Reply
 
Thread Tools Search this Thread Display Modes
 
Old 01-22-2003, 07:51 AM   #1
Rivet Master
 
Join Date: Oct 2002
Posts: 917
Question Optimum gas mileage

I've asked this question within other threads, but never really received a response, so I thought I'd try it here .

Question: Optimum gas mileage- am I correct in my thinking that while towing at the rpm in which the horsepower and torque curves intersect is where one would get their best gas mileage?

Mine intersects @ 2500rpm, which translates to 55mph in 3 gear. At this speed, rpm & gear-I'm getting 13mpg.

So if any of you engine gruru's could set me straight, I'd be greatful-John
__________________

__________________
John is offline   Reply With Quote
Old 01-22-2003, 02:45 PM   #2
3 Rivet Member
 
Join Date: Apr 2002
Posts: 184
Images: 55
What engine/drivetrain/rearend combo do you have? To some extent, you are correct regarding the relationship between torque and HP. Remember though, its torque that actually moves you. Its the gear multiplication through the transmission and rear end that allow freeway speeds. Different engines respond differently to different loads and so on.

I tow a 25' airstream with a 1/2ton suburban with a 350 and 3.73 rear end. I usually get 12-13mpg regardless of speed. I ususally don't tow in OD unless its REALLY flat ground. Its much harder on the engine to lug it rather than rev it.


Tripp
__________________

__________________
Tripp is offline   Reply With Quote
Old 01-22-2003, 05:33 PM   #3
Rivet Master
 
Join Date: Oct 2002
Posts: 917
Tripp,

I have a 2000 5.3 4:10 auto. I did Ocsar's forumal and I come up with the above infor.

John
__________________
John is offline   Reply With Quote
Old 01-22-2003, 06:28 PM   #4
Retired Moderator
 
john hd's Avatar
 
1992 29' Excella
madison , Wisconsin
Join Date: Aug 2002
Posts: 4,644
Images: 40
stioch

john

you get optimum fuel milage when your motor reaches "stioch" or 14.7to1 air to fuel ratio.

with modern fuel management and automatic transmissions the onboard computer makes the most of a given load and speed situation.

your mileage to a point will be the same until your reach peak efficency. after that, the faster you go the more fuel you will use.

so your observation about rpm and torque makes sense.

that is assuming everything is in working order in your truck.

john
__________________
john hd is offline   Reply With Quote
Old 01-22-2003, 06:35 PM   #5
Rivet Master
 
Join Date: Oct 2002
Posts: 917
I guess that's I'm looking for form the forum.

Instead of driving at different speed's/rpm's and checking gas mileage to find the best fuel mileage at the highest speed.

Thanks-John
__________________
John is offline   Reply With Quote
Old 02-14-2003, 11:09 AM   #6
1 Rivet Member
 
Join Date: Jan 2003
Posts: 10
FYI, The rule of thumb that I've heard, as it applies to cars, is that for best mileage you should shift at your torque peak, and for best performance, shift at your horsepower peak.
__________________
blackbox is offline   Reply With Quote
Old 02-14-2003, 12:55 PM   #7
Rivet Master
 
RoadKingMoe's Avatar
 
2001 34' Limited
The State of , Ohio
Join Date: Sep 2002
Posts: 1,605
Images: 23
The intersection of HP and TQ lines is meaningless. You can make the lines intersect wherever you want by making the scales unequal.

HP=TQ at 5,252 rpm

Here's one dyno run of my hot-rod Harley, with the scales equal:
Attached Thumbnails
Click image for larger version

Name:	dyno1.jpg
Views:	220
Size:	31.1 KB
ID:	1476  
__________________
Maurice
RoadKingMoe is offline   Reply With Quote
Old 02-14-2003, 12:56 PM   #8
Rivet Master
 
RoadKingMoe's Avatar
 
2001 34' Limited
The State of , Ohio
Join Date: Sep 2002
Posts: 1,605
Images: 23
Here's the SAME dyno run with me tweaking the torque scale to make the lines intersect at 4,000 rpm.
Attached Thumbnails
Click image for larger version

Name:	dyno2.jpg
Views:	212
Size:	31.1 KB
ID:	1477  
__________________
Maurice
RoadKingMoe is offline   Reply With Quote
Old 02-14-2003, 01:01 PM   #9
Rivet Master
 
RoadKingMoe's Avatar
 
2001 34' Limited
The State of , Ohio
Join Date: Sep 2002
Posts: 1,605
Images: 23
Maximum gas mileage doesn't necessarily occur at stoichometric 14.7:1. It depends on the individual engine and my actually occur at 15:1 or greater. i.e. the power fall-off from being a bit leaner is not as great as the reduced fuel usage.

