The gearbox is directly behind the engine, and a tubular propeller shaft, under the transmission "tunnel", transmits the drive to a solid live rear axle.

The rears were drum brakes (a technological backwards step from the 124, which used discs all round), utilising leading and trailing shoe design operated by a dual piston fixed slave cylinder.

[8] Trim levels were two; the entry-level 131 Mirafiori (also known as "Normale" or "Standard") had single square headlamps, wheels and dished hubcap from the 124, and simplified interior furnishings.

Next was the better appointed 131 Mirafiori Special (or simply "S"), which could be distinguished from the base model by its quadruple circular headlamps, specific grille, side rubbing strips, chrome window surrounds, and rubber bumper inserts.

[7] Inside it added different instrumentation with triple square dials, a padded adjustable steering wheel, cloth upholstery, and reclining seats.

[7] Additionally the more sophisticated options — such as air conditioning, tachometer, limited slip differential and vinyl roof — were exclusive to the Special.

Initial sales were disappointing, since the car was launched at the height of the oil crisis and at a time of strikes and hyperinflation at home.

US market versions had a SOHC 1.8 litre inline-four, carried over from the earlier 124 but now with more emissions control equipment, and were available with a GM three-speed automatic transmission.

Abarth and Bertone had the strict order to stay as close as possible to the base model, both in parts choice and in physical appearance.

The cars were fully painted and trimmed and then delivered back to the Fiat special Rivalta plant where they received the Abarth mechanicals.

The street version of the car used a DOHC 4 valves per cylinder derivative of the standard twin cam inline-four engine, equipped with a double downdraught 34 ADF Weber carburetors producing 140 PS (138 bhp; 103 kW) at 6400 rpm and 172 N⋅m (127 lb⋅ft; 17.5 kg⋅m) of torque[17] at 3600 rpm.

The street cars used the standard gearbox with no synchromesh (Rally type regulations required the use of the same type of synchromesh on the competition cars as on the street versions) and stock front brake discs, which performed better because the redesigned Bertone front and hood allowed for more air to cool them.

[18] Competition cars used dry sump lubrication and eventually Kugelfischer mechanical fuel injection.

In race specifications, the engine produced up to 240 PS (237 bhp; 177 kW) in 1980, being driven to World Championship status by Walter Röhrl.

[19] The biggest changes exterior-wise for the Series 2 were larger rectangular shaped front lights (quad round headlights in the US), new bumpers (for the Supermirafiori), new bigger rear lights, while a new interior (CL, Supermirafiori) included the dashboard and a chunky, single-spoke steering wheel.

An important introduction for the Italian and many other continental European markets was the new diesel engined versions, built in Sofim's brand-new factory in Foggia.

[20] The diesel had been previewed and tested in competition already, with three diesel-engined cars prepared by Abarth for the 1977 London–Sydney Marathon rally.

[20] As introduced on the Series 2, 131 Diesels received four, equally sized round headlights and a noticeable (and characteristic) bump in the hood to accommodate the taller engine.

[24] The January 1991, edition of Popular Mechanics in the United States listed the 1979 Fiat 131 in the "Overall Worst" category as the most "trouble prone" car ever recorded in their Owner Report histories.

The car received a slightly updated interior (instruments, single-piece glovebox lid), and thanks to changes to the furniture and headliner, the passengers had a little more room.

Fiat 131 Rally's precursor the 3.5-litre Group 5 Abarth SE 031 won 1975 Giro d'Italia automobilistico.

[34] Between 1976 and 1981 the Fiat 131 won 20 WRC events; other notable drivers were Sandro Munari, Timo Salonen, Attilio Bettega and Michèle Mouton.

Prior to the introduction of the 131 Diesel, Fiat had Abarth prepare three diesel-engined cars with two-door, Series 1 bodyshells for the 1977 London–Sydney Marathon rally to promote its reliability and robustness.

The car carried sponsorship from Anheuser-Busch Natural Light beer, which had been introduced the previous year.

These vehicles enjoyed a very long production run (1986–2002 at Turkey, 1991–2009 at Egypt, 2006–2010 at Ethiopia), and were later replaced by newer Fiat models.

The Fiat 131 was also produced at Helwan, in Egypt, by El Nasr since at least 1982,[37] on the basis of complete knockdown (CKD) kits.