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Towing MPG Calculator

Estimate fuel economy and trip cost while towing, from road-load physics rather than a rule of thumb — and see why a trailer’s shape costs more than its weight.

How to use this calculator

  1. 1Use a real solo economy figure at a real speed, from your own trip computer over a long highway run. Everything else scales from it, so the window sticker will produce a confident wrong answer.
  2. 2Estimate frontal areas as height × width. For the tow vehicle, multiply by about 0.85 to allow for the rounded corners; for a square trailer, use the full rectangle.
  3. 3Pick the trailer type for its shape rather than its name — what matters is how tall and square it is, not what it is called.
  4. 4Run the speed table before the trip. Ten miles an hour is usually worth 15 to 20% of the fuel bill, and it costs only time.
  5. 5Compare the shape table if you are still choosing a trailer. Two trailers of the same weight can differ by half in what they cost to move.

How the calculation works

Road load force = ½ ρ Cd·A v² + Crr × weight Road load power (hp) = force × speed ÷ 550 Cd·A towing = Cd·A of the vehicle + Cd of the trailer × (trailer area − vehicle area) + interference Fuel economy is calibrated against your own solo figure: mpg = (v ÷ v₀) × solo mpg × (P₀ + P_fixed) × efficiency gain ÷ (P + P_fixed)
ρ
Air density, 0.002377 slug/ft³ at sea level and 59°F
Cd·A
Drag coefficient times frontal area. The single quantity that determines aerodynamic drag
Crr
Rolling resistance coefficient — about 0.008 for truck radials, 0.010 for trailer tyres
P_fixed
A 4 hp allowance for accessories, engine friction and driveline drag that does not scale with road load
Efficiency gain
Engines run at a better brake specific fuel consumption under load. Up to 10% for gasoline, 15% for diesel

A trailer is only charged for the frontal area it presents beyond the tow vehicle’s shadow, plus an interference allowance. This is why a low open trailer costs so little and a tall box costs so much.

Calibrating against the driver’s own observed solo economy removes the need to model absolute engine efficiency, which is the least reliable part of any fuel calculation.

The efficiency gain reflects a real effect: a lightly loaded petrol engine is throttled and inefficient, and loading it moves it toward a better operating point.

Worked example

A half-ton pickup towing a 7,000 lb travel trailer 1,000 miles at 65 mph

  1. 1.At 65 mph the truck alone has a Cd·A of 0.42 × 33 = 13.9 ft², giving 150 lbf of drag, plus 52 lbf of rolling resistance — about 35 hp of road load.
  2. 2.The travel trailer presents 60 ft², of which 27 ft² stands beyond the truck’s 33 ft² shadow. At a drag coefficient of 0.85 plus 2 ft² of interference, it adds 25 ft² of Cd·A.
  3. 3.That extra Cd·A is worth 270 lbf of drag. The trailer’s rolling resistance adds only 70 lbf.
  4. 4.So 79% of the extra effort is aerodynamic, and 21% is weight — which is why a trailer’s shape decides the fuel bill.
  5. 5.Total road load rises from 35 hp to 94 hp, a factor of 2.7.
  6. 6.Applying that to the observed 20 mpg, with a small allowance for the engine running more efficiently under load, gives about 8.4 mpg.
  7. 7.Over 1,000 miles: 119 gallons at $3.60 = $428, against $180 solo. The trailer costs $248 in fuel, or 43 cents a mile.
  8. 8.The same 7,000 lb as an open utility trailer would return about 14.4 mpg — 71% better, on identical weight.

Result: 8.4 mpg and $428 of fuel — 79% of the extra effort is drag, not weight

Exactly the same 7,000 lb, on an open car hauler instead

  1. 1.Same truck, same speed, same 7,000 lb, same 1,000 miles. Only the shape changes.
  2. 2.A car hauler presents about 28 ft², which is less than the truck’s own 33 ft². It sits entirely inside the tow vehicle’s shadow.
  3. 3.So it is charged nothing for exposed frontal area — only the 2 ft² interference allowance for the gap and the underbody.
  4. 4.That adds 22 lbf of drag against the travel trailer’s 270 lbf.
  5. 5.Rolling resistance is unchanged at 70 lbf, because the weight is unchanged.
  6. 6.Now only 24% of the extra effort is aerodynamic and 76% is weight — the exact inverse of the travel trailer, from the identical load.
  7. 7.Road load rises from 35 hp to 51 hp rather than to 94 hp, and fuel economy lands at 14.4 mpg instead of 8.4.
  8. 8.Over 1,000 miles: 69 gallons and $250, against 119 gallons and $428. The shape is worth $178 on one trip.

