Dodge Diplomat

Dodge Diplomat: which version to buy

Buy a Dodge Diplomat on arithmetic, not on nameplates. There is one generation (1977–1989), one body (Coupe), one layout (Front, longitudinal, RWD), one tank (74 l), and two engine families. The 12 versions shown from 45 catalogue lines split into six 3.7 cars (86–112 hp) and six 5.2 cars (122–147 hp).

Buy a Dodge Diplomat on arithmetic, not on nameplates. There is one generation (1977–1989), one body (Coupe), one layout (Front, longitudinal, RWD), one tank (74 l), and two engine families. The 12 versions shown from 45 catalogue lines split into six 3.7 cars (86–112 hp) and six 5.2 cars (122–147 hp). 0–100 km/h, top speed, CO2, eco class, fuel grade, gross weight, ground clearance and wheel size are not measured on every line, so they cannot break a tie. What does break a tie is power per litre, kilograms per horsepower, and the mixed consumption where it exists.

Short list. Cheapest-to-run six with a stick: 3.7 MT 101 hp (11.2 l/100 km mixed, 8.4 l/100 km highway). Strongest six: 3.7 MT 112 hp or 3.7 AT 112 hp (30.42 hp/l). Lightest V8 for the mass: 5.2 AT 122 hp (12.65 kg/hp, 1543 kg). Best measured V8 economy: 5.2 AT 132 hp (11.8 mixed, 9.0 highway). Most torque: 5.2 AT 137 hp (339 Nm). Lightest kg/hp in the table: 5.2 AT 147 hp (11.16 kg/hp) — consumption on that line is not measured. Skip 3.7 AT 91 hp (1600 kg, fuel not measured) and skip 3.7 AT 101 hp if you care about fuel (14.7 mixed, worst in the set).

Generations: there is nothing to choose between

1 generation ran 1977-01-01 to 1989-12-31 as a Coupe. The catalogue counts 45 modifications and shows 12. Chassis language does not change across those 12: Independent, torsion front, Dependent, leaf spring rear, Disc front brakes, Drum rear, Carburetor, boost No. You are not choosing a later generation. You are choosing output, gearbox and mass inside one generation.

Engines and specs

All engines are Gasoline. The 3.7 family is 6 cylinders, Inline, 3682 cm³, bore × stroke 86.4 × 104.8, compression 8.4, power at 3600 rpm. The 5.2 family is 8 cylinders, V-shaped, 5210 cm³, bore × stroke 99.3 × 84.1, compression 8.5. Automatics are Automatic 3-speed; manuals are Manual 4-speed. Drive is RWD on every line.

Specific output (hp per litre of the stated displacement): 3.7 AT 86 hp = 23.36 hp/l; 3.7 AT 91 hp = 24.71; both 101 hp sixes = 27.43; both 112 hp sixes = 30.42 (best in the table). 5.2 AT 122 hp = 23.42 hp/l — almost the same as the weakest six, despite 1528 cm³ extra. Then 5.2 AT 132 hp = 25.34; 5.2 AT 137 hp = 26.30; both 142 hp eights = 27.26; 5.2 AT 147 hp = 28.21. The 112 hp 3.7 still beats every 5.2 on power per litre.

Performance and economy (mpg)

0–100 km/h is not measured on all 12 versions. Top speed is not measured on all 12. CO2 is not measured. Eco class is not measured. Fuel grade is not measured. Do not invent a sprint time.

Mixed consumption that is measured, best to worst:

• 3.7 MT 101 hp: 11.2 l/100 km (city 13.1, highway 8.4 — the lowest highway figure)
• 3.7 AT 86 hp: 11.8 (13.1 / 10.7)
• 3.7 MT 112 hp: 11.8 (13.8 / 9.4)
• 5.2 AT 132 hp: 11.8 (13.8 / 9.0)
• 5.2 AT 142 hp: 11.8 (13.8 / 9.4)
• 3.7 AT 112 hp: 12.4 (13.8 / 10.7)
• 5.2 AT 122 hp: 13.1 (15.7 / 10.2)
• 5.2 AT 137 hp: 13.1 (14.7 / 10.2)
• 5.2 MT 142 hp: 13.1 (15.7 / 9.4)
• 3.7 AT 101 hp: 14.7 (16.8 / 13.1) — worst mixed and worst city
• 3.7 AT 91 hp: city, highway, mixed all not measured
• 5.2 AT 147 hp: city, highway, mixed all not measured

