so then the back seat leg room is the same in the 140 and 180 assuming the same year?
No, they are not.
The 140 and 180 up to 1973 were the same length fuselage.... In 73 the Challenger was the first "180" with the 5 inch fuselage stretch, and the old wing. The "Archer" was the Challenger with the new taper wing. All 140's have the same "shorter" length fuselage. and the same wing.
The -140 and early (pre-1973) -180 have the same
external dimensions, but the rear cabin bulkhead of a -140 is one station further forward, so the -140's cabin is considerably smaller behind the front seats.
This is the 1965 Cherokee 140 "2+2 Cruiser". Beginning in 1969 there was an option for a molded plastic rear bulkhead that formed a hat shelf and tiny baggage area behind the snap-in seats, but the room for the rear victims ... er, passengers ... was the same.
And here's the Cherokee 180 (also the Cherokee 150, 160, 235 and Arrow). Beginning in 1971 the rear seats were individual bucket seats.
The convoluted history of the Cherokee line helps to understand the differences.
The Cherokee was designed as an entry level, four-seat family airplane to replace the Tri-Pacer in the Piper line. The first PA-28 to be certified and go to market was the four-seat PA-28-160 in mid 1961, followed a few months later by the PA-28-150. Despite somewhat lower performance, the -150 proved more popular because it could use less-expensive 80-octane fuel. The PA-28-180 came along in late 1962, and the -235 at the end of 1963. All these had identical interior dimensions (same as the blue one above); in fact the same airframe, N2800W, was used as the flight test prototype for the -160, -180 and -235 in succession.
Meanwhile, Piper was getting its brains beat out by Cessna in the primary trainer market. Piper had nothing to compete with the popular C-150, other than the fabric-covered Colt and Super Cub. Piper had planned a new two-seat, low-wing trainer (PA-29 Papoose) to replace the Colt, but its plastic-composite construction was not ready for prime time (or even direct sunlight, as it turned out), and it was scrubbed. To get a trainer to market as quickly as possible, Piper took the rear seats out of a PA-28-150, moved the rear bulkhead forward by one station, eliminated the baggage door, and put a cruise prop on it which derated power to
140 hp (2450 rpm), and
voilá, the PA-28-140 "Cherokee 140" was introduced in early 1964. MGW was initially limited to 1,950 lb., which seemed plenty for a two-seater.
Desperate to build brand loyalty with a basic trainer, Piper aggressively priced the -140 at $8500, about 25% less than the four-seat Cherokee 150.
Within a year, though, Piper added optional snap-in rear seats for the -140 (the "2+2 Cruiser" option package), repitched the prop so power was equivalent to the PA-28-150 (2700 rpm), and raised the MGW to 2,150 lb., also equivalent to the Cherokee 150.
The "2+2" Cherokee 140 (intended for the flight school market) and full four-seat Cherokee 150 and 160 were built side-by-side until 1967, when the -150 and -160 were discontinued. Beginning in 1964, though, the Cherokee 150, 160 and 180 had the restyled fiberglass cowl that made them easily distinguishable from the -140.