Well, it's been 3 weeks now since they've cut my truck from 68mph to 64mph. The main thing I've noticed is that my fuel mileage hasn't gotten any better. In fact, it's dropped.

Before they cut my speed, I had an average of 5.8mpg on the truck. It stayed almost identical no matter what freight (weight) I was hauling. Now, since the speed drop, my average is about 5.1mpg. If I haul a light load (10-15k lbs) I MIGHT get 5.5-5.6mpg.

My company has us fill out a paper each time we fill up. It shows the total miles since the last fillup and the total gallons of fuel we purchased. So, I know firsthand what the mileage on the truck is on a daily basis (I have to fill up daily).

Any clues to why the truck would lose mpg after slowing down? I'm not idling the truck whatsoever, except for maybe the first 15 minutes in the morning when I'm getting ready to start my day, so that's not an issue.

Cummins used to have a little liquid crystal window called a "ROAD RELAY". You could set it to see what your fuel economy was continuously. I noticed that as you down-shift, your economy also goes down. I don't remember exactly what the numbers were when pulling hills, but in relative terms...

10th gear, 4.8 mpg
9th gear, 4.2 mpg
8th gear, 3.9 mpg
7th gear, 3.4 mpg
6th gear, 2.8 mpg and so on.

When you hit the bottom of a hill at 68 mph, you go farther up the hill before you down-shift. And, you are generally in a higher gear when you get to the top. If you can only do 64 when you hit the bottom of a hill, you will be down-shifting sooner, and you will be going slower at the top of the hill, which means you will be in a lower gear and using more fuel.

On level ground, like areas of OH, IN, and IL, that are flat, you will see better fuel economy. But, in areas where you have a variety of "ROLLER-COASTER" ride because of hills, you will find yourself down-shifting where you did not do so before. When you have to "PULL" yourself up the hill instead of coasting most of the way up, you use much more fuel.

Like I said, I do not remember the exact numbers. But I do remember that when I drove a truck governed at 60, I was able to see what the fuel economy was whenever I looked at the little window. On one hill out west, I was down to 1.3 mpg. That was with a 350 hp L-10. And, the overall fuel economy was only about 5.4 mpg. With my own truck, 500 hp, N-14 PLUS, and not governed, I was running the same roads at over 7 mpg. And, the weight I was pulling was about the same. While both had 10 speeds, with the L-10, I was dropping down to 6th and even 5 gears. With the N-14, I seldom dropped below 8th gear. The rear ratios were different as well. The L-10 had 3:73 rears and the N-14 had 3:42 rears. On I-70, through CO, I would go an extra mile up a grade with the N-14 before I was anywhere close to being in the same gear as the L-10. That made a very big difference.

Take note of things like, how much sooner up a hill are you down-shifting, and how much farther down you have to down-shift to make the top of the hill. That's where your fuel is going.

I'm not an engineer, so the company big-shots won't listen to me. But when paying my own fuel, I know how I had to drive to make my fuel stretch farther. And, when the truck is governed down like that, I can't do it. If that's what they want, let them pay the price.

For what it's worth... I don't have my own truck anymore. The company I pull for did cut their trucks two months ago, and didn't realize any savings at all. The truck now runs 75 again. They've put them back up again.

Wind it's because you're using the same horsepower but going slower. In those situations, instant readout is actually completely useless.

Also, instant readout is useless for accelerating. The only time it's useful is in steady state situations.

When you're going up a hill, you're storing energy. Instant readouts don't account for that.

So no, taking a run at a hill definitely won't save fuel. Even if you have to downshift. Think about it, instead of using fuel on the hill, you're using it BEFORE the hill as well. You're also traveling faster, and that's what wastes power.

For the OP, I don't know what would cause that. I don't even think it's possible to be honest. Unless it has something to do with specs.

Has anything else changed?

All trucks have a "sweet spot" where they will get the best mpg and for some reason you can take trucks of the same make, model, year and spec's and each one may very well have a different sweet spot.

