Diesel vs Gas Cost Comparison
Compare annual fuel costs between diesel and gasoline trucks based on price, MPG, and mileage.
Results
Visualization
How It Works
The diesel-vs-gas decision pivots on three numbers: pump price spread, MPG ratio, and engine-life ratio. Diesel's 13% higher energy density per gallon gives it a permanent thermal-efficiency edge — a Cummins X15 returns 38-42% brake thermal efficiency vs 28-32% for a comparable PSI 6.0L gas engine. The pump premium swung from $0.69/gal (Q1 2026 EIA: diesel $3.79 / gasoline $3.10) to as wide as $1.95/gal (June 2022). For Class 8 over-the-road, diesel wins on TCO every time. For Class 4-6 medium-duty (delivery boxes, dump bodies, contractor trucks), the math is genuinely close — and Ford's 7.3L Godzilla, GM's 6.6L L8T, and Ram's 6.4L Hemi have made gas competitive at lower acquisition cost.
The Formula
Variables
- Diesel Price — Current ULSD pump price (EIA weekly average $3.79/gal Q1 2026)
- Gas Price — Current regular gasoline price (EIA $3.10/gal Q1 2026)
- Diesel MPG — Real-world fill-to-fill MPG of the diesel option
- Gas MPG — Real-world fill-to-fill MPG of the gas option
- Annual Miles — Realistic annual operating mileage
Worked Example
Class 8 over-the-road comparison at 120,000 annual miles. Diesel Cascadia at 6.7 MPG and $3.79/gal: 17,910 gal x $3.79 = $67,879/yr fuel + $1,810 DEF (2.7% of diesel volume at $4.40/gal) = $69,689 total. Hypothetical gas Class 8 at 4.2 MPG (no production gas Class 8 exists, but scaled from gas medium-duty): 28,571 gal x $3.10 = $88,571/yr — gas LOSES by $18,882 even at the lower per-gallon price because the MPG gap dominates. Now Class 5 box-truck comparison: diesel Isuzu NPR-HD at 13 MPG vs gas Ford F-650 6.7L Godzilla at 8 MPG, 30,000 mi/year. Diesel: 2,308 gal x $3.79 = $8,747 + $235 DEF = $8,982. Gas: 3,750 gal x $3.10 = $11,625. Gas costs $2,643 MORE, but Ford F-650 acquisition is $52K vs Isuzu NPR-HD at $61K — recovery period roughly 3.4 years.
Practical Tips
- Run total cost of ownership, not just fuel cost. Diesel Class 8 engines (Cummins X15, Detroit DD15, Volvo D13) routinely log 1.0-1.2 million miles before in-frame overhaul; comparable gas large-displacement engines are designed for 250-400K. On a 7-year ownership cycle at 120K mi/year (840K total), diesel survives one engine; gas needs 2-3.
- DEF cost is real but overstated. At 2-3% of diesel volume and $4.40/gal jug DEF (Walmart/TA Petro pricing March 2026), a 1,500-gal/month diesel operation adds $135-$203/month or $0.013-$0.020/mi. This isn't a deal-breaker — it's a line item.
- Gas medium-duty saves on emissions hardware. No DPF regen cycles (which can cost 0.3-0.5 MPG and require occasional forced regens that take a truck out of service for 30-90 minutes). No SCR system to fail. Class 4-5 fleets running urban delivery have moved heavily back to gas since 2020 for this reason.
- Cold-start advantage favors gas in extreme cold. Diesel below -10F can struggle to start without block heaters and winter-blend fuel; gas trucks fire reliably down to -30F with normal cold-weather oil. For ND/MN/Alaska operators in light-medium duty, this matters enough to influence the choice.
- Resale matters more than people think. KBB Commercial 2024 data: 5-year-old Class 8 diesel sleeper retains 38-42% of MSRP vs 25-30% for comparable gas medium-duty. The depreciation gap on a $180K Class 8 = $36K-$50K over 5 years — closes some of the higher acquisition gap.
- Battery-electric Class 8 is still niche in 2026. Tesla Semi production miles are limited; Freightliner eCascadia and Volvo VNR Electric have 230-275 mile real-world ranges with payback only on dedicated short-haul routes with charging at both ends. For typical OTR, diesel remains the only viable choice on 2026 economics.
Frequently Asked Questions
Why is diesel more expensive per gallon than gasoline?
Three reasons: (1) federal excise tax is $0.244/gal on diesel vs $0.184 on gas, plus state taxes typically $0.05-$0.15 higher on diesel; (2) ULSD refining requires hydrotreating to <15 ppm sulfur, which adds $0.10-$0.20/gal in refinery cost; (3) demand competition from heating oil (chemically nearly identical to diesel) and global shipping creates seasonal pull that gasoline doesn't face. Q1 2026 EIA spread: $3.79 diesel vs $3.10 gas = $0.69 gap.
Is diesel always better for heavy trucking?
For Class 7-8 long-haul, yes — universally. The MPG ratio (diesel 6.5-7.5 vs theoretical gas 4.0-4.5) means diesel uses 38-44% fewer gallons per mile, which more than compensates for the per-gallon premium. No major OEM produces a Class 8 OTR sleeper with a gas engine in 2026 — the economics simply don't work.
How much longer do diesel engines actually last?
Detroit DD15 and Cummins X15 design life is 1.0-1.2 million miles to in-frame overhaul; in-frame rebuild adds another 600K-800K miles for $20K-$30K. Comparable Ford 7.3L Godzilla gas: 250-400K miles to major service. The longevity ratio of 3-4x is the historical reason diesel dominates commercial truck residuals — and the resale market still prices it in.
Does DEF significantly hurt diesel operating economics?
No — it's a 0.013-0.020 dollar/mile additive. DEF runs 2-3% of diesel volume in modern SCR systems (post-2010 EPA mandate). At $4.40/gal jug DEF and 7.0 MPG, that's less than 2 cents per mile. Annoying when it freezes in winter (DEF crystallizes at +12F), but not a TCO factor.
Are battery-electric Class 8 trucks cheaper to run than diesel in 2026?
On energy cost alone, yes — Tesla Semi and Freightliner eCascadia run roughly $0.20-$0.30/mi in electricity vs $0.55/mi diesel. But acquisition cost is $400K-$500K vs $180K-$220K for diesel, and real-world range under load is 230-280 miles. Payback only works on dedicated short-haul circuits with charging at both endpoints. For typical OTR multi-day runs, the math doesn't close yet.
Should I buy gas or diesel for a Class 5 work truck?
Run the breakeven. Take the acquisition-cost gap (gas typically $5K-$10K cheaper) and divide by the annual fuel-cost gap. If you'll keep the truck longer than the breakeven period, diesel wins. Below 30,000 miles/year and 6-year ownership horizon, gas usually wins on TCO. Above 50,000 miles/year, diesel pulls ahead by year 3.