Fuel Economy
#11
12-23-2012, 08:20 PM
maybe, maybe not, it is very possible to get a nice air bubble in the tank, this becomes more likely if you fill up the filler neck (along w/ the very good chance of ruining your charcoal canister by filling up the neck, but thats a whole other story).
im also curious about enviromental conditions, ambient temp (higher is better for mpg), wind (im expecting you had a tailwind most of the way) amount of traffic, smooth driving w/o slow downs is obviously better, all things that need to be taken into account when thinking about gas mileage
for example, where i live now is mostly flat, so in normal driving i get around 17 around town. lately i have been getting 14.5-15, normally this kind of drop would be a concern, however i can account for a number of negative factors,
1) cold weather (denser air contains more oxygen, requires more gas), also longer open loop run time and lower air pressure
2) lots of idling in my commute, i have to idle through a guarded access gate every morning and evening, this totals btwn 5 and 20 minutes of idling a day.
3) winter gas mix is less efficient than summer gas because of the additive package used to help prevent gas tank/line freeze up,
also i have heard of people getting a gallon of gas into the filler neck, which on a 25 gallon tank is just under 5%, not huge, but far from insignificant.
i am not doubting you, just bringing up pieces of the puzzle that many people overlook when talking about gas mileage (im sure ill come up with more as time passes)
im also curious about enviromental conditions, ambient temp (higher is better for mpg), wind (im expecting you had a tailwind most of the way) amount of traffic, smooth driving w/o slow downs is obviously better, all things that need to be taken into account when thinking about gas mileage
for example, where i live now is mostly flat, so in normal driving i get around 17 around town. lately i have been getting 14.5-15, normally this kind of drop would be a concern, however i can account for a number of negative factors,
1) cold weather (denser air contains more oxygen, requires more gas), also longer open loop run time and lower air pressure
2) lots of idling in my commute, i have to idle through a guarded access gate every morning and evening, this totals btwn 5 and 20 minutes of idling a day.
3) winter gas mix is less efficient than summer gas because of the additive package used to help prevent gas tank/line freeze up,
also i have heard of people getting a gallon of gas into the filler neck, which on a 25 gallon tank is just under 5%, not huge, but far from insignificant.
i am not doubting you, just bringing up pieces of the puzzle that many people overlook when talking about gas mileage (im sure ill come up with more as time passes)
I guess I should clarify what I meant by filling the filler tube I never fill the tube up to the top, I usually just run it up to the nearest dollar after it stops.
As for the conditions, it was between 30 and 40 degrees the whole trip. The weather was pretty calm for the first half of the 534 mile trip. For the second half of the trip there was heavy rain along with strong crosswinds. I was driving from South to North and the storms were going from West to East, so there wasn't really a tailwind.
The ground was fairly level through South Carolina and North Carolina, and the mountains started right on the Virginia/North Carolina border and continued the whole way up to Pennsylvania. Traffic was fairly light for most of the trip, with the exception being Charlotte, NC which was starting to pile up for rush hour. I was driving between 60 and 70mph the entire time. It took me 8 and a quarter hours to go 534 miles so that works out to 64.7mph average.
I topped the tank off once in South Carolina, once in Virginia, and once in Pennsylvania; I'm guessing that the gas from the latter two were winter mix. When I topped of the tank the last time in PA the math gave me 17.2mpg, 17.0 when I topped off in VA, and I forgot to do the math when I topped off in SC.
Last edited by that_guy; 12-23-2012 at 08:27 PM.
#12
12-23-2012, 08:50 PM
THE ULTI-MOD
supposedly 60-65 is the most efficient speed from an aerodynamic standpoint, which could definitely explain why your number were higher than usual, however another factor is the engine speed, every engine has a speed at which it runs most efficiently (most efficiently converts fuel to power/torque), if i can find out where i read that and the equation ill let you know what that engine speed is for your engine, i remember that the 4.7 is around 2100 rpm (roughly 80 mph for my gearing)
#14
12-23-2012, 09:16 PM
THE ULTI-MOD
ok found it optimal engine speed is 1000-1400 ft/min piston speed
for the 5.2l engine the stroke is 3.31 inches
3.31*2= 6.61in
6.62/12 =0.552 ft/rev
1000ft/min divided by .552ft/rev = 1811.xx rpm
1400ft/min divided by .552ft/rev = 2536.xx rpm
with the ideal place being 1200 ft/min so
1200ft/min divided by .552ft/rev = 2173.xx
so now i ask are these roughly the engine speeds that you are seeing cruising at highway speeds?
or cruising at about 70% (+/- of course) throttle
https://dodgeforum.com/forum/1036359-post14.html
for the 5.2l engine the stroke is 3.31 inches
3.31*2= 6.61in
6.62/12 =0.552 ft/rev
1000ft/min divided by .552ft/rev = 1811.xx rpm
1400ft/min divided by .552ft/rev = 2536.xx rpm
with the ideal place being 1200 ft/min so
1200ft/min divided by .552ft/rev = 2173.xx
so now i ask are these roughly the engine speeds that you are seeing cruising at highway speeds?
or cruising at about 70% (+/- of course) throttle
https://dodgeforum.com/forum/1036359-post14.html
Last edited by shrpshtr325; 12-23-2012 at 09:23 PM.
