Oct 31 at 20:20

The 757 was designed after the oil shock of 1973 and represented the best which was possible in fuel efficiency. Therefore, it's range is much greater, and with ETOPS certification it can be used on longer overwater trips. Besides, it shares the cockpit layout of it's twin aisle sibling 767, so flight crews could be certified on both with a single training. It replaced the Boeing 727, which is a bigger airplane than the 737.

The 737 is a much older design, but has since then been redesigned twice. The latest models (737-600 to 737-900) were redesigned in the 1990ies and have a new wing and new engines, so their fuel efficiency is again state of the art. Both the 737 and the 757 inherited their fuselage cross section from the 707. With the stretched fuselage of the 737-900, the latest 737 it is almost as big as a 757-200.

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Could you elaborate on how the 737's cross section was inherited from the 707? Otherwise, nice answer and +1 :) –  shortstheory Jul 20 at 16:37
    
@shortstheory The 707 and 737 share the same nose for instance. –  MikeFoxtrot Jul 20 at 17:59
    
Thanks, MikeFoxtrot! @shortstheory, please see this link. The 727 also inherited the 707 fuselage, which had a constant cross section for most of its length, so the designers simply cut out sections. They could use doors, galleys, the interior paneling and many fittings from the 707. The commonality is approx. 60%, which helped to get the plane from project launch to first flight in just two years. –  Peter Kämpf Jul 20 at 18:38
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I just learned from this thread at airliners.net that the forward fuselage cross section of the 757 is smaller than the aft section. Interesting stuff! The forward section was taken from the 727, and the aft section from the 737. The upper part was the same for all three; the difference was in the lower part. –  Peter Kämpf Jul 20 at 19:46

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