Diesel Engine vs Petrol Engine

If you’re choosing between a diesel and a petrol engine for your truck, the short answer is: diesel wins for heavy hauling, fuel economy, and longevity, but petrol costs less upfront and suits lighter duty. This guide breaks down the real differences so you can pick the right powerplant for your operation.

Power and Torque: Diesel Pulls, Petrol Revs

Diesel engines produce more torque at lower RPMs, which is what you need to get a loaded rig moving and keep it rolling up grades. A typical modern diesel in a Class 8 truck makes 1,200 to 2,000 lb-ft of torque, while a petrol engine in a medium-duty truck might make 300 to 500 lb-ft. That torque advantage means diesel trucks can pull heavier trailers without straining.

Petrol engines rev higher and make their power at higher RPMs. They work fine for lighter loads, like box trucks or delivery vans, but they’ll struggle and wear out faster if you regularly haul near maximum payload.

Fuel Economy and Cost per Mile

Diesel fuel contains about 10% more energy per gallon than petrol, and diesel engines are more thermally efficient. That translates to better miles per gallon. A diesel truck might get 6 to 8 mpg pulling a loaded trailer, while a comparable petrol truck gets 4 to 6 mpg under the same load. Over 100,000 miles a year, that difference adds up.

Fuel prices vary by region, but as of 2026, diesel averages $3.80 to $4.50 per gallon, and petrol averages $3.20 to $3.80. Even with diesel costing more per gallon, the better fuel economy often makes it cheaper per mile. For example, at $4.00 diesel and 7 mpg, you pay about $0.57 per mile. At $3.50 petrol and 5 mpg, you pay $0.70 per mile. That’s a $0.13 per mile difference, which on 120,000 miles a year is $15,600.

Upfront Cost and Resale Value

Diesel engines cost more to buy. A new diesel pickup truck might run $8,000 to $12,000 more than the same truck with a petrol engine. In medium-duty trucks, the premium is $10,000 to $20,000. But diesels hold their value better. After five years, a diesel truck might retain 60% of its original value, while a petrol truck retains 45% to 50%.

If you finance the truck, the higher upfront cost means higher monthly payments. But if you keep the truck for 10 years or more, the resale value helps offset that.

Maintenance and Longevity

Diesel engines are built heavier to handle compression ratios of 16:1 or higher, versus about 10:1 for petrol. That means they last longer: 500,000 to 1,000,000 miles is common for a well-maintained diesel, while a petrol engine might go 200,000 to 300,000 miles.

But diesel maintenance costs more. Oil changes for a diesel take 3 to 5 gallons of oil, costing $100 to $200, versus $50 to $80 for petrol. Diesel engines also have more complex emissions systems: diesel particulate filters (DPF), diesel exhaust fluid (DEF), and EGR systems. Those can fail and cost $2,000 to $8,000 to replace. Petrol engines have simpler emissions, so fewer expensive repairs.

Routine maintenance for a diesel also includes fuel filter changes every 15,000 to 25,000 miles, and you need to keep the DEF tank filled, which adds about $10 to $20 per 1,000 miles.

Noise, Vibration, and Harshness (NVH)

Modern diesels are much quieter than old ones, but they still clatter at idle and vibrate more than petrol engines. If you’re in the cab for 10 hours a day, that can matter. Petrol engines run smoother and quieter, which reduces driver fatigue. Some owner-operators prefer petrol for local delivery where you’re in and out of the truck all day.

Environmental and Regulatory Factors

Diesel engines produce more nitrogen oxides (NOx) and particulate matter, which is why they need DEF and DPFs. Petrol engines emit more carbon dioxide (CO2) per mile, but less NOx and soot. If you operate in states with strict emissions rules, like California, diesel trucks may require additional compliance, such as CARB certification. That can add cost and paperwork.

Also, check your local idling laws. Many cities restrict diesel idling to 3 to 5 minutes, while petrol trucks may have fewer restrictions. That affects how you run your truck in cold weather.

Comparison Table: Diesel vs Petrol for Trucks

Feature	Diesel	Petrol
Torque	1,200-2,000 lb-ft (Class 8)	300-500 lb-ft (medium-duty)
Fuel economy (loaded)	6-8 mpg	4-6 mpg
Fuel cost per gallon (2026)	$3.80-$4.50	$3.20-$3.80
Cost per mile (example)	$0.57	$0.70
Upfront cost premium	$8,000-$20,000	Baseline
Resale after 5 years	55-65% of original	45-50%
Engine lifespan	500,000-1,000,000 miles	200,000-300,000 miles
Oil change cost	$100-$200	$50-$80
Emissions system cost	DPF/DEF/EGR, $2,000-$8,000	Simpler, lower cost
Noise/vibration	Louder, more vibration	Quieter, smoother

Which Should You Choose?

If you haul heavy loads, drive long distances, or plan to keep your truck for years, diesel is the better investment. The higher upfront and maintenance costs pay off in fuel savings and longevity.

If you do local deliveries, carry light loads, or want the lowest possible starting cost, petrol might be enough. You’ll save on purchase price and maintenance, but you’ll lose on fuel economy and resale.

Before you buy, run the numbers for your specific route: miles per year, average load weight, fuel prices in your area, and how long you plan to keep the truck. That will tell you which engine makes financial sense.

FAQ

Q: Can I use a petrol engine for long-haul trucking? A: Technically yes, but it’s not practical. You’ll get worse fuel economy, less torque for hills, and the engine will wear out faster under constant heavy load. Most long-haul trucks are diesel for a reason.

Q: Are diesel engines more reliable than petrol? A: Diesel engines are built stronger and can last longer, but they have more complex emissions systems that can fail. With proper maintenance, both can be reliable, but diesel repairs are more expensive.

Q: What is DEF and do I need it? A: DEF (diesel exhaust fluid) is a urea solution injected into the exhaust to reduce NOx emissions. All modern diesel trucks require it. You’ll need to refill the DEF tank every few thousand miles, costing about $10-$20 per 1,000 miles.

Q: Is diesel fuel more expensive than petrol? A: Yes, diesel typically costs $0.30 to $0.70 more per gallon than petrol. But because diesel engines get better fuel economy, the cost per mile is often lower.

The Bottom Line

Diesel engines are the standard for trucking because they offer superior torque, fuel economy, and lifespan. They cost more upfront and in maintenance, but they pay off for heavy haulers and long-haul drivers. Petrol engines are cheaper to buy and maintain, but they only make sense for light-duty, local operations. Crunch your own numbers, and choose based on your payload, mileage, and budget. If you’re still unsure, talk to a mechanic who works on both types, and test drive both if you can. That’s the most practical way to decide.