What the extra gearbox “buys” on the same power. 3.7 AT 101 hp versus 3.7 MT 101 hp: same 101 hp, same 224 Nm, same 1525 kg; the Automatic uses 3.5 l/100 km more mixed (14.7 vs 11.2) and 4.7 more on the highway (13.1 vs 8.4). That is the largest auto-vs-manual gap in the table. 3.7 AT 112 hp versus 3.7 MT 112 hp: mixed 12.4 vs 11.8 (0.6 l), highway 10.7 vs 9.4 (1.3 l) — the gap shrinks. 5.2 AT 142 hp versus 5.2 MT 142 hp: the Automatic is thriftier (11.8 mixed vs 13.1; city 13.8 vs 15.7). On the 5.2 142 hp pair, the 4-speed Manual does not buy economy.

Tank is 74 l on every version. Range from mixed figures, where mixed is measured: 3.7 MT 101 hp ≈ 74 / 11.2 × 100 = 661 km; 3.7 AT 101 hp ≈ 74 / 14.7 × 100 = 503 km — 158 km less from the same tank.

Dimensions and weight

Kerb weight is the only size figure given. Gross weight, ground clearance and wheels are not measured on all 12.

Mass span is 140 kg (1500–1640). The 5.2 AT 122 hp at 1543 kg is 57 kg lighter than the 112 hp sixes at 1600 kg, and only 43 kg heavier than the lightest 3.7 AT 86 hp. Stepping to 142/147 hp costs 1640 kg — 97 kg more than the 122 hp eight.

Which version to choose

If you want the lowest measured fuel use: 3.7 MT 101 hp. Mixed 11.2, highway 8.4, 1525 kg, 27.43 hp/l. Same engine family as the 86 hp car, +15 hp, same 224 Nm, +25 kg. The matching 3.7 AT 101 hp is the car to avoid on economy: 14.7 mixed is 3.5 l worse than the Manual and 2.9 l worse than the 86 hp Automatic.

If you want the strongest six: 3.7 MT 112 hp (or 3.7 AT 112 hp if you refuse a Manual). 112 hp and 244 Nm are the six-cylinder peaks. 30.42 hp/l is the catalogue high. You pay 100 kg versus the 86 hp car (1600 vs 1500) and 75 kg versus the 101 hp pair. Mixed on the Manual 112 is 11.8 — equal to the 86 hp Automatic, 0.6 better than the 112 Automatic. Torque arrives later (2000 rpm vs 1600 on the smaller sixes).

If you want a six and an Automatic and you do not want the thirsty 101: 3.7 AT 86 hp. Lightest Diplomat shown (1500 kg), mixed 11.8, 23.36 hp/l. 3.7 AT 91 hp is a bad neighbour: +5 hp, −7 Nm (217 vs 224), +100 kg, kg/hp worse (17.58 vs 17.44), fuel not measured. There is no measured reason to pick 91 over 86.

If you want the V8 without the heaviest shell: 5.2 AT 122 hp. 332 Nm at 1600 rpm — 88 Nm more than the best six (244 Nm) and 108 Nm more than 3.7 AT 91 hp. Mass 1543 kg, 12.65 kg/hp (already better than every six). Mixed 13.1 is 1.3 l worse than 3.7 AT 86 hp and 0.7 worse than 3.7 AT 112 hp. You buy torque and a better kg/hp, not a better fuel sheet. Specific output (23.42 hp/l) is essentially the 86 hp six’s figure (23.36) on 1528 cm³ more.

If you want V8 economy that matches a six: 5.2 AT 132 hp. Mixed 11.8 equals 3.7 AT 86 hp and 3.7 MT 112 hp; highway 9.0 is second-best after the 101 hp Manual. You give up 20 Nm versus the 122 hp eight (312 vs 332) and peak torque moves to 2000 rpm; you gain 10 hp and 0.85 kg/hp (11.80 vs 12.65). Among measured eights this is the fuel pick.

