torquefan

I always wondered the same thing, then I found a site that will do the calculation.

http://www.sciencemadesimple.net/fuel_economy.php

Duallydog

Red Feather have a look at this website, it will tell you the difference tire sizes make on speed. http://www.miata.net/garage/tirecalc.html

Crazy_Farmer, I usually keep good records on my mileage too, I've noticed that my mileage can change just because of parking uphill or downhill when putting fuel in. When I park downhill it seems to take on more fuel for some reason, which doesn't make sense because the filler neck goes into the front section of the fuel tank.
Head winds make a lot of difference as do cross winds, these trucks are aerodynamic dependents. I'll bet you were behind some traffic most of the way when you got the good mileage and were by yourself on the way back, not to mention the windows down make a lot of difference too.
Fuel quality also makes a difference, I was talking to someone in town who operates works with a vehicle fleet of F-250s both gas and diesel. He said that fueling up at HEB here usually nets them 2mpg less for the diesels. I haven't been able to prove that since the credit card I use for fuel won't give me a % back if I fuel up there so its been 2 years since I've pulled up to their pumps.
RPM, aerodynamics, weight, turbo efficiency, fuel quality, how you fill up, and right foot all make a difference on these trucks.
RPM looks to be best in a range from 1700-2000 empty, it seems to come down to how efficiently your truck produces power at low throttle, it has to stay in a range that keeps it from turning more RPM than is needed but prevent lugging, the more weight you have the more rpm to prevent lugging. I can't run 55mph very well because that is too low in the rpm band for my 6th gear, it doesn't take much incline at 55mph before it starts bucking a little, but it is too fast for 5th gear which would turn 2200 or so rpm. I believe weight has something to do with this though and I will know better after I take the 1300# flatbed off and put the 400-600# stock box back on.

For me my best consistent mileage has been netted at 1750 to 1800 rpm which means I'm running around 60-63, part of the reason for it is the lower rpms, the other part is the aerodynamics, there is less aerodynamic pressure at 60mph than there is at 70mph. I have had some exceptions to that though, I have followed a bunch of high powered trucks flying through Houston traffic and still got 18mpg, but the difference there was the drafting effect caused by the vehicles in front of me. I have noticed that when I go to Ft Worth as well, the higher traffic areas I get better mileage (assuming there isn't a traffic jam and I have to sit there idling). That drafting negates my inefficient aerodynamics between the big honking bumper and grille guard and one of the most unaerodynamic flatbeds made for a Dodge. The less air that those big chunks of iron have to deal with the better.

The turbo makes a difference, I noticed that with my old He 341 and I kept it at 7psi I would get good mileage, but when I put the H3 351 on it was easier to keep it at 7psi but I would get 2-4 mpg less. The 351 seems more efficient at higher boost (i.e. boost levels necessary to tow a loaded trailer) whereas the 341 was more efficient at low boost levels. Here in the next month or so I will find out how efficient the He 351 VE off a 6.7 is.

Like I have had a professor say about his class "don't believe me I am just showing you my perspective", and that is the case here, this is what I have observed and since reality is just one's perception I could be completely off base with what I have just written, but this is what I have seen.

JOE.SBG

my most recent trip figures. left Long Island NY just me and my truck 07 5.9 QC
totally stock 27000m. Went to nashville Tenn. and back. trip there down 81 to 40. back up through kentucky WV, maryland back to 81. Total miles 2055 total fuel 100.9 gals. 20.3 mpg. By the way the mileage squares with Garmin gps. regular mileage around town driving is about 14-15. Towing my 21' camping trailer 12.5- 14.5.. On the Tenn. trip the overhead was about 1-1.5 high. Normally reads 1.5-2.5 high. just some facts I'm passing along.

Joe

RFR2212

I just towed my Jeep on my 18ft steel deck 7k trailer. The trailer weighs about 5500lbs loaded. Not sure of my truck weight (I've got the scale sheet somewhere) and I got 12.6 on the IL flat lands. This was running about 70mph. Unloaded into a headwind, I was 15.12 hand calc. It's alot less than what I was hoping for and unfortunately, my old 02 ford f250 did better milage. I sold that and took a huge payment increase and hoped for better milage in the deal. At least I've got less rust and alot better ride. Oh, this is with a cat back 4in kit as well from the previous owner. I guess I'll just continue to mod to improve milage. Or buy back my original dodge 12v that got 23mpg empty on the road...

-WHEELMAN-

I have a 2wd lowered with K&N FIPK, straight through exhaust and a DS Preditor which I feel produces the best mileage out of any programmer on economy performance mode. The best I got was 26 un loaded of course going from Southern California to Las Vegas and back consistently at 65MPH not breaking 70MPH. I could have gotten lucky with a tail wind both ways and generous pumps but numbers dont lie I calculated twice in disbelief. This was my first long trip with upgrades and had mileage in mind the whole way. It made me extremly happy to see those kind of numbers. There is many people that swear by the DS Preditor for mileage which was the main reason that is what I went with.

Duallydog

Wheelman, what is your truck? Year, transmission, cab, etc.

CutlassRacer

Wheelman is your truck an Automatic? What year is your truck also?

I am debating between the Smarty JR, Xzillaraider 2 and the Diablo Sport Predator. Main concern is mileage and low smoke.

isu elvi

FWIW

Just did a hand calc on the first tank in the wifes truck. Worked out to about 17.2 mpg, overhead says 20.8. This is using the wifes recorded gallons per miles traveled, and her stile of driving, about a 25 mile hwy and 2 mile city drive twice a day, plus a few short trips to town from the house ( about 15 miles) per tank. Thats also figuring that she says the speedo is low by 2 mph, per two different speed trap signs. I need to stick a gps in it and see what it actually is. This is on the mega in my sig with 265/17 tires.

If past experience tells me anything, I was always able to get about 1-2 mpg + better than her in the 99 (we take a lot of repetitive 200 mile trips to the same destination), so driving habits are a huge factor in mpg.

-WHEELMAN-

2006, 48RE, Megacab, SRW

Hard to go wrong with a Smarty but for mileage.... DS Preditor. Smokes when your really getting into it but none during normal driving. Do some research there is a lot of guys running the DS.

CutlassRacer

Seems like everybody on this site recommends the Smarty Jr for mileage. I am trying to get as many opinions as I can before I purchase any parts. I would love to get that kind of mileage in my truck!