62 MPH rule? What is the reasoning behind it?

True . Don't companies know how to spec for maximum efficiency at the speed they mostly run at ? Weren't 68 m.p.h. trucks ordered to be in the sweet spot at close to 68 m.p.h. ? Then how would turning them back to 62 and running at an rpm below the sweet spot improve mpg ?

 

My point exactly.

 

Yeah, my company tells us that the engine is most efficient at 1600RPM (you can argue the number, but that's what they tell us) then they govern us where the engine is running at 1500RPM (65mph in our trucks). Makes no sense to me.

 

The general thought was go slower = less fuel burned. They got nervous when the economy took a dive and diesel hit $4/gal. OTOH, with the possibility that fuel will hit those kind of prices again, it'd make more sense for a fleet to spec new tractors with those lower speeds in mind.

 

The only thing I can add to this is when traveling at a slower speed you can reduce the possiblity of getting a speeding ticket. Police focus on faster moving vehicles and ones that are being driven aggressively. Traveling at 60 mph or 62 mph will normally keep you from having to pass other vehicles. Fewer traffic clusters to deal with. You will find yourself between the packs most of the time.

The only problem that I see with the slower speed is you then have to keep in the right lane of the interstate (slower vehicles keep right) and will encounter traffic from entering/exiting vehicles at each interchange.

As a general rule, the fewer interactions you have with other vehicles on the highway the greater your chace of maintaining a safe driving record.

 

Fewer interchages mean driving in the Plains States, What about us poor souls that get sent to NJ, ML, VA, Just another reason to have my gov. speed at 72, No. I don't run 72 everywhere Cept. TN, KY, MO, KS, OK and any other state that will allow it. Otherwise I am within 2 MPH of the speed limit nearly every where I go. My Fuel is not to bad either.

 

They were probably talking about maximum power output vs fuel usage, and when you're just cruising on the highway, you're looking more for "enough power to sustain this speed" vs fuel milage. Shrug. Do they also teach progressive shifting? that's a big one to help with fuel milage, unless youre going up a hill, most low range gears on a 9 or 10 speed can be shifted at an extremely low RPM, obviously saving gas (because the next gear has enough torque at the reduced rpm to pull it in). A higher RPM always uses more fuel than a lower RPM, but....you have to have the torque to do what you are doing.

if all this was true about companies speccing the trucks to run at the most efficient rpm at governed speed, what about all of these examples?

Covenant has a policy where if you get a speeding ticket, your governor is reduced to a lower speed (yes, im serious, and i have the employee handbook to prove it)

Lots of companies have in recent years decreased governed speeds with rising fuel costs. Some smaller companies even set governors on trucks that were previously not governed. They didn't change the gearing.

My truck, having a 9 speed, doesn't have enough gears. A 10 speed would improve fuel milage, because there are some speed limits where to obey, i have to either lug the engine in a higher gear (baaad) or keep the engine at an excessive RPM. Or speed or go slower, of course, but if the traffic is light and the speed limit is 20, im not slowing down!

By the way, what kind of engine are you running? I have a detroit series 60 515. I drove with an owner op once, who told me to shift his truck at 1900 RPM and never let it fall below 1500! Sounds crazy but it seemed like that truck needed to be revved that high to run right. That was a 2004 CRST international with a cummins engine and an eaton fuller lightning 10. and we got better fuel milage on that thing than i do now.

 

if youre talking about my post, so do i, and you cant do anything about the speed limits. i also have a transmission problem that the company so far will not touch. my top 2 gears are supposed to be automatic but it just lugs into the top gear. i estimate this is costing me about 1mpg because sometimes it doesnt do it and when it doesnt, i can see the mpg difference.

 

Sounds logical. Mind if I ask who you work for?

 

Concerning the statement from the Mack engineer:

You're not going to take a truck spec'd to run 70 mph and turn it down to 65 mph and expect it to get better fuel mileage. In fact, it will get worse mileage for the reasons he stated. Most companies spec their trucks to run at 65 (or whatever their chosen governed speed) and they will get better mileage than a truck spec'd for 70 or 75. For instance, if you were to take that truck the Mack engineer was referring to and reduce its governed speed to 65 but at the same time spec it with 3.55 or 3.58 rears, it would allow the engine to turn at about 1400 rpm (its sweet spot) and get better mileage than 3.36 rears at 70 mph because of less wind resistance. The point is, there is more to fuel economy than just driving slower. They have to be designed to operate at those speeds or the results will be undesireable.