Is the fuel economy of the HPCR engines really that bad compared to the 12v motors?

[CCiatteo]

I just ran my first full load of fuel through my new (to me) truck and refueled today after work. I purposely ran the truck down to the chime/light just to get a feel for how close to empty you really are when those warnings go off. Turns out, they are pretty accurate because I took 34. 129 gallons, but the trip meter only showed 536 miles = 15. 71mpg???? This is combined driving, empty, light foot and easy on the highway at about 70mph. I was not ready for a such a steep fall off in the fuel economy? I have the last 15,000 miles of data on my old 97 truck and on its worst days, still did better than that. And to add insult to injury, the darn overhead in the new truck was leading me to believe I could expect 21+? Reading old posts, I knew the overhead (a. k. a. lie-o-meter) was padding the stats, but to the tune of 5 mpg???



I did some digging on here and the 15. 71 seems to be within reason (unfortunately), but should I be looking for problems or is this the norm? I'm really curious what the next tank load reveals, maybe the hose clicked off early last fill up and I didn't realize it, but I'm not holding my breath on that one... .....

 

[Prairie Dog]

My 2WD 93 got as much as 24 at low highway speeds and my 03 that weighs at least a ton more got 23 before ULSD.
So in some cases the CR's do better. The very newest 6. 7's seem to be doing much better as well.

 

[Joseph Donnelly]

As Prairie Dog said, ULSD hurts mpg around 5% or more; winter fuel likewise. The heavier weight of the newer trucks also hurts, especially on stop and go or frequent acceleration. My 97 Club cab 8' bed at 70-75 gave 17-19 stock timing and maybe 2 more when timing was set up to 15. 3 deg. That was 3. 54 gears with 255-85-16 tires. My 2004 quad cab 8' bed gives around 17-19 at same speed, stock tire size (265 17s) and 3. 73s. The 2004 shipping weight is 7098 vs. 6025 for the 1997. A similar 2013 crew cab but short bed weighs 7337 shipping and early results are the same mpg range. In all cases, if you are happy at 60 or a max of 65 mph and don't vary speed much if at all, and are on the flats, you can get even better mpg with any of them. If there is a "worst" it probably is the 2007. 5-2012 when used under conditions where it regens the cats a lot.

 

[rscurtis]

I've had both- my signature truck and a '95 2500 4X4/47RH with 3. 54 gears. The '95 was more economical around town, it would do about 17MPG. It was lighter and had the benefit of a lower TC lockup speed. The '03 with its modifications is a stellar performer on the highway, but a little thirstier in local (Long Island) driving, where it would deliver about 14 MPG. "Local" driving in PA nets around 19-20 MPG, as very little time is spent idling in traffic. My last trip to LI in May netted a hand-calculated 22. 5 MPG, and that includes about 100 miles of local driving in Nassau County, land of "traffic, taxes, and red-light cameras". I also returned with about 500# of junk to be repaired and resold. On the return trip, I filled the fuel tank to the top- it would not hold another cup of fuel, and still netted the above figure.

 

[DNDracing]

For whatever it's worth Chris, my truck history is somewhat similar to yours. I had a 95 3500, 4x4, 5 speed, that had some larger injectors and an upgraded fuel pump on it. I would consistently get 18 - 20 MPG with that truck "running around town" empty which was mostly 2 lane road driving, with some highway, maybe 30% or so.

My current truck, the one in my signature which is an automatic 2500 has never done that well. I bought the truck used with 75,000 miles on it, and now it has 165,000. Like you, my computer always reads over 20, but manual calculations (which I do almost every tank) tell me it gets 16 - 18 mpg. Again that is empty same driving as listed above. I'm usually pretty conservative with my foot, but every so often like to hear my exhaust and blow a little puff of black smoke.

Sounds to me like you are in the ball park.

 

[Lordsman]

You should get better mpg I would think. Maybe the 89k miles have been too easy and its not broken in quite yet? Wonder what rear end you have and assume its a 305 output?
I have the std. output but with a smarty jr on SW1/timing 1 and with 4. 10's I get 18. 5 combined(supposedly raised to a 305 status) Wish I had the 3. 73's to lower highway rpm. I use Amagamated on each fill up and get about a 1 mpg increase. My overhead is unreliable so always hand calculate. yesterdays fillup was 27 gallons into 495 miles/18. 33mpg. I also did a fresh air intake on bottom filter box to get more air in and it stopped causing the filter minder to always show 65% filter use even on a brand new filter.
For some reason my overhead on fillups the OH only says 390 miles till empty and says I only get 16. 7 or so mpg. I would like to do the tank vent mod. someday. That ought to thouroghly confuse the OH even more! I will say the temp. readout/compass is pretty accurate.
Do a fresh air intake, change fuel filter, clean fuel system/injectors with a dose of BG/44k, maybe raise the timing a bit and see what happens.
Just my . 02

 

[rscurtis]

The tank vent mod does not affect the OHD display as far as I can tell- mine is usually about 5-10% optimistic. What is nice is to be able to drive 200 miles on the highway before the fuel gauge even moves off of the "full" mark.

 

[CCiatteo]

I've had both- my signature truck and a '95 2500 4X4/47RH with 3. 54 gears. The '95 was more economical around town, it would do about 17MPG. It was lighter and had the benefit of a lower TC lockup speed. The '03 with its modifications is a stellar performer on the highway, but a little thirstier in local (Long Island) driving, where it would deliver about 14 MPG. "Local" driving in PA nets around 19-20 MPG, as very little time is spent idling in traffic. My last trip to LI in May netted a hand-calculated 22. 5 MPG, and that includes about 100 miles of local driving in Nassau County, land of "traffic, taxes, and red-light cameras". I also returned with about 500# of junk to be repaired and resold. On the return trip, I filled the fuel tank to the top- it would not hold another cup of fuel, and still netted the above figure.

