The core question for modern BMW six-cylinder models: gasoline B58 (340i, 540i, X5 40i) or diesel B57 (340d, 540d, X5 40d)? Both are built on the modular architecture with 500 cc per cylinder, both produce 286–374 hp, and both are considered structurally stout. Where they diverge is elsewhere — maintenance costs, real-world fuel economy, range, and resale value against the backdrop of the ongoing diesel debate.
Below is a breakdown of which engine suits which driving profile.
Technical Comparison
| Parameter | B58 (Gasoline) | B57 (Diesel) |
|---|---|---|
| Layout | Inline-6, gasoline, twin-scroll turbo | Inline-6, diesel |
| Displacement | 2,998 cc | modular architecture, 500 cc/cylinder |
| Power (factory) | 320–382 hp | — |
| Torque | 332–370 lb-ft | 479–515 lb-ft |
| WLTP fuel consumption | 26–34 mpg | 36–47 mpg |
| CO₂ emissions | 160–200 g/km | 130–170 g/km |
| Range | 400–500 miles (60 L tank) | over 600 miles |
| Emissions standard | — | Euro 6d-Temp / 6d |
| Residual value after 3 years | 55–62% | — |
Both engines belong to BMW’s modular family with 500 cc per cylinder. That defines the shared engineering signature: inline-six, integrated cooling and forced-induction solutions, unified electronics. The differences start at the combustion process level.
Design and Working Principle
B58 — a turbocharged inline-six gasoline engine with a twin-scroll turbo. Displacement 2,998 cc, factory power 320–382 hp, torque 332–370 lb-ft, consumption 26–34 mpg on the WLTP cycle, CO₂ emissions 160–200 g/km. With a full 60 L tank, range is 400–500 miles. The engine runs on gasoline without additional aftertreatment: no AdBlue, no diesel particulate filter (DPF), no restrictions on entering urban low-emission zones.
B57 — a diesel inline-six, the direct successor to the proven N57 and M57 line. It delivers strong torque from low rpm (479–515 lb-ft), returns 36–47 mpg on the WLTP cycle, and emits 130–170 g/km CO₂. With a full tank, range exceeds 600 miles. It uses 1.5–3.0 L/100 km less than its gasoline sibling. It requires AdBlue, refilled every 5,000–9,000 miles, and is fitted with a DPF that’s sensitive to driving patterns.
Torque (Summary Table)
| Version | Torque |
|---|---|
| B58, factory | 332–370 lb-ft |
| B57, factory | 479–515 lb-ft |
| B57, Stage 1 | 555–625 lb-ft |
| B57, maximum tune | over 590 lb-ft |
The torque gap defines the character of each car. The B57 pulls from the very bottom and is built for load; the B58 revs higher and responds to throttle more eagerly.
Real-World Fuel Economy
The WLTP cycle rates the B58 at 26–34 mpg and the B57 at 36–47 mpg. In real-world driving, the gap widens noticeably.
| Mode | B58 | B57 |
|---|---|---|
| City | 17–21 mpg | 26–34 mpg |
| Rural highway | 24–29 mpg | 34–43 mpg |
| Autobahn at 81 mph | 25–28 mpg | 34–39 mpg |
| Full load | 11–15 mpg | 20–26 mpg |
At 12,000 miles per year, the diesel saves a substantial amount on fuel — the key economic argument in its favor. The diesel premium at new-car purchase pays for itself in 2–4 years under active use.
Common Faults
B58 (Gasoline)
- PCV whistle — the classic B58 problem: failure of the crankcase ventilation membrane.
- Water pump — critical at 50,000–75,000 miles.
- Coolant loss — more common with tuning or high loads.
B57 (Diesel)
- AdBlue system — NOx sensors age, and the dosing module can fail.
- DPF clogging — with purely city driving (trips under 9 miles), DPF regeneration doesn’t trigger.
- Injector coking — possible with poor-quality diesel or high mileage (over 124,000 miles).
Reliability and Longevity
Both engines are equally stout. The B57 is a direct descendant of the proven M57/N57 line, consistently developed. The B58 mostly has service items (PCV, water pump) but no structural killers. Expected service life for both is 155,000–217,000 miles with careful maintenance.
After 124,000 miles, the diesel shows wear-related issues on components (injectors, high-pressure fuel pump), the gasoline engine on the water pump and PCV. Frequency is comparable, so neither has a clear longevity advantage.
Tuning Potential
A standard Stage 1 for the B58 is 420–450 hp, and tuning is simple and well-established. The B58 supports tuning up to Stage 2, 2+, E85 conversion, and turbo upgrades, and the community offers a noticeably wider range of solutions than for the B57.
The diesel B57 at Stage 1 produces 380–430 hp and 555–625 lb-ft of torque. After modifications, torque exceeds 590 lb-ft, making it ideal for towing and heavy loads. Stage 2 on the diesel is often accompanied by DPF deletion — a setup not legal for use on public roads.
Service and Running Costs
| Item | B58 | B57 |
|---|---|---|
| Oil service (LL-04) | moderate cost | moderate cost |
| AdBlue fill | not required | every 5,000–9,000 miles |
| Road tax | lower | higher |
| Coolant | level check | level check |
Cost of Ownership
At 12,000 miles per year, the diesel saves a substantial amount on fuel — the key economic argument in its favor. The diesel premium at new-car purchase pays for itself in 2–4 years under active use.
The gasoline B58 is cheaper to buy, has lower road tax, and no AdBlue costs. The diesel B57 costs more up front but is cheaper to run thanks to fuel economy, while tax and AdBlue consumption are higher.
A back-of-the-envelope estimate over 62,000 miles. Assumed: M340i (B58) vs. M340d (B57). The B58 averages 25 mpg, the B57 36 mpg. The fuel budget difference over that distance is significant and favors the diesel. Working against it are road tax (favoring the gasoline engine), AdBlue costs, and the diesel premium at