MPG Calculator
Calculate your truck's actual miles per gallon and fuel cost per mile at various diesel prices.
Results
Visualization
How It Works
Your MPG is the denominator on every cost-per-mile equation in trucking — get it wrong and every other number is wrong. Real-world Class 8 fleet MPG per NACFE's 2023 Annual Fleet Fuel Study averaged 7.32 MPG (best-in-class fleet) down to 5.51 MPG (lowest reporting fleet), with the median at 6.55 MPG. The dashboard readout from your Detroit DD15 or Cummins X15 is calculated; the only number that matters is the fill-to-fill: miles driven between two complete fill-ups divided by the gallons it took to top off the second time. Track it weekly, log it in a spreadsheet, and your maintenance schedule writes itself.
The Formula
Variables
- Miles Driven — Odometer delta between two complete fill-ups
- Gallons Used — Pump-meter gallons at the second fill-up
Worked Example
Top off in Effingham IL on Tuesday at odometer 547,229. Run a 480-mi load to Indianapolis, deadhead 88 mi to a Louisville KY pickup, then deliver in Knoxville TN — total trip 991 miles to odometer 548,220. Top off again at the Knoxville Pilot: 142.7 gallons. Fill-to-fill MPG = 991 / 142.7 = 6.95 MPG. At the EIA Q1 2026 national diesel average of $3.79/gal, that's $0.545/mi fuel cost. Compare to the dash readout that morning: 7.4 MPG. The dash overstates by 0.45 MPG because it doesn't account for idle gallons. The fill-to-fill is the truth — and the basis for every IFTA, FSC, and load-pricing calculation.
Practical Tips
- Always top off to the auto-shutoff click. A "rounded" $200 fill-up that doesn't fully fill the tank corrupts the next MPG calculation by 2-4 gallons of error, which on a 700-mile leg shifts the calculated MPG by 0.2-0.4 — enough to mask a real maintenance trend.
- Track an idle-included MPG and a "miles-only road MPG" separately. Most ELD platforms (Samsara, KeepTruckin/Motive, Geotab) export both. Road MPG above 7.0 with combined MPG below 6.4 means idle is eating 0.6+ MPG — replace bunk idling with an Espar D5 diesel-fired heater (about $2,500 installed; ROI inside 18 months at $4/gal diesel).
- Use a 4-week rolling average, not single-tank readings. Cross-state fills with different temperature-corrected gallons (warm-state pumps deliver fewer actual BTUs per pump-gallon) create single-tank noise. The 4-week trend is the real signal.
- Cold weather pulls MPG hard. -10F operation cuts MPG by 0.4-0.7 vs +60F per Argonne National Laboratory winter testing. Bunk heater + plugging in your block heater overnight if available recovers most of it.
- Tire condition is the second-largest controllable lever after speed. New low-rolling-resistance tires (Michelin XZA3+ Energy, Bridgestone R283 Ecopia) deliver 0.3-0.5 MPG over standard SmartWay-compliant designs. At $0.55/mi fuel, a 0.4 MPG gain over 100,000 miles = ~$3,400 — well over the $200/tire premium across 18 wheel positions.
- A 0.5+ MPG drop in a 4-week window with no weather/load explanation is a maintenance flag. Top three causes in order: clogged air filter (cheap fix, $80 part), worn or sticky variable-geometry turbocharger actuator (Detroit DD15 known issue, $1,200-$2,400 repair), and clogged DPF (regen issue, $1,800-$4,500 if forced regen fails).
Frequently Asked Questions
What is a typical Class 8 semi MPG in 2026?
NACFE 2023 Fleet Fuel Study: median 6.55 MPG, best-in-class 7.32 MPG, lowest reporting 5.51 MPG. New 2024+ aerodynamic sleepers running interstates average 7.0-7.4 MPG. Older 2010-2015 trucks without skirts run 5.8-6.3 MPG. Heavy haul, multi-stop local, and reefer with high run-time pull the number down 1.0-1.5 MPG.
Why is my MPG lower than the OEM-rated number?
Manufacturers test on flat track at constant 65 mph empty. Real-world MPG is 1.0-1.5 MPG below OEM rated. Your specific MPG depends on load weight (every 5K lbs above empty trailer costs ~0.1 MPG), terrain, ambient temperature (cold air is denser, cuts MPG), and idle time. A truck rated 8.5 MPG that returns 7.0 in real operation is performing normally.
How much does speed actually cost me?
NACFE Run on Less data: average MPG drops from 8.5 at 62 mph to 7.1 at 70 mph — that's a 16.5% efficiency loss for 13% more speed. On 100,000 annual miles at $3.79/gal, the difference between 62 and 70 mph cruise is roughly 2,800 gallons or $10,612/year. Set the cruise at 63 and stop using the 5+ over.
Does idle time get included in calculated MPG?
Yes — fill-to-fill MPG inherently includes idle because the gallons you burn idling come out of the same tank that powered your road miles. Eight hours of high-idle in a Detroit DD15 (0.8 gal/hr) burns 6.4 gallons, which on a 500-mile day at 7.0 MPG appears as a drop to 6.5 MPG. ELD platforms can split road-only MPG so you see both numbers.
How do I calculate MPG correctly fill-to-fill?
Fill the tank to the auto-shutoff click. Note the odometer. Drive normally. Refill to auto-shutoff click again — the pump-meter gallons divided into miles since first fill is your true fill-to-fill MPG. Repeat at every fill-up and log it. Single readings are noisy; the 4-week rolling average is what you trust.
Will an APU pay for itself in MPG savings?
Yes, on most long-haul applications. A Thermo King TriPac diesel APU at $9,500-$11,500 installed runs 0.18 gal/hr vs main-engine high idle at 0.8-1.0 gal/hr. Over 1,800 idle hours/year that's ~1,260 gallons saved or about $4,775 at $3.79/gal — roughly 2-2.5 year payback, with the APU lasting 8-10 years.