By Norman Ford · Updated October 2026

Speed limiter impact calculator

Average highway speed now (mph)Governed or cruise speed (mph)Highway miles per week (mi)MPG at your current speed (mpg)Assumed MPG gain per mph slower (mpg)A commonly quoted rule of thumb; your truck may differDiesel price ($/gal)Your average rate per mile ($/mi)

RESULT · SPEED LIMITER IMPACT

Speed limiter impact result
Extra driving hours per week for the same miles	1.7 h
Or: miles lost in the same hours	110 mi
Revenue at risk if those miles are lost	$254
Estimated MPG at the lower speed	6.60 mpg
Fuel saved per week	$70
Weekly fuel at 60 mph	$1,373 (7.10 mpg)
Weekly fuel at 62 mph	$1,413 (6.90 mpg)
Weekly fuel at 65 mph	$1,477 (6.60 mpg)
Weekly fuel at 68 mph	$1,548 (6.30 mpg)
Weekly fuel at 70 mph	$1,598 (6.10 mpg)
Weekly fuel at 75 mph	$1,741 (5.60 mpg)

The MPG change per mph is an editable assumption, not a measured value for your truck. Hours of service limits decide whether extra hours are available.

Show the math

Hours = miles / speed; extra hours = miles / governed - miles / current
Miles lost = weekly miles - governed speed x current hours
MPG at speed = current MPG + (current - speed) x MPG per mph
Fuel = miles / MPG x price

Slower trucks still need full weeks. We plan loads with realistic transit times, at 5% of gross, and you approve every one.

Start dispatch

This speed limiter impact calculator returns the extra hours or lost miles at a governed or lower cruise speed, the revenue at risk, the estimated fuel saved, and a table of weekly fuel cost at several cruise speeds.

Whether your truck is governed by a fleet, a lease or your own choice, speed is a trade between time and fuel. Slower saves fuel and is easier on the truck. Faster covers more miles in the hours you are allowed to drive. Which side wins depends on your truck, your lanes and how many hours you actually use.

How it works

Hours = miles / speed. The extra hours are the difference between the governed and current speeds for the same weekly miles.
Miles lost in the same hours = weekly miles - governed speed x hours at your current speed
MPG at a lower speed = current MPG + mph slower x the assumed MPG gain per mph
Fuel cost = miles / MPG x diesel price

The MPG gain per mph is an assumption. The default, 0.1 MPG per mph, is a commonly quoted rule of thumb. Your truck may do better or worse. Change it once you have your own numbers.

A worked example

A truck averages 68 mph on the highway, runs 2,500 highway miles a week at 6.3 MPG, and pays $3.90 for diesel. It is governed down to 65 mph.

Time: 2,500 miles takes about 36.8 hours at 68 mph and about 38.5 hours at 65, so about 1.7 extra hours a week.
Fuel: with the assumed 0.1 MPG gain per mph, MPG rises to about 6.6, saving about 18 gallons, or about $70 a week.
If the driver cannot add the extra hours, the truck covers about 110 fewer miles in the same time. At $2.30 a mile, that is about $250 in revenue at risk.

So the result depends on hours. If the extra 1.7 hours fit inside the driver's legal hours and schedule, slowing down saves money. If they do not, the lost miles can cost more than the fuel saved.

Hours of service decide it

For CDL drivers, hours of service rules cap driving time. A driver who regularly runs out of hours will lose miles at a lower speed. A driver with hours to spare will mostly see longer days. Be honest about which describes your week.

Brakes, tires and safety

Lower speeds mean less energy to stop, less brake heat and shorter stopping distances, and they are easier on tires. Our guide to the slack adjuster on a truck explains how brake adjustment holds up under load, and the guide to the drag link on a semi truck covers a key steering part that takes a beating at speed on rough roads.

Using the result in your rate

A slower truck covers fewer miles per hour, so each hour of the week needs to earn more per mile to stay level. The rate per mile calculator shows what a load pays across every mile, and if a fuel surcharge is part of your pay, the fuel surcharge calculator shows how it moves with diesel prices.

Reading the speed table

The table at the bottom of the results shows weekly fuel cost at six cruise speeds, using your MPG and the same assumption. It is a quick way to see how much each few miles per hour costs over a week. Pair it with your hours: if moving from 68 to 65 saves a meaningful amount and your schedule has room for the extra time, it is an easy change to make on your own truck.

Finding your truck's best cruise speed

Run a few tanks at one cruise speed, then a few at another, on similar routes and loads. Record miles and gallons from the receipts. After a month you will know how your truck responds to speed, and you can replace the rule of thumb in the calculator with your own number.

Where we fit

We plan loads with transit times that match your speed and legal hours, so a slower truck still has full weeks. You say yes, counter or no to each load, and the broker sends the rate confirmation to you to sign. Single-truck carriers can read how we dispatch owner-operators.

Questions

How much fuel does slowing down save in a truck?

It depends on the truck, load, aerodynamics and terrain, but aerodynamic drag grows quickly with speed, so a few miles per hour slower often saves noticeable fuel at highway speeds. A commonly quoted rule of thumb is about 0.1 MPG for each mph over a moderate highway speed. The calculator lets you change that assumption to match your own records.

Does a speed limiter cost me money?

It can cost time. At a lower speed, the same miles take longer, and hours of service limit how many hours you can drive. If you were already using all your legal driving hours, a lower speed means fewer miles per week. If you were not, it may mean longer days with fuel savings. The calculator shows both sides.

What speed gives the best fuel economy for a semi truck?

There is no single answer, because it depends on the engine, gearing, aerodynamics and load. Many trucks are most efficient somewhere in the low to mid 60s mph on flat highway, but your truck's gearing and engine speed at cruise matter. Track MPG at a few cruise speeds to find your truck's sweet spot.

Do speed limiters affect brakes and tires?

Lower speeds put less energy into each stop, which can reduce brake heat and wear, and they can reduce tire heat on long runs. They also cut stopping distances. These benefits are real but hard to put in dollars, so the calculator focuses on time, miles and fuel.