Result: 14.4 mpg on the same weight — $178 cheaper over 1,000 miles

Why "towing halves your mileage" is only true for one kind of trailer

The halving rule is repeated everywhere, and for a conventional travel trailer behind a petrol pickup it is roughly correct. Generalised beyond that, it is badly wrong in both directions, because it is keyed to the fact that a trailer is present rather than to the thing that actually costs the fuel.

At highway speed, the effort of moving a vehicle divides into two terms. Rolling resistance is proportional to weight and roughly constant with speed. Aerodynamic drag is proportional to frontal area and to the square of speed. For a typical rig at 65 mph, the aerodynamic term is three to four times the rolling term.

That ratio is what decides everything. A trailer that adds a lot of weight and very little frontal area — a low open utility trailer, a boat on a trailer, a flatbed with a tractor on it — adds mostly to the smaller of the two terms. A trailer that adds a lot of frontal area adds to the larger one, and it does so whether it is loaded or empty.

Run the numbers on a 7,000 lb load behind the same truck at the same speed and the point makes itself: an open utility trailer returns around 14 mpg, an enclosed cargo box around 9, a travel trailer around 8. Identical weight, and the fuel bill differs by more than half.

The tow vehicle’s shadow, and why area beyond it is what costs

A trailer does not travel through undisturbed air. It travels in the wake of the tow vehicle, and air that has already been pushed aside does not need pushing aside twice.

This is why frontal area alone is misleading and why the useful quantity is the area a trailer presents *beyond* what the tow vehicle already covers. A 26 ft² pop-up camper behind a 33 ft² pickup is essentially free aerodynamically — it hides completely in the truck’s shadow, and it pays only a small interference penalty for the gap between them and for its own underbody. A 60 ft² travel trailer behind the same truck stands 27 ft² proud of that shadow, and pays for every square foot of it in clean, fast air.

The practical consequences follow directly. Towing the same trailer behind a taller, wider vehicle costs less fuel than towing it behind a smaller one, all else equal — the shadow is bigger. Closing the gap between vehicle and trailer helps, which is part of why fifth wheels do better than their height suggests. And a roof-mounted air deflector, the item most often mocked, is doing real work when the trailer is genuinely taller than the tow vehicle, and nothing at all when it is not.

It also explains the folding camper’s reputation. A pop-up weighs less than a hard-sided trailer, but that is not mainly why it tows so much more cheaply. Folded down it disappears into the wake; the weight difference is the smaller half of the story.

Speed is the only free improvement

Every other way of improving towing fuel economy involves buying something or changing something. Slowing down involves neither, and because drag rises with the square of speed, it works better than anything else available.

Dropping from 70 mph to 60 reduces the aerodynamic term to 73% of its value. On a rig where drag is 80% of the extra load, that is a 15 to 20% improvement in fuel economy — larger than any bolt-on device will ever deliver, and free.

There are three further arguments for the same choice. Special Trailer tyres are load-rated at a maximum of 65 mph, so travelling faster puts you outside the assumption behind the number on the sidewall. Braking distances with a trailer are considerably longer than solo, and they too scale with the square of speed. And trailer sway becomes more likely as speed rises, because the destabilising forces grow with speed while the trailer’s restoring geometry does not.

The cost is time, and it is smaller than people expect. Over 1,000 miles, dropping from 65 to 55 mph adds under three hours — spread over the two or three days such a trip usually takes, it is one shorter stop a day.

What else changes the number, and by how much

Several factors sit outside a steady-state road load model but matter on a real trip.

  • Wind — the drag term uses airspeed, not ground speed. A steady 15 mph headwind at 65 mph produces the drag of 80 mph — around a 50% increase in the aerodynamic term. Crosswinds are worse than they look, because they increase the effective frontal area as well as adding a sway input.
  • Terrain — sustained grades dominate everything else while you are on them. Climbing a 6% grade at 60 mph with a 13,000 lb combination needs roughly 140 hp for the grade alone, more than the entire flat-ground road load. Some of it comes back on the descent, but never all of it.
  • Altitude — thinner air cuts drag, which helps, and cuts naturally aspirated engine power, which does not. Turbocharged and diesel engines keep most of their output and so see a net gain at altitude; naturally aspirated petrol engines roughly break even.
  • Stop-start and traffic — every acceleration is energy spent on kinetic energy that the brakes then throw away, and the mass being accelerated has doubled. Towing economy in traffic is far worse than the steady-state figure.
  • Tyre pressure — under-inflation raises rolling resistance measurably, and the trailer is where it is most often overlooked. It is also, separately, the leading cause of trailer tyre failure.
  • Roof loads — a cargo box on the tow vehicle adds frontal area at the point where the air is fastest, and unlike the trailer it gets no shadow to hide in. It is one of the more expensive small additions available.

