Speed Limiter vs Idle Timer: Which Saves More Fuel?

It depends on how your trucks spend their day. A heavy truck idling burns about 0.8 gallons an hour, and a truck at 65 mph instead of 55 mph can use up to 20% more fuel. The first waste happens while the truck sits still, the second while it moves. For a mostly highway truck the speed limiter usually saves more. For a truck that idles 1,500 hours a year, idling can cost more than speeding.

How much fuel does idling waste?

The EPA’s SmartWay program says idling a heavy-duty engine burns about 0.8 gallons an hour. That is an average, and it varies with engine, load and temperature.

Smaller vehicles burn less. The Department of Energy says idling can use a quarter to a half gallon of fuel per hour, depending on engine size and air conditioner use.

An idle timer shuts the engine down after a set time, so the hours it removes are hours of fuel you stop buying.

How much fuel does speeding waste?

Speed waste is spread across every highway mile. The EPA’s SmartWay program says a heavy truck driving 65 mph instead of 55 mph can use up to 20% more fuel, and its example truck does 6 mpg. The Department of Energy says gas mileage usually decreases rapidly at speeds above 50 mph.

Across a mixed fleet the average saving is smaller than 20%, because only the fast miles change. A one-year trial of 177 vans and 30 trucks measured an average 5% fuel saving after speed limiters were fitted (CE Delft for Novem, 2009).

Worked example: one truck at $6.352 diesel

This is an illustration with stated assumptions, not a measurement of your fleet.

Assumptions

• Diesel at $6.352 a gallon, the EIA’s weekly on-highway average for the week of October 26, 2026.
• One heavy truck covering 100,000 miles a year at 6 mpg.
• Speed limiter saving: 5% of fuel, the fleet average from the trial above.
• Idling: 0.8 gallons an hour (EPA SmartWay).
• Idle timer: cuts idling hours in half. This is our assumption, not a published figure.

Annual fuel

100,000 miles at 6 mpg is about 16,670 gallons, or about $105,900.

Speed limiter

5% of 16,670 gallons is about 830 gallons, or about $5,290 a year.

Idle timer, at three levels of idling

Idle hours a year	Fuel idling (0.8 gal/hr)	Cost of idling	Saved if hours are halved
500	400 gal	about $2,540	about $1,270
1,000	800 gal	about $5,080	about $2,540
1,500	1,200 gal	about $7,620	about $3,810

At 1,000 idle hours, halving idling saves about $2,540. The speed limiter saves about $5,290. Both together at that level: about 1,230 gallons, or about $7,830 a year, per truck.

At 1,500 idle hours, the idle waste alone is about $7,620, more than the limiter saves, so a timer that removes most of it would beat the limiter. Neither tool wins everywhere. Your own idle hours and highway share decide.

So which saves more for your fleet?

Use these rules of thumb.

• Mostly highway, long legs, few idle hours: the speed limiter saves more, because the waste is in the miles.
• Local delivery, stop-start work, long loading waits, cold or hot cabs: idling often outweighs speeding. The idle timer may lead.
• Vans and pickups: idling burns a quarter to a half gallon an hour, so a given number of idle hours is worth less than on a heavy truck. Speed usually carries more of the saving.
• Mixed fleets: you almost certainly have both problems.

To test it, pull two numbers per vehicle from your telematics or engine data: idle hours and miles above your target speed. Our fuel cost calculator turns mileage and mpg into dollars at the current EIA price.

Can you use both?

Yes, and many fleets do. They fix different waste, and one does nothing for the other. An idle timer does not slow a truck down on the interstate, and a speed limiter does not stop an engine running in a loading bay.

A speed limiter does two things an idle timer cannot. It caps the risk side of speed: NHTSA’s Speeding: 2024 Data counts 11,288 deaths in speeding-related crashes in 2024, 29% of all US road deaths. And it gives you a sealed cap you can show an insurer or customer. An idle timer cannot do either. See engine setting vs aftermarket governor for how the cap is set.

Frequently Asked Questions

How much fuel does a truck waste idling?

The EPA’s SmartWay program puts it at about 0.8 gallons an hour for a heavy-duty engine. At $6.352 a gallon that is about $5.08 for every hour of idling.

Does a speed limiter save more than an idle timer?

For a mostly highway truck, usually yes. At 100,000 miles a year and 6 mpg, a 5% saving is about $5,290 at $6.352 diesel, against about $2,540 from halving 1,000 idle hours. With heavy idling and little highway driving, the idle timer can win.

Can I use a speed limiter and an idle timer together?

Yes. They address different waste: one while moving, one while stopped. In our worked example, both together save about $7,830 a year per truck.

How much does speed affect fuel use?

The EPA says a heavy truck at 65 mph instead of 55 mph can use up to 20% more fuel. A one-year trial of 177 vans and 30 trucks measured an average 5% fleet saving after speed limiters were fitted.

Does a speed limiter reduce idling?

No. It caps road speed only. It has no effect on how long an engine runs while the vehicle is stationary.

How do US fleets buy AutoKontrol speed limiters?

Through Scorpion, our parent group’s US team. Start with the quote form and fitting is agreed per job, by your own technicians or our engineers and partners.

Cut the waste on the move

Idling is waste you can see. Speed is waste hidden in every mile, and at $6.352 diesel it adds up fast. Tell us what you run and how many vehicles, and we will match a limiter to each one and price it. Get a quote or book a free consultation. How US fleets buy is set out on our US speed limiters page.

AutoKontrol has made speed limiters in Britain since 1985 and supplies fleets in more than 50 countries, with approvals including ISO 9001, UTAC, TÜV and VCA.