Why Fuel Use Is Hard to Compare Across Vehicles

Why Fleet Fuel Comparisons Break So Easily

Picture two vehicles from the same model year assigned to the same region. One reports 10.2 MPG, while the other reports 7.8 MPG. At first glance, the second vehicle looks inefficient. But then the details appear. It carries heavier loads, hits more traffic lights, idles during jobsite waits, and missed its last tire pressure check.

That is why raw MPG or raw fuel spend is one of the most misleading numbers in fleet management. NACFE reported that top study fleets averaged 7.8 MPG while the national average was 6.9 MPG. Across 50 trucks at 100,000 miles per year and $4 per gallon, that gap can represent roughly $334,000 in annual fuel impact. The mistake is assuming the gap belongs to the vehicle alone.

A fuel comparison only becomes useful when it answers one question: is the vehicle actually underperforming, or is the benchmark unfair?

Raw MPG Was Never Built for Mixed Fleets

MPG works best when every vehicle uses the same fuel, runs similar routes, carries similar loads, and operates under similar conditions. Few fleets look like that anymore. Many now run diesel trucks, gasoline vans, electric vehicles, and CNG units side by side.

The measurement problem starts before operations even enter the picture. An EV is often measured in kWh per 100 miles. A gasoline vehicle uses MPG. A diesel truck may be reviewed by gallons burned, cost per gallon, or MPG. CNG is commonly measured through GGE, or gasoline gallon equivalent. LNG may be measured through DGE, or diesel gallon equivalent. The Alternative Fuels Data Center explains that these equivalency methods exist because fuel types have different energy content and measurement units.

The EPA uses MPGe to normalize energy content across fuel types. One gallon of gasoline is treated as equivalent to 33.7 kWh of electricity for MPGe calculations. That helps with broad comparison, but fleet operations need more than energy equivalence because MPGe does not account for payload, terrain, idle time, driver behavior, charging loss, or service condition.

For mixed fleets, the most common measurement problems include:

• Fuel type differences across diesel, gasoline, EV, CNG, and LNG assets
• Billing unit differences such as gallons, GGE, DGE, and kWh
• Route differences that change fuel demand even at the same mileage
• Payload differences that change engine load
• Idle and stop density differences that raw MPG hides

That is why a fleet fuel management software setup should not only collect fuel entries. It should help managers compare fuel use against the right vehicle context.

The MPG Illusion Fleet Managers Fall For

MPG is not linear. A move from 10 MPG to 20 MPG saves 50 gallons every 1,000 miles. A move from 30 MPG to 40 MPG saves only about 8 gallons every 1,000 miles. Both look like a 10 MPG improvement, but the fuel savings are completely different.

The EPA has pointed out that MPG can mislead comparisons because gallons used over a set distance do not move in a straight line as MPG changes. That is why gallons per 100 miles gives a clearer view of actual consumption.

Fleet managers make the same mistake when they compare percentage MPG improvements across vehicles with different base efficiencies. A low MPG truck improving by one MPG may save more fuel than a light van improving by five MPG. For budget decisions, gallons per 1,000 miles and fuel cost control across fleets usually show the impact more clearly.

The Five Variables That Make Vehicle to Vehicle Fuel Comparisons Unreliable

A useful comparison needs to explain why one vehicle consumed more fuel than another. These five variables usually decide whether the comparison is fair.

Payload Weight

Payload directly changes how hard the engine works. A fully loaded 80,000 pound Class 8 truck will burn far more fuel than the same truck at 34,000 pounds on the same route. Oak Ridge National Laboratory research found that fuel efficiency decreases as vehicle weight increases, especially for heavy loads and certain grade conditions.

Overloading beyond the engine's ideal operating range can create a major efficiency penalty. That means two trucks can look different in MPG while both are performing normally for their assigned load. For payload heavy operations, gallons per 1,000 miles and cost per loaded mile give a better read than raw MPG, especially when the goal is to reduce fleet fuel costs without blaming the wrong asset.

Route Terrain and Grade

Terrain can turn a good vehicle into a bad looking one. A truck assigned to hilly territory will almost always underperform a truck running flat interstate lanes. The same vehicle, speed, and driver can produce different fuel results when grade changes.

Route planning matters because climbs, traffic signals, congestion, and unnecessary detours all increase fuel demand. A fair comparison must tag routes by terrain type before judging the vehicle.

Fleet managers should classify routes before comparing fuel performance:

• Flat highway routes
• Rolling regional routes
• Mountain or grade heavy routes
• Dense urban routes
• Mixed jobsite and road routes
• Off road or construction routes

Duty Cycle and Stop Density

A regional delivery truck with frequent stops cannot be compared with a highway truck running long uninterrupted miles. Accelerating from a complete stop is one of the most fuel intensive events for a heavy vehicle. The DOE estimates aggressive driving can reduce fuel economy by 10 to 40 percent in stop and go conditions.

Stop density also changes how useful MPG becomes. A van making 30 stops per day will burn more fuel per mile than a similar van making five stops, even if both drivers are doing their jobs properly. Trip and mileage tracking helps separate actual distance from the operating conditions behind that distance.

Driver Behavior

Driver behavior can create major fuel gaps between similar vehicles. Speeding, hard acceleration, harsh braking, and poor anticipation all increase fuel use. The DOE estimates aggressive driving reduces fuel economy by 15 to 30 percent at highway speeds.

