BMW 420d vs 420i: Diesel Economy or Petrol Simplicity?
420d or 420i is the 4 Series coupe version of the classic diesel-versus-petrol question, and the same logic applies as on the 3 Series: annual mileage, ULEZ exposure and engine-build year decide it. The 420d (N47 then B47 diesel) is the long-distance economy pick; the 420i (N20 then B48 petrol) is the simpler, no-DPF choice. This covers the F32 (2013 to 2020) and G22 (2020 onwards) generations for UK ownership.
Quick verdict
420d if you cover over roughly 12,000 miles a year and the V5 shows Euro 6: the diesel's 50 to 60 mpg pays for itself on long runs. 420i if you do lower mileage or mostly short urban trips, where the petrol's simplicity and lack of a DPF win. Pre-2015 N47 diesel and pre-2015 N20 petrol both carry timing-chain risk; post-2015 B47 and B48 builds are the safer used picks. Verify the emissions class for ULEZ on any pre-2015 car of either fuel.
Side-by-side comparison
| Dimension | 420d | 420i | Notes |
|---|---|---|---|
| Real-world UK MPG | 50 to 60 mpg combined | 34 to 44 mpg combined | The 420d is far more economical on longer journeys. |
| Power / torque | 184 to 190 bhp / 380 to 400 Nm | 184 bhp / 270 to 300 Nm | The diesel has much more torque; the petrol revs more freely. |
| ULEZ compliance | B47 (post-2015) yes; N47 no | Yes on B48; verify pre-2015 N20 on the V5 | The single biggest London-buyer factor; check the emissions class. |
| Timing-chain risk | Pre-2015 N47: documented; B47: low | Pre-2015 N20: documented; B48: low | Both pre-2015 engines share the modular chain weakness. |
| DPF / diesel emissions | DPF, EGR, and AdBlue on later builds | None (petrol) | The 420i is the city and short-journey friendly choice. |
| Insurance group (UK) | Around 26 to 32 | Around 24 to 30 | The petrol is typically a touch cheaper to insure. |
| Body styles | Coupe (F32 / G22), plus Gran Coupe and Convertible siblings | Same body range | Same choice of coupe, four-door Gran Coupe and drop-top on both fuels. |
| Best-value used pick | Post-2015 F32 LCI 420d (B47) | Post-2015 F32 LCI 420i (B48) | LCI B-series cars avoid the chain and ULEZ questions. |
420d: deeper look
<p>The 420d is the 4 Series coupe's default long-distance engine: 184 to 190 bhp, 380 to 400 Nm, and a real-world 50 to 60 mpg that makes it cheap to cover big miles in. The two engine generations matter: N47 (F32 pre-2015) carries the familiar timing-chain risk and is Euro 5 (not ULEZ-compliant on early builds), while B47 (F32 post-2015 and the later G22) resolved the chain issue and is Euro 6 ULEZ-compliant. Later cars add AdBlue/SCR.</p><p>Buying a used 420d in 2026 is a value play if you do the mileage to justify it. Prioritise a post-2015 B47 car for the chain and ULEZ resolution, verify the emissions class on the V5, and check DPF health on any car that has lived on short urban journeys (a clogged or cracked DPF is an expensive fix). Standard diesel items apply: EGR, NOx sensors on higher-mileage cars, and the usual modular service pattern. For the motorway-heavy buyer, the 420d is the rational choice.</p>
420i: deeper look
<p>The 420i is the simpler petrol pick: 184 bhp, a real-world 34 to 44 mpg, and no diesel emissions hardware to maintain. The two engine generations follow the 3 Series pattern: N20 (F32 pre-2015) carries the same modular timing-chain risk as the N47, while B48 (F32 post-2015 and the later G22) is the cleaner, well-proven unit. Petrol cars are ULEZ-compliant from the B48 onwards, and pre-2015 N20 cars should have their emissions class verified.</p><p>As an ownership proposition the 420i suits lower-mileage and urban drivers who do not need diesel economy and want to avoid the DPF conversation entirely. It shares the modular petrol service items (oil filter housing gasket, PCV, electric water pump, and walnut blasting for intake carbon on higher-mileage cars) but nothing diesel-specific. It is a touch cheaper to insure than the 420d and lighter over the nose, which suits the coupe's balance. For a city-bound buyer, the 420i is the more sensible 4 Series.</p>
Decision logic
Annual mileage is the first filter, exactly as on the 3 Series. Under roughly 8,000 miles a year, the 420i wins on total cost of ownership: no diesel-specific wear items, no DPF risk, and a simpler ULEZ story on B48 cars. Over roughly 12,000 miles a year, the 420d wins on fuel cost alone, its 50 to 60 mpg saving real money on long runs. Between those, the build year matters more than the fuel: a post-2015 B47 420d or B48 420i is the right buy, avoiding the pre-2015 N47 and N20 timing-chain and ULEZ questions. For a London buyer, prioritise a post-2015 diesel or any B48 petrol and verify the emissions class before purchase.
