do you think cutting mph saves fuel?

10 speed OD?

 

I agree totally. If I'm running 55 or so, especially loaded I never use the top gear. I stay in 17th and get the best mpg. If I drop to direct or 18th mpg drops big time. I actually wish I went with 4.10s or 4.33s (vs 3.90) cause I seldom use top gear. Even on the big road it will get better mileage at 70-73 than 65. 70 puts the engine where it wants to be. This is my truck though, grossing close to 120k. What works for me probly doesn't for most

 

You'll get best fuel mileage running in the engine's sweet spot RPM at your cruise speed AND in direct gear.
Direct is the most efficient gear in the trans.

I'd spec it with rears that allow you to run direct between 55-60 mph for max fuel economy. Use the OD gears for when you need higher speeds if needed.

 

Simple physics. It takes more energy to move mass at a faster rate. Gain .1 for every 1mph slowed down... on avg.

 

My new truck will be specd'ed with a direct drive tranny (DT-12) and a 264 rear, with 22.5's, BUT, I have seen instances were it seems a motor running faster or higher rpm does get at least the same or better mileage that running real slow/low rpm usually it has to do with a small cube low tq motor.

 

Yes

 

There are soooo many variables that determine how a truck will do for mpg at given speeds. If the motor is run too low, overcoming rotational inertia can be a problem, as can excessive back pressure in the exhaust. Direct vs OD has an effect. Tires have an effect, Aero has an effect. The key is to get all of these things just right. Most fleets don't. An O/O who is diligent about spec'ing can do some interesting things that make for good mpg and still not have to drive 55 everywhere.

Speed has a detrimental effect on a lot of things besides mpg. The tire manufacturers are on record that tire wear goes up 10-15% right away if running speeds above 65. I have no concrete proof that it is true, but not sure why they would lie about that.

I just dumped my ECM a while back while getting an ABS sensor worked on. For 390,000 miles I had averaged 7.83 mpg. That is staying up north all year round, light and heavy, up to 46K in the box, with a fair amount of hilly 2 lane running. Now I have 413,000 miles and still have the original drive tires and they have 10/32nd tread left with no uneven wear. The first set of steers got changed at 210,000, and the current set has 203,000 and 11/32nd's tread left with no uneven wear. I usually run right around 62 mph / 1425 RPM with my Detroit 60 hooked up to an 18 speed running in 16th direct drive, turning 2.64 rears. Summer time is great, in that I regularly see 8 and even 9 mpg averages between fill ups. Think I might have hit a good combination when I spec'd this truck. Might not be the best for every situation that can occur in trucking, but works well for me.

 

Try this. When you have time, take your foot out of the tank. Slow down to about 58mpg. If you have a digital gauge, watch the MPG go up. Now drive no faster than 60 for as long as you can. Use cruise control on the flats only. OR, Nurse the throttle and think you only have 1 gallon of fuel in the truck and you need to save as much as possible. At the end of the day, you will have gone further then you thought you could.

I can bobtail close to 18MPG on the flats slowing way down. With an empty trailer, around 11 - 14mpg. With a load, I get around 8.5mpg. Of course, traffic, hills will cause this to go away.

 

That would work, on flats. I have run slower when I am not bucking high winds, heavy, and on rolling hills. But taking my foot out of it and slowing to 58, I am turning about 1325. Everything is lost on any little hill that comes up. I am not averse to slowing down, and I do it when it can be work. It all depends on the terrain, load, and weather. If I start to run even the slightest hill at 1325, EGT's rapidly climb and motor is fighting with itself. This is a Detroit 60 12.7, not a DD15. It does not have the traditional waste gate turbo either.

Oh, I did a 10.5 mpg today running about 60 with around 15K in the box. Got just under 10 yesterday running about 62 with about 25K in the box and quite a bit of rainstorm running. But that was flatland stuff and light. I can pull off close to 9 on same ground, good weather, with 45K in the box sometimes. Either way, averaging 7.83 for the life of the truck, with the staying up north all winter long (and dealing with a lot of cold, very windy days), an average overall payload of around 37K, is not too bad. Imagine what that average lifetime mpg would be like if I ran I-10 a lot!

 

Running slower does not guarantee you better fuel mileage! I get much better mileage at 58-59 mph than I do at 55-56. It is a function of how the truck is geared. But yes at 59 I am gettting much better mileage than at 70mph.