A fuel comparison should not review vehicles without reviewing driver assignment. Harsh braking wastes momentum, and harsh acceleration burns fuel to rebuild that same momentum. Fleet GPS tracking and telematics can make this comparison more useful by showing speed, idle time, route, and behavior patterns beside fuel records.

Driver behavior usually shows up in fuel reports through:

• More speeding events per mile
• Frequent harsh acceleration
• Repeated harsh braking
• Longer idle sessions
• Poor route discipline
• Late response to warning lights or vehicle issues

Vehicle Age and Maintenance Condition

Deferred maintenance quietly drains fuel. Low tire pressure increases rolling resistance. Dirty filters, poor alignment, wrong oil, aging tires, and dragging brakes can all reduce efficiency before a breakdown happens.

An older truck with overdue service will often look worse than a newer truck in raw MPG. That does not mean it needs replacement. It may need a tire check, alignment, filter replacement, or service review. Comparing MPG trends against vehicle service history gives managers the diagnostic context many fuel reports miss.

Why Mixed Fuel Type Fleets Can't Share a Single Benchmark

Fleets transitioning to EVs and alternative fuels now operate two energy systems at once. Internal combustion vehicles rely on fuel cards, fuel receipts, and fueling networks. EVs rely on chargers, electricity rates, charging behavior, and energy reporting.

A diesel truck measured in MPG, an EV measured in kWh per 100 miles, and a CNG van measured in miles per GGE cannot share one simple benchmark. Cost per mile works better because it converts energy consumption into dollars traveled. Even then, cost per mile must be grouped by route and mission type.

A mixed fleet benchmark should include:

• MPG for gasoline and diesel units
• kWh per 100 miles for EVs
• Miles per GGE or DGE for natural gas units
• Cost per mile across every fuel type
• Idle percentage and route type for operating context

For teams running mixed vehicle classes, an online fuel management system needs clean fuel, mileage, and vehicle level records rather than disconnected transaction logs.

How Electrification Is Making This Problem Worse Before It Gets Better

EVs make fuel comparison harder before they make it cleaner. They have zero tailpipe emissions, but energy production, charging rates, infrastructure cost, and vehicle purchase premiums all affect the business case. A simple MPGe comparison does not show those costs.

A fleet manager comparing EV energy cost to diesel fuel cost must include local electricity rates, charger installation, charger maintenance, charging downtime, utility demand charges, and vehicle premium. Without those factors, the fleet can overestimate or underestimate EV savings. For trucking and logistics fleet management, that mistake can affect route planning, replacement cycles, and capital budgets.

What a Fair Fleet Fuel Comparison Actually Looks Like

A fair comparison starts with peer groups. Compare vehicles only against others with similar duty cycles, payloads, routes, and vehicle types. A single fleet average hides more than it reveals.

Use this framework before judging fuel performance:

• Group vehicles by mission type such as highway, regional delivery, urban service, and construction
• Compare vehicles only within the same peer group
• Set baseline fuel ranges for each group
• Flag vehicles 10 percent or more below group average
• Review idle percentage, cost per mile, fuel variance, and maintenance status
• Assign one action before assuming the vehicle is the problem

NACFE's Fleet Fuel Study shows the value of disciplined tracking. Fleets averaging 7.8 MPG versus the 6.9 MPG national average did not get there by reading fleet wide averages. They used vehicle level tracking, fuel efficiency practices, and consistent benchmarking.

The Metrics That Actually Tell You Something Useful

The right metrics should explain what happened and what action to take next. Fleet wide MPG alone cannot do that.

Track these metrics at the vehicle level:

• MPG per vehicle: Shows trend direction for each asset instead of hiding outliers inside an average.
• Cost per mile: Normalizes fuel types and fuel price changes across diesel, gasoline, CNG, and EVs.
• Idle percentage: Shows avoidable waste that often comes from routing, jobsite waits, and driver habits.
• Fuel variance: Compares purchased fuel against expected fuel use.
• Driver behavior score: Tracks harsh acceleration, harsh braking, speeding, and idle events per mile.
• Maintenance correlation: Connects fuel drops to inspections, tires, oil, filters, and repairs.

Fleet fraud also belongs in the fuel comparison conversation. CFO.com reported in 2024 that leaders estimated 19 to 22 percent of fleet spend was being lost to theft and fraud, including fuel fraud and purchases outside approved categories.

The act of tracking these metrics at vehicle level drives savings because visibility changes behavior. Drivers respond differently when fuel waste is visible. Managers respond faster when outliers appear in a fleet reports dashboard instead of a disconnected spreadsheet.

A simple workflow can turn fuel comparison into action:

How Fragmented Data Makes Fuel Comparisons Even Harder

Most fleets do not struggle because they lack fuel numbers. They struggle because fuel transactions sit in one system, mileage sits in another, maintenance records sit somewhere else, and driver assignments are not always documented.

That fragmentation makes every low MPG report inconclusive. A manager cannot tell whether the vehicle needs driver coaching, a route review, a payload check, or a maintenance inspection. The report identifies a gap, but it does not explain the cause.

A connected fleet maintenance platform like AUTOsist can bring fuel records, mileage, service history, and vehicle details into one place. That allows managers to spot outliers, review inspection records, assign a corrective action, and confirm whether fuel efficiency improves after the change.

The goal is not to compare every vehicle against every other vehicle. The goal is to compare each vehicle against the right standard.