I would be very happy with 18 mpg, and that's kinda what I expected going into this. I was aware the new trucks don't fair as well as the older truck with respect to mileage, but didn't think the drop off was in the range of 3 to 4 mpg.

For whatever it's worth Chris, my truck history is somewhat similar to yours. I had a 95 3500, 4x4, 5 speed, that had some larger injectors and an upgraded fuel pump on it. I would consistently get 18 - 20 MPG with that truck "running around town" empty which was mostly 2 lane road driving, with some highway, maybe 30% or so.



My current truck, the one in my signature which is an automatic 2500 has never done that well. I bought the truck used with 75,000 miles on it, and now it has 165,000. Like you, my computer always reads over 20, but manual calculations (which I do almost every tank) tell me it gets 16 - 18 mpg. Again that is empty same driving as listed above. I'm usually pretty conservative with my foot, but every so often like to hear my exhaust and blow a little puff of black smoke.



Sounds to me like you are in the ball park.

I'm vigilant about tracking my mileage on a spreadsheet, as I sort use it see if something is wrong with the truck if trends appear over time. And I am (more than I would like to admit) a creature of habit, so my driving routine probably differs very little leading me to believe the numbers on the new truck are valid.

You should get better mpg I would think. Maybe the 89k miles have been too easy and its not broken in quite yet? Wonder what rear end you have and assume its a 305 output?

I have the std. output but with a smarty jr on SW1/timing 1 and with 4. 10's I get 18. 5 combined(supposedly raised to a 305 status) Wish I had the 3. 73's to lower highway rpm. I use Amagamated on each fill up and get about a 1 mpg increase. My overhead is unreliable so always hand calculate. yesterdays fillup was 27 gallons into 495 miles/18. 33mpg. I also did a fresh air intake on bottom filter box to get more air in and it stopped causing the filter minder to always show 65% filter use even on a brand new filter.

For some reason my overhead on fillups the OH only says 390 miles till empty and says I only get 16. 7 or so mpg. I would like to do the tank vent mod. someday. That ought to thouroghly confuse the OH even more! I will say the temp. readout/compass is pretty accurate.

Do a fresh air intake, change fuel filter, clean fuel system/injectors with a dose of BG/44k, maybe raise the timing a bit and see what happens.

Just my . 02

The truck is a 325 HO engine with 3. 73's, and it is a very nice truck, well taken care of and shows its age very very well. I would just like to see the milage at or close to 18mpg and I'd be a happy camper. My old truck rolled around at about 6600lbs empty (truck box and gear in the bed), so this new truck empty (no truck box yet) isn't that much heavier to have that drastic an effect on mileage, I don't think. Your suspicion that the truck saw light duty is probably valid. It was owned by an older gent (mid to late 50's), had a tarp style tonneau cover and looks like it never had dirt in it. The tires have good tread remaining, but are showing some dry rot around the rims (noticed when i washed it this weekend). Combined that with the low mileage for the year (9 years old and only 90,000 mi) and I think its safe to assume the truck say mostly light duty, with maybe some medium duty occasional towing (brake controller in cab).





Thanks for everyone's input. You collectively talked me off the cliff, at least for now. We'll see how the next load of fuel goes. I've got some gauges (boost, pyro, fuel pressure) ordered to compliment the transmission temp gauge already installed and take inventory of everything as its running down the road. I'm mildly considering a Smarty Jr. , but first want to get a good baseline on the truck. I'll keep ya posted.

 

[AH64ID]

Your mileage sounds normal, but the chime on the low fuel light seems off. My light goes off with 3. 5 gallons remaining in the tank.

The Smarty Jr will give you a nice little bump in mileage, but if you just cruise at 1500-1800 rpms in OD don't expect too much.

The OH on my truck is inconsistent, but averages ~17% error each year.

 

[Wingate]

FWIW, my 06 model got a consistent 1. 5MPG bump from adding the Smarty Jr on SW1. Your fuel mileage seems normal to me. My truck seems to get better fuel mileage after being worked hard.

 

[rscurtis]

The Rokktech crank sensor will help a 325 engine significantly- it's cheap and easy to install, and you're not messing with the fueling. IIRC, there was also a reflash for the early "600" engines that also helped FE.

 

[Joseph Donnelly]

The Roktech may not be able to change timing very much without the software having an issue with crank/cam synchronization, and you only get one setting afaik. . I haven't tried it, however. I do use a Smarty "Senior" and it is effective, giving an early 2004 about a half mpg at a guess. Smarty also allows choosing among several timing curves to optimize mileage for the use, such as solo, towing, etc.

 

[AH64ID]

The Rokktech crank sensor will help a 325 engine significantly- it's cheap and easy to install, and you're not messing with the fueling. IIRC, there was also a reflash for the early "600" engines that also helped FE.

I have seen the stock timing tables, and heard the stock timing rattle and personally wouldn't want a rokktech. I cannot see how 1-1. 5 degrees of timing will be significant, especially when stock runs too much timing below 1400 anyhow. Above 30% load the stock timing is 10-20 degrees lower than it should be and 1. 5 won't so squat.

I am pulling better mileage than stock with less timing in certain cruise speeds, and where I am running more its a lot more than the little bit the rokktech gives.