If you want peak torque: 5.2 AT 137 hp, 339 Nm at 1600 rpm. That is 5 Nm more than the 122, 27 Nm more than the 132, 6 Nm more than the 142/147. Mixed 13.1 matches the 122, not the 132. Mass 1569 kg, 11.45 kg/hp. Power 137 hp at 4000 rpm. Choose this when Newton-metres at 1600 rpm matter more than the 11.8 mixed of the 132.

If you want the highest horsepower with a measured fuel line: 5.2 AT 142 hp, not the Manual. 142 hp, 333 Nm, 1640 kg, 11.55 kg/hp, mixed 11.8. 5.2 MT 142 hp is the same power and mass with mixed 13.1 and city 15.7 — 1.3 l and 1.9 l worse. The 4-speed Manual on this eight does not pay for itself in litres.

If you want the best kg/hp in the table: 5.2 AT 147 hp (11.16 kg/hp, 147 hp, 108 kW, 333 Nm, 1640 kg, 28.21 hp/l). Fuel is not measured, so you cannot claim it is cheaper or thirstier than the 142 Automatic. Versus 3.7 AT 86 hp you gain 61 hp and 109 Nm and you add 140 kg; kg/hp improves from 17.44 to 11.16.

What the extra displacement buys (no prices in the data)

There are no prices, so “extra money” is extra cubic centimetres and extra kilograms.

• 86 hp 3.7 AT → 112 hp 3.7 AT: +26 hp (+30%), +20 Nm, +100 kg, mixed +0.6 l (11.8 → 12.4). Specific output jumps 23.36 → 30.42 hp/l.
• 112 hp 3.7 AT → 122 hp 5.2 AT: +10 hp, +88 Nm, −57 kg, mixed +0.7 l. The eight is the lighter car.
• 122 hp 5.2 AT → 132 hp 5.2 AT: +10 hp, −20 Nm, +14 kg, mixed −1.3 l (13.1 → 11.8). This step buys economy, not torque.
• 132 hp 5.2 AT → 137 hp 5.2 AT: +5 hp, +27 Nm, +12 kg, mixed +1.3 l (back to 13.1). This step buys torque, not economy.
• 137 hp 5.2 AT → 142 hp 5.2 AT: +5 hp, −6 Nm, +71 kg, mixed −1.3 l (13.1 → 11.8). You buy 5 hp and better mixed fuel, and you pay the full 1640 kg kerb.
• 142 hp 5.2 AT → 147 hp 5.2 AT: +5 hp, same 333 Nm, same 1640 kg; fuel not measured. The last 5 hp improve kg/hp from 11.55 to 11.16 and nothing else that this table can prove.

Problems and reliability (what the sheet can and cannot say)

The data do not record faults, recalls or durability. What they do record is that every shown Diplomat is Carburetor, boost No, RWD, Disc/Drum, torsion/leaf. Reliability talk that needs failure rates is not measured. The practical risk that is in the numbers is buying a line with holes: 3.7 AT 91 hp and 5.2 AT 147 hp have no consumption; nobody should call those two “economical” or “thirsty.” The 3.7 AT 101 hp does have numbers, and they are the worst mixed and city figures in the set — that is a measured problem for a buyer who wanted a modest six.

Decision tree

1. Need a Manual and lowest litres: 3.7 MT 101 hp. 2. Need a Manual and the strongest six: 3.7 MT 112 hp. 3. Need Automatic, lightest car, known 11.8 mixed: 3.7 AT 86 hp. 4. Need Automatic and the strongest six: 3.7 AT 112 hp. 5. Need V8 torque without 1640 kg: 5.2 AT 122 hp (332 Nm, 1543 kg) or 5.2 AT 137 hp (339 Nm, 1569 kg). 6. Need V8 with 11.8 mixed: 5.2 AT 132 hp or 5.2 AT 142 hp. 7. Need the catalogue peak hp: 5.2 AT 147 hp, knowing fuel is not measured. 8. Do not pick 3.7 AT 91 hp over 3.7 AT 86 hp. Do not pick 3.7 AT 101 hp for economy. Do not pick 5.2 MT 142 hp over 5.2 AT 142 hp for economy.

That is the whole choice set inside 1 generation of the Dodge Diplomat Coupe, on the 12 versions the catalogue shows.