[slowmover]

After joining Team Cummins a while back, and carefully reading what Diesel_Dave was doing on his truck I made some changes.

Several years ago had settled into a routine that was easy (the point of it) where all miles were covered between 1,700-1,900-rpm, town or country. The gearing of my truck (transmission and rear axle) meant that speeds of from 33-63 mph were easily covered by driving in 4th, and rarely in Direct in town, and OD for country (1,725-rpm at 58-mph). This might mean 33-mph in a 40-mph zone. A 7,500-lb truck is hard to stop, and keeping a bit of engine rpm to accentuate engine braking was found optimal by me. On a 45-mph road with little chance of cross-traffic I would upshift to Direct and run a bit faster.

This brought my town mpg from 18 to 19+ mpg, annual average over several years, and country mpg to 24 in all solo conditions (traffic, weather, payload, etc).

I gave myself room to be sloppy. And left room for when the weather was hottest (extended idling to maintain cab cool). The point of simplicity is that it is easy to follow with a minimal set of guidelines, and might be followed even if sick or injured or just plain tired.

I have always driven for mpg since acquiring a drivers license nearly 40-years ago, so a pretty fair amount of this -- EM suggestions -- was not new. More important to me -- much more important -- than mpg is vehicle longevity, followed by reliability. Mpg is third at best. Questionable practices -- EOffC -- were never considered.

So the second change I'd instituted just prior to joining here a couple of years ago was to reduce the total number of trips, even if each trip was longer by several measures available. The vehicle that doesn't move doesn't burn fuel. And when it was turned on, a route to all places was in hand with a considerably long warm-up drive favoring the local freeways to the farthest point first and then working back to the start point (home).

So far, so good. But I noted Dave's numbers, and -- even though I am not a commuter with set departure times and routes -- I felt I could improve my majority (at this time) town mpg. When he noted that Cummins had stated that the (later) 6.7L engine had it's best FE in the 1,300-1,500 rpm range and that he rarely saw above 1,600-rpm, and that it is possible to have higher town than country mpg the lightbulb went on.

So I moved things downward. All miles (as possible) at 1,300-1,500 rpm and shifts at about 1,500-rpm (and a hair upwards) to bring the engine back in at around 1,100-rpm. I was uncomfortable previously in bringing it back in at lower than 1,300-rpm (torque peak is 1,600-rpm) as that was part of my training in big trucks (respecting peak torque). City freeways are now covered at about 53-4 mph at 1,600-rpm.

I've recently averaged above 23-mpg "town" over more than a thousand miles with, again, these simple changes.

Thank you, Dave. First for me among the many around here as our vehicles are so similar; setting a larger context for me to read more carefully what others are doing.

For you other CTD drivers I've been able to up my average mph to 28+ in this period. Anything above 25-mph is optimal to the best of my experience.

Now I'm stoked to change my rear end gearing to 3.42 from the present 3.73 as that will bring me into the 1300-1500 sweet spot for nearly all town driving, and quite close on country driving. I'll give up a little perfomance, but as I am full-timing (living/working from a 32' travel trailer) this matched pair rarely exceeds 16,000-lbs -- low for this trucks capabilities; not considered "heavy" -- the change makes sense. A marginal change that will add up over the expected years and miles this vehicle will be in use.

I've also yet to explore slightly advanced engine timing and it may be that both changes will occur this calendar year (along with some small changes on intake and exhaust that mirror later factory practice).

I may also do some changes to reduce NVH (and hopefully extend vehicle life) that could have an effect on mpg, namely, changing to a lighter one-piece driveshaft and to a fluid-filled harmonic dampener (as the factory has, again, done in models later than mine). The effect may simply be indirect: a more alert driver on long trips.

A winter front, engine heater cord and a partial grille block have long been on the list. 2012 ought to be fun (more of the easy gains).

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