When the shop foreman told me that he'd cut the speed I asked if they'd done anything else - cut horsepower, changed the max rpm, etc. He said that the only change they made was only setting the top speed of the truck.

The truck is an '05 Volvo 660. It has a 400 ISX and straight 10spd. I don't know the rear axle specs, but it's running at 1450rpm @ 64mph. It was at 1550rpm @ 68mph before they dropped it.

One of my previous employers turned up our trucks to get better mileage. We went from 65mph to 70mph.

Thats the difference in having a Manager that actually drove and truck compared to these idiots with business degrees running trucking company's. Granted if your running flat ground running slower will get you better MPG's but if your running hills heavy it just doesn't work.

And, collective intelligence said "you can not put a dead short across a live circuit" and it took someone diagnosed as a boarderline retard, mentally slow, to invent the light bulb. (source, Paul Harvey, the rest of the story)

That's why we don't have inexpensive hydrogen, because collective intelligence says it can't be done.

Don't you think I experimented with my own truck to see how I got the very best fuel economy? Well before the fuel prices spiked to their current level, I was able to get far better economy by using my foot in hills than by using the cruise control. On level ground, I could not improve on the cruise control, Going through NV on I-80, setting the cruise at 70 gave me better economy than setting it at 65. I didn't do as much down-shifting with the cruise set at 70.

When you're pulling a hill, have your engine pulling max hp, and you're in 6th gear instead of 8th, you're using the same amount of fuel per power stroke of the engine. The same amount of fuel per revolution of the engine, but you're not getting the feet per revolution that you get in a higher gear. So, you're either lugging the engine at 800 or 900 rpm in a higher gear, or you're down-shifting and using more fuel to climb the hill. That is NOT offset by coasting down the next one.

Like I said. I'm not an engineer, so I'm wrong. The OP lives in WV, not NE where the ground is flat. The hills, and having to down-shift sooner, make the difference. I did use the instant read-out, but looking at my average fuel economy and fuel expense for the week backed up the "useless" instant read-out. How much of the hill you're able to maintain high gear, and what gear you're in at the top most certainly does make a difference.

I've tried it with this truck as well. Same results, and it's an 03 with a Detroit. When they set the speed down, the economy went up in FL, but when running I-81 in TN, VA, and PA, the consumption went up enough that the overall average actually went down. I was getting 6.2 before they cut the speed, and 5.7 after. Now, the speed is back up and so is the economy. Every time I fuel up, I enter the information on DDL. When I look at the fuel report, I get fuel economy between fuel stops, as well as the overall economy to date. And, it still backs up what I was getting on that "useless" instant read-out. So, while I may not be an engineer, I most certainly can understand the numbers for gallons purchased and distance traveled. No, I'm not paying for the fuel anymore, but I'm still in the habit of looking at the numbers. Being able to do 50 to 55 at the top of the hill most certainly does not use as much fuel as only doing 40 at the top because of the difference in gears and distance traveled per revolution of the engine.

Or are you going to tell me that you drop two more gears and climb the hill at half throttle? Half the distance traveled per revolution of the engine and half the fuel per revolution still saves you nothing. But, as I recall, you're paying for your fuel, so you drive as you see fit.

Heh - if I have to slow down any more on some of the hills around here I'll be at a dead stop. There are a couple of hard pulls we do that bring my truck down to about 25-27mph. Those hills are all in the range of 5 to 8 miles of hard straining on the truck.

I would love to be a fly on a wall in a Corporate meeting with some of the bigger company's. I would bet everything I own that this conversation has actually taken place.

Director: "Rear End collisions are up 18% and it's costing us an extra $7.2 Million dollars to operate our 10,000 trucks. Any suggestion on how we can prevent this? "

Eager Safety Director looking to climb the management food chain: Yes Sir. I have a solution!!!!! It would cost us $4.7 to equip our fleet with larger brake calipers that will make our fleet safer with a reduction on 20ft of stopping distance saving our shareholders $2.5 Million Dollars in insurance claims"

:lol: :lol: :lol: :lol: