MPG question

tmsnyder said:
I'm sure driving habits have a lot to do with mpg. However, it's impossible to get a read on the mpg with a single tank of gas. Especially in a GMC. Too much chance for error in the fill-up.

I like the Fuelly.com app b/c it keeps track of every drop of fuel I put in all my vehicles.

If I kept my foot out of it and drove that way for 3-4 tanks of gas in a row and recorded 10mpg, I might start to believe that number. One tank, definitely not. Two tanks, I might believe 1/2 the improvement.

My last two tanks were 9.97 and 10.26. I don't have a lot of faith in those numbers b/c of the inconsistency of filling the tanks. You can see the noise in the fuelly data.

The 8.5mpg over 7200 miles of driving, _that_ number is accurate. That's 27 fill-ups, so any inconsistent fills are averaged into the overall volume pumped. If the final fill-up was off 5-10 gallons vs the total pumped (857 gallons) it's only going to be off 0.5 to 1% on the mpg number.
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I kinda wonder how a NP205 would do. Those are pretty bullet-proof units. Ubiquitous and cheap. The way they're put together is kinda modular--I don't think it'd be that hard to get rid of the range box portion, build a custom aluminum plate adapter, and get the input shaft cut/splined as needed.
 
If you move the fill vent line T up to the cab floor, fills will be much more consistent and therefore you calculated fuel rate will be too. We can run down to very near dry and not have a problem. I have taken on in excess of 48 gallons several times.
Matt
 
I like it......
That is about perfect. I am not at all concerned about MPG specifically, but as a pilot of both aircraft and water craft, fuel inventory is a concern and it just carries over. Moving that T makes fueling easier and more consistent.
Matt
 
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pvfjr said:
I kinda wonder how a NP205 would do. Those are pretty bullet-proof units. Ubiquitous and cheap. The way they're put together is kinda modular--I don't think it'd be that hard to get rid of the range box portion, build a custom aluminum plate adapter, and get the input shaft cut/splined as needed.
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The NP205 was my first choice (still have two at home) They are a gear driven box which would probably add to the decibels up front and the are at least twice the weight. Getting the right combination of case/output/input shafts can be a bit daunting but possible. The B/W unit is more flexible for our application being shorter meaning less intrusion into the cabin as well as the ability to be configured as over or under drives as the need requires. As I understand it, the NP205 is in fact stronger than the Atlas replacement and much cheaper.
Hal
 
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Hal said:
The NP205 was my first choice (still have two at home) They are a gear driven box which would probably add to the decibels up front and the are at least twice the weight. Getting the right combination of case/output/input shafts can be a bit daunting but possible. The B/W unit is more flexible for our application being shorter meaning less intrusion into the cabin as well as the ability to be configured as over or under drives as the need requires. As I understand it, the NP205 is in fact stronger than the Atlas replacement and much cheaper.
Hal
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A balancing act of concessions, for sure. I'm just going to keep the possibility tucked into the back recesses of my mind as reassurance that someday, if the need arises, it could provide a solution. I'd feel pretty good with a Vortec 8100, 4L80, and Np205 hodge podged together somehow. It would always be easy to fix and get parts for, at least. For now, I take enjoyment out of having a stock, original drivetrain.
 
Matt Colie said:
If you move the fill vent line T up to the cab floor, fills will be much more consistent and therefore you calculated fuel rate will be too. We can run down to very near dry and not have a problem. I have taken on in excess of 48 gallons several times.
Matt
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48 gallons at the fill-up? Well done, sir! That's something for me to aspire to. I think I've topped out at 35 gallons, but the hills are making me nervous, and I have low confidence in my fill-up capacities. I'll have to see about moving that tee as you mention. It seems a couple milliradians of slope change at the gas station may have significant effect on my filling ability. Not to mention, we have to let some random bozo take a stab at filling for your in OR. Not my favorite thing about this state...
 
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You might want to look at my thread on LiPO4 batteries. I am pulling all of the ac units, storage box, railing and ladder of the coach and install (2) direct current 12,000 btu heat pump mini splits to reduce air resistance.YMGI Exterior Units Install 070921.jpg
 
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The key to increasing mpg is to reduce energy use and expand useable energy storage with LiPO4 batteries. No electric hot water heating, ac air conditioning, ac refrigeration, no led lighting, inefficient alternator, etc. These heat pumps use 48v dc power and with the (12) SUnPower semi flexible (frameless) solar panels can have a SEER of 32. Most rooftop ac units have SEER of 5 to 10. Also, dash ac blows hard with a 12v dc compressor. Remove all parasitic power loads off the 455.
 
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Matt Colie said:
And yes, the block that was used in the wind tunnel testing did not have mirrors or a roof top A/C unit. I believe that model is on the top of a cabinet at the RV Hall of Fame in Elkhart.
Matt
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Shoot, I was just there and didn't realize I should have been looking for that! Maybe someone can try and find it if they visit soon?
 
pvfjr said:
A balancing act of concessions, for sure. I'm just going to keep the possibility tucked into the back recesses of my mind as reassurance that someday, if the need arises, it could provide a solution. I'd feel pretty good with a Vortec 8100, 4L80, and Np205 hodge podged together somehow. It would always be easy to fix and get parts for, at least. For now, I take enjoyment out of having a stock, original drivetrain.
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My BIL has a Class C running all over with a Chevy Lx 6.0L and 4L80e. No issues in the hills and he weighs as much as the GMC with 50% more frontal area. With the GMC being smaller frontal area, I wouldn't think a 8.1L would be necessary. In fact, dare I say a 5.3L would be adequate and they are ubiquitous.

Maybe in a couple years after I'm done working my day job I'll mess around with this idea. At the moment I just rebuilt the 455 and will run that for a while.
 
tmsnyder said:
I wouldn't think a 8.1L would be necessary
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I agree completely! I'd venture to say the 8.1 in my Silverado isn't necessary either. But I have it, and I like it a lot.

I also like the idea of not pushing things to their limits. My 8.1 has 310k on the clock with just the regular maintenance. A 5.3 or a 6.0 would surely move the GMC around, but I do wonder if they'd last as long.
 
pvfjr said:
I agree completely! I'd venture to say the 8.1 in my Silverado isn't necessary either. But I have it, and I like it a lot.

I also like the idea of not pushing things to their limits. My 8.1 has 310k on the clock with just the regular maintenance. A 5.3 or a 6.0 would surely move the GMC around, but I do wonder if they'd last as long.
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I get it, we have a saying, anything worth overdoing is worth doing lol
 
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tmsnyder said:
I get it, we have a saying, anything worth overdoing is worth doing lol
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That's the age old argument, which is better, a barely used big engine with a hardly open throttle or a small engine with a wide open throttle. For MPG the small engine wins, for a wider range of operations the bigger engine wins.

I once was on a team of 6 drivers that set a world record for the longest distance traveled on a single tank of gas in a standard production car. We used a 1st generation Honda Insight and went 2254 miles to average 164 MPG. What we found out is that modifying the car makes MINOR changes to the MPG. Modifying driving habits makes MAJOR changes.

Tomorrow we leave for the Buellton GMCMI convention in our 74 GMC with our fresh specific RV built Cad 500. It has a RV cam, forged pistons with slightly more compression and 1/16 in rings, bigger valves, bigger chambers, full balance, light weight crank pulley. It also has electronic ignition, fuel injection, 3.7 FD with limited slip, and big exhaust, which it had before. It's got 3500 miles on it now and feels pretty well broke in. We'll see if the mods make much difference in the MPG. The power is noticeably up from the previous Cad 500, but we still drive it at around 60-65. We used to get 8 MPG towing or 9 without.
 
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Louis said:
That's the age old argument, which is better, a barely used big engine with a hardly open throttle or a small engine with a wide open throttle. For MPG the small engine wins, for a wider range of operations the bigger engine wins.
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It takes about 90 hp to run a GMC down the road at 65mph iirc. A 5.3L Lx engine is rated at 300+ hp, so it wouldn't be WOT.
 
I don't check the gas mileage, but we do keep track of all the gas we buy and all the miles driven. On average we get quite a bit less than the rumored 8 - 10 mpg. WE always tow and my wife ends to load up the towed car and the coach is probably a little overweight.

I had a 1968 Dodge Charger RT with a 440 Magnum, automatic, all the bells and whistles, and (I think) 325 rear end gears. It got about 10 - 12 MPG, even on road trips. All this in spite of the car weighing under 4500 lbs and being somewhat more low and sleek compared o the GMC. I'm not at all surprised at the mileage I get in the GMC.
 
tmsnyder said:
7200 miles tracked so far, 8.5mpg: https://www.fuelly.com/car/gmc/tze/1976/tmsnyder/963498

This fine individual (also on this forum) is getting about the same as me, also 7200 miles tracked, 8.1 mpg: https://www.fuelly.com/car/gmc/motorhome/1977/76spirit/962644
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my most recent trip was 250 (RT)
mostly highway traveling at about 60mph
temps in the upper 80's and humid
30 miles of it was full of short and dare I say somewhat steep hills (river bluffs)
Since I have not yet upgraded my fuel lines - I always have run no-oxy fuel
--my fuel economy was 8.7mpg--
 
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