UK 2026 pricing comparison
| Scenario | 420d | 420i | Notes |
|---|---|---|---|
| 2015 F32 420d M Sport, 80k miles (pre-LCI) | £8,000 to £11,000 | £8,500 to £12,000 (420i N20 equivalent) | Pre-LCI; verify chain history and emissions class on both. |
| 2018 F32 LCI 420d M Sport, 60k miles (B47) | £13,000 to £16,500 | £13,000 to £16,500 (420i B48 equivalent) | Post-LCI; ULEZ-compliant on both; broadly at parity. |
| 2021 G22 420d M Sport, 40k miles | £24,000 to £30,000 | £24,000 to £30,000 (420i equivalent) | Current generation; petrol and diesel at parity. |
| Gran Coupe / Convertible body | £9,000 to £32,000 (420d, by year and body) | £9,500 to £32,000 (420i, by year and body) | The four-door Gran Coupe and Convertible add a small premium over the coupe. |
Recommendation by scenario
| Scenario | Pick | Why |
|---|---|---|
| London commuter (low mileage, inside ULEZ) | 420i (B48) | ULEZ-compliant, no DPF risk on short journeys, and a simpler service profile. The petrol is the low-mileage city choice. |
| Motorway warrior (over 15,000 miles annual) | 420d (post-2015 B47) | 50 to 60 mpg makes the diesel far cheaper to run at high mileage. Choose a post-2015 build for the chain resolution and Euro 6 ULEZ compliance. |
| Mixed use (8,000 to 12,000 miles) | Post-LCI 420i B48 or 420d B47 | Either works. The 420i wins on simplicity, the 420d on motorway economy. Personal preference matters more than the spreadsheet at this mileage. |
| Enthusiast / weekend coupe | 420i (or step to a six-cylinder 440i) | For a low-mileage, character-led coupe the petrol suits the balance. If budget allows, the six-cylinder 440i or M440i is the sweeter enthusiast pick. |
| Tight budget (sub-£11,000) | Pre-LCI 420d or 420i with chain documented | The cheapest way into a 4 Series coupe. Verify chain history and the emissions class; reject or renegotiate if neither is documented and the price does not reflect the risk. |
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Frequently asked questions
Is the 420d more expensive to maintain than the 420i?
Modestly, yes. Beyond the shared service items, the 420d carries diesel-specific costs: DPF health (a clogged or cracked filter is expensive), EGR, NOx sensors on higher-mileage cars, and AdBlue/SCR on later builds. The 420i has the usual modular petrol wear items but no diesel emissions hardware. The annual difference is broadly a few hundred pounds in the petrol's favour, before fuel is considered.
How many miles do I need to do for the 420d to be cheaper?
At current UK pump prices, with the 420d returning 50 to 60 mpg and the 420i 34 to 44 mpg, the breakeven is around 10,000 to 12,000 miles a year. Below that, the petrol tends to win on total ownership cost once its lower service and insurance costs are included; above it, the diesel's fuel saving pulls clearly ahead.
Is the 420d ULEZ-compliant?
It depends on the build. N47 420d (typically pre-2015 production) is Euro 5 and not ULEZ-compliant; B47 420d (typically post-2015) is Euro 6 and compliant. Always verify the emissions class on the V5 before buying. If London access matters, prioritise a post-2015 B47 car or a B48 petrol 420i.
Do the pre-2015 420d and 420i have the same timing-chain risk as the 3 Series?
Yes. The F32 4 Series shares the 3 Series engines, so pre-2015 N47 (420d) and N20 (420i) carry the same documented timing-chain weakness. Post-2015 B47 and B48 builds resolved it. Buy a pre-2015 car only with documented chain history, or negotiate the roughly £1,500 to £2,500 risk into the price.
Should I buy a pre-2015 420d or a post-2015 420i?
The post-2015 420i wins on simplicity and ULEZ certainty: no timing-chain conversation, no ULEZ concern, and no diesel emissions hardware. The pre-2015 420d only makes sense if you do high mileage and the cheaper purchase price reflects its chain and ULEZ risk. For most buyers the post-2015 B-series car of either fuel is the safer choice.
Is the 4 Series 420d different from the 3 Series 320d?
Mechanically they are near-identical: the 4 Series is the coupe (and Gran Coupe and Convertible) version of the same platform and engines. The 420d and 320d share the N47 and B47 diesels, so the buying logic, chain risk, ULEZ rule and running costs are the same. The 4 Series commands a small style premium and has a slightly sportier body, but the drivetrain story is common to both.
Should I just get a six-cylinder 440i or M440i instead?
If budget allows and you want an enthusiast coupe, the six-cylinder 440i (F32, N55 or B58) or the current M440i (G22, B58) is the sweeter, quicker car, but it costs meaningfully more and is thirstier than the 420i or 420d. The 420d vs 420i decision is about the efficient, everyday four-cylinder 4 Series; the six-cylinder cars are a separate, higher-budget conversation.