#15
12-23-2012, 10:11 PM
ok found it optimal engine speed is 1000-1400 ft/min piston speed
for the 5.2l engine the stroke is 3.31 inches
3.31*2= 6.61in
6.62/12 =0.552 ft/rev
1000ft/min divided by .552ft/rev = 1811.xx rpm
1400ft/min divided by .552ft/rev = 2536.xx rpm
with the ideal place being 1200 ft/min so
1200ft/min divided by .552ft/rev = 2173.xx
so now i ask are these roughly the engine speeds that you are seeing cruising at highway speeds?
or cruising at about 70% (+/- of course) throttle
https://dodgeforum.com/forum/1036359-post14.html
for the 5.2l engine the stroke is 3.31 inches
3.31*2= 6.61in
6.62/12 =0.552 ft/rev
1000ft/min divided by .552ft/rev = 1811.xx rpm
1400ft/min divided by .552ft/rev = 2536.xx rpm
with the ideal place being 1200 ft/min so
1200ft/min divided by .552ft/rev = 2173.xx
so now i ask are these roughly the engine speeds that you are seeing cruising at highway speeds?
or cruising at about 70% (+/- of course) throttle
https://dodgeforum.com/forum/1036359-post14.html
Yeah, 60mph is right over 1700rpm, 65mph is right around 1900rpm, 68mph is right on 2000rpm, and 70mph is right under 2100rpm, so me cruising between 60 and 70mph with an average speed of 64.7mph falls within that range.
#16
12-24-2012, 03:54 PM
The 1998 owner manual says 21 gallons. I thought for sure half tank ran near 200 miles that it would get close the second half. Nope! Sometimes it likes the faster speed. The 35s bring the 3.92 gears down to 3.55. 75 is about 1800. Running 2-2200 helps sometimes. I found that I can get better MPG without cruise.
#17
12-24-2012, 04:39 PM
THE ULTI-MOD
yes you should be able to get better mpg w/o cruise bc cruise doesnt care about compensating for terrain it keeps you at your set speed +/- a preset value, if you control the speed yourself you can let the truck slow some going up a hill, and accelerate again going back down the other side.
also the fuel gage goes slower over the first half because it doesnte ven start dropping until you burn a couple of gallons out of it, you need to burn enough fuel for the float to be released from the top of the tank before it moves, and from there it drops relatively consistently (still not perfect since its an electronic gauge, but pretty good)
also the fuel gage goes slower over the first half because it doesnte ven start dropping until you burn a couple of gallons out of it, you need to burn enough fuel for the float to be released from the top of the tank before it moves, and from there it drops relatively consistently (still not perfect since its an electronic gauge, but pretty good)
#18
12-24-2012, 05:19 PM
The 1998 owner manual says 21 gallons. I thought for sure half tank ran near 200 miles that it would get close the second half. Nope! Sometimes it likes the faster speed. The 35s bring the 3.92 gears down to 3.55. 75 is about 1800. Running 2-2200 helps sometimes. I found that I can get better MPG without cruise.
From my build sheet:
NF4S 25 Gallon Fuel Tank
The cruise is alright on flat ground, but like shrp said it's better to control the throttle yourself if you're driving up and down hills. I hardly ever use mine, and didn't use it at all on my trip.
#19
12-25-2012, 04:09 PM
Record Breaker
The 1998 owner manual says 21 gallons. I thought for sure half tank ran near 200 miles that it would get close the second half. Nope! Sometimes it likes the faster speed. The 35s bring the 3.92 gears down to 3.55. 75 is about 1800. Running 2-2200 helps sometimes. I found that I can get better MPG without cruise.
Your owners manual must have a typo because mine says 25 and the FSM says 25...
#20
12-29-2012, 02:36 AM
Hello, ya'll might remember me. I'll give a hint at the end of the post.
I drive a 2001 Durango SLT 4x4 (but I usually keep it in 2WD).
On my last two road trips (both to the same location ~175 miles away) I clocked 18.1 mpg this last November for hunting and 18.5 mpg late December for Holidays with family (me, wife, two daughters and all their stuff...). First trip was at 60 mph whole way (all highway) and the second was at 55 mph whole way (all highway).
I currently now get ~15 mpg ( 14.7 mpg and 15.3mpg) over the course of a month with city driving (to work then home or to school then work then home).
I can get up to 55 mph @ 1200 rpms and 65 mph @ 1600 rpm.
I am quite pleased with the way my truck is preforming. As long as I stay out of 4WD (which really sucks for mpg) I do pretty good for a 6k lb truck.
I have 162000 on the body and 3k on the engine. Yup, I'm the guy who was stressing over an engine replacement few months back.
I drive a 2001 Durango SLT 4x4 (but I usually keep it in 2WD).
On my last two road trips (both to the same location ~175 miles away) I clocked 18.1 mpg this last November for hunting and 18.5 mpg late December for Holidays with family (me, wife, two daughters and all their stuff...). First trip was at 60 mph whole way (all highway) and the second was at 55 mph whole way (all highway).
I currently now get ~15 mpg ( 14.7 mpg and 15.3mpg) over the course of a month with city driving (to work then home or to school then work then home).
I can get up to 55 mph @ 1200 rpms and 65 mph @ 1600 rpm.
I am quite pleased with the way my truck is preforming. As long as I stay out of 4WD (which really sucks for mpg) I do pretty good for a 6k lb truck.
I have 162000 on the body and 3k on the engine. Yup, I'm the guy who was stressing over an engine replacement few months back.