[7] The drive to enter a product in this class came from the increased affluence of the French market which Peugeot felt could accommodate a car more expensive than the 504.

The 604-era was perhaps the last point at which PSA Peugeot Citroën could theoretically have focused resources on becoming a viable long-term competitor in this lucrative market segment.

Contemporary journalists attributed this relative failure to a variety of reasons, such as the conservative styling, the lack of technical innovation, modest performance, and a reputation for problems with rust.

In September, the somewhat quicker 604 TI model with Bosch K-Jetronic fuel injection and a five-speed manual transmission was added.

[35] In 1980 the 604 underwent a minor facelift, featuring a new rear-view mirror, while the bonnet badge was removed, the indicators were changed to orange, and there was also a new plastic dashboard and gearstick.

[39] As a partial replacement, a V6 version of the 505 soon appeared, and in 1989 the front-wheel-drive Peugeot 605 arrived to fill the empty slot left by the 604.

The 604 is noted for being the first production turbodiesel car ("604 D turbo") to be sold in Europe,[40] the Mercedes-Benz 300 SD having been marketed in North America since 1977.

Accommodation inside the car was viewed as being "fully up to international standards for a four-five seater touring saloon with room in the rear seat for two people to travel long-distances in real comfort".

[54] Autocar's 1977 long-term test of the SL model commented that as well as the cabin being able to accommodate "up to five people, the boot was "one of the largest [the author] could remember, even with the big spare wheel strapped upright on the right hand side.

Autocar went on to add that the boot was "not only long, fore and aft, but wide and deep so that full-size suitcases can be packed upright".

[7] Throughout its career it was rated above all for its ride quality which Car magazine described as being superior to the 1980 Mercedes S-Class W-126 and the equal of Jaguar.

[21] As a classic car, the 604 has acquired the lasting image of being "forgotten" and for being less successful than Peugeot hoped,[62] but it is also noted for its "outstanding" ride and accurate, communicative steering.

Car praised the refinement of the suspension and the smoothness of the engine which was "an exceptionally flexible unit that has the ability to rev to dizzy heights without uttering the slightest complaint at either extremity".

[63] The article concluded that "the Jaguar XJ6, the BMW 525/528, the Fiat 130, Mercedes 280E and Volvo 264 GL are going to have to close ranks against a formidable new intruder".

[65] Detail criticism was focused on the ergonomics (awkward window switch placement, poor driver's seat adjustment, "fussy" speedometer markings), mediocre fuel consumption, and some minor assembly faults.

Comparatively, the more expensive Jaguar XJ6 3.4 was rated more highly for its ride and noise suppression while the BMW 520i was judged to be more sporting though overpriced.

[66] The US magazine Motor Trend described the rack-and-pinion steering as being "of surpassing excellence and must be experienced to be appreciated" in a review of the 604 SL.

[66] Turning to the suspension, Motor Trend wrote that "springing is soft with a great deal of wheel travel.

[68] The only weak area was performance, the 604 "not coming up the standards of excellence set by ride, handling and stopping ability".

The review added that "acceleration and speed are adequate to keep from being overrun by other traffic but by no means can performance be considered brilliant".

[68] This was countered by the car's ability to maintain high average speeds on account of its capacity to cope with "twisting, poorly surfaced roads".

[68] Overall, given the car's remit as a luxury saloon, Motor Trend viewed this performance limitation as being of secondary importance.

[68] In November 1977 the UK's respected Car magazine ranked the 604 first in its Giant Test of the Peugeot, BMW 728 and Mercedes 280E.

[69] Autocar's long-term test in 1977 of the 604 SL concluded it was very refined, let down mainly by the design of the front seats and minor mechanical defects.

[70] In the following year, Car compared the fuel-injected 604 Ti with the Lancia Gamma and Rover 2600 and declared the Peugeot the best car out of the trio: "So strong is its appeal [...]that even the problems some of our team have with the driving position can´t turn us away from it, enhanced as it now is by the extra silkiness that the fuel injection brought, and the greater quietness when cruising that the five-speed transmission allows".

Whilst the Peugeot received commendation for its ride ("outstandingly good"[72]), "superb" headlamp performance, the comfort of the rear passenger compartment and the generously-sized boot, it lost out to the Royale which was viewed as a more rounded overall design.

Of the four cars, the Peugeot 604 Ti had the highest top speed (119 mph), best 0-60 performance (9.7 seconds) and best fuel consumption figures.

[75] Overall, the car's main advantages were its comfort for driver and rear passengers, the large boot capacity, low-noise levels and the quality of the steering.

[77] Classic & Sportscar carried out a dual test of the Peugeot 604 SL four-speed manual with the Lancia Gamma Berlina (though did not rank them).

[23] The tri-choke Solex carburettor arrangement also received a critical commentary, being responsible for leaving the driver "with an impression of a mid-range flat spot" though the car still "surges forward with brisk authority and a smooth lusty hum....".