Budgeting a real trip

Fuel is usually the largest single variable cost of a towing trip, and it is straightforward to estimate once the economy figure is honest.

Take the towing mpg from this page, divide the trip distance by it, and multiply by the fuel price you will actually pay — not the price at home. Towing routes cross states with different taxes, and a long rig frequently cannot enter the cheapest stations, so a premium of 20 to 40 cents a gallon over the best local price is realistic.

Then add a margin. The steady-state model assumes level ground, still air and a constant speed, and a real trip has none of those. Ten to fifteen percent above the calculated figure is a sensible allowance for terrain, wind and traffic on most routes, and more if the route crosses mountains.

Range planning matters as much as cost for anything towing. A truck that comfortably covers 500 miles on a tank solo may manage barely 200 while towing, which changes where the stops fall — and a large combination cannot use every station it passes. Work out the towing range before departure rather than discovering it at a quarter tank.

What this assumes, and where it stops

Assumptions

  • Steady-state road load on level ground in still air, at constant speed.
  • Air density is taken at sea level and 59°F, 0.002377 slug/ft³.
  • The tow vehicle’s drag coefficient is assumed to be 0.42, typical for a modern pickup or large SUV.
  • The trailer is charged only for frontal area beyond the tow vehicle’s, plus a 2 ft² interference allowance.
  • Rolling resistance is 0.008 for the tow vehicle and 0.010 for trailer tyres.
  • Fuel economy is calibrated against the entered solo figure, with a fixed 4 hp allowance for non-scaling losses and an efficiency gain of up to 10% for petrol and 15% for diesel as load rises.

Limitations

  • Terrain is not modelled. Sustained grades dominate fuel consumption while you are on them, and a mountainous route will consume considerably more than this predicts.
  • Wind is not modelled, and it is the largest unpredictable factor. A 15 mph headwind at 65 mph produces the drag of 80 mph.
  • Stop-start driving, traffic and acceleration cycles are excluded entirely. Towing economy in town is much worse than the steady-state figure.
  • The efficiency gain under load is a simplification of a real but engine-specific effect, and individual vehicles vary.
  • Frontal areas and drag coefficients are typical figures for each trailer category. A specific trailer can differ substantially, particularly if it has been shaped for aerodynamics or has equipment mounted on its front wall.

Common questions

How much mpg do you lose towing a travel trailer?

Typically 50 to 60% for a conventional travel trailer behind a petrol pickup — around 8 mpg from a 20 mpg baseline at 65 mph. The figure depends far more on the trailer’s frontal area than its weight: the same weight as a low open trailer costs around 25 to 30% instead, because most of the loss is aerodynamic.

Does trailer weight or size matter more for fuel economy?

Size, at highway speed, and by a wide margin. Aerodynamic drag rises with frontal area and the square of speed, while rolling resistance rises only with weight. For a typical rig at 65 mph, roughly 80% of the extra effort a trailer creates is aerodynamic. Weight dominates only in hills and in stop-start driving.

How much fuel do you save by towing at 55 instead of 65?

Around 20 to 25% for a tall trailer. Drag scales with the square of speed, so 55 mph produces 72% of the aerodynamic drag of 65 mph. Over 1,000 miles that is roughly 20 gallons on a typical travel trailer setup, and it also puts you inside the 65 mph speed that ST trailer tyre load ratings assume.

Do diesel trucks get better mileage towing?

They lose a smaller share of their solo economy, yes. Diesel engines have flatter brake specific fuel consumption across the load range, so working harder costs them proportionally less than it costs a petrol engine, which is throttled and relatively inefficient at light load. Diesel fuel also carries more energy per gallon. The advantage is real and it is largest under sustained heavy load.

Does a roof air deflector help when towing?

Only when the trailer is genuinely taller than the tow vehicle, which is exactly when it is most often fitted. A deflector works by directing air over the trailer’s leading edge rather than into it. If the trailer already sits inside the tow vehicle’s shadow — a pop-up, a low utility trailer, most boats — there is nothing for a deflector to do.

How do I estimate fuel cost for a towing trip?

Divide the trip distance by your towing mpg, multiply by the fuel price you will actually pay on the route rather than at home, then add 10 to 15% for terrain, wind and traffic. Also work out your towing range: a truck good for 500 miles a tank solo may manage barely 200 towing, which changes where the stops have to fall.

Sources

Formula and content last reviewed on .

Results are estimates for information only, not professional advice.

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