The ratio at which maximum hp occurs also depends on the engine. It may be as rich as 12.0, or as lean as 13.8, but generally occurs somewhere between 12.8-13.2:1

There's a school of thought that says max fuel mileage occurs at the torque peak, because that's the engine's point of peak efficiency, but that's obviously too high to cruise at with other than diesel engines.

There is no easy ROT that fits all engines. Best to use trial and error to find it yourself.
__________________
Maurice
RoadKingMoe is offline   Reply With Quote
Old 02-14-2003, 01:47 PM   #10
Moderator Emeritus
 
Pick's Avatar
 
1972 31' Sovereign
High Springs , Florida
Join Date: May 2002
Posts: 2,310
Images: 36
Send a message via AIM to Pick
Well, it looks like Maurice's post puts the 100 MPG Fish Carburator myth to rest!
__________________
ARS WA8ZYT
2003 GMC 2500HD 4X4 D/A Ext. Cab
Propane Powered Honda EU2000i
Lots of Hot Sauce!
Air # 283
WBCCI 1350
Pick is offline   Reply With Quote
Old 02-14-2003, 05:22 PM   #11
Rivet Master
 
2003 25' Safari
Eden Prairie , Minnesota
Join Date: Nov 2002
Posts: 765
Images: 3
Optimizing engine RPM is only part of the equation. Modern engine controls keep fuel/air mixture close to optimum all the time, and four/five speed transmissions keep the rpm's in a narrow range. Lower RPM's reduce internal friction and improve mileage - but may not produce enough torque to get the rig up the hills without downshifting, which is hard on the tranny.

The most significant variables are weight and speed. You have limited control over weight (don't bring your anvil collection). Speed is about all you can control. Since drag increases four times when speed doubles, generally slower is better. However, at idle you have zero wind resistance, but you are going nowhere - thus getting zero mpg. In general - the optimal speed will be somewhere just above where the highest gear becomes usable. While towing, the highest gear (overdrive) may never become usable except on very flat roads so the next highest gear would be used.
__________________
Dan
dmac is offline   Reply With Quote
Old 11-06-2007, 09:11 PM   #12
Vintage Kin
 
slowmover's Avatar
 
Fort Worth , Texas
Join Date: Nov 2006
Posts: 7,603
Images: 1
Wind resistance is the killer, followed by weight. An old rule of thumb for gasoline motors was to cruise a few hundred rpm below peak torque for the best balance of power and economy (in the days when V8's were at torque max at 2600-3200 rpm).


HORSEPOWER ESTIMATOR
Horsepower Estimator Worksheet

Optimum mileage is a tradeoff. We can't run the Interstates at 45 mph nor would we want to. I use 62-63 mph as a good speed that features very good reaction times, etc. It also allows me to easily come to a stop.
__________________
1990 35' Silver Streak Sterling; 9k GVWR.
2004 DODGE Cummins 305/555; 6-manual; 9k GVWR.
Hensley Arrow. 9-cpm solo, 15-cpm towing
Sold: Silver Streak Model 3411
slowmover is offline   Reply With Quote
Old 11-06-2007, 10:45 PM   #13
Rivet Master
 
Bob Thompson's Avatar
 
Corpus Christi , Texas
Join Date: Sep 2003
Posts: 936
Images: 67
Strictly my own observation, but optimum gas mileage occurs when the smallest vacuum space occurs behind the vehicle or the towed vehicle. The size of the vacuum space increases with speed. Example: my Airstream Excella 25 weighs about 5400 lbs but because of it's ability to slip thru the air, I get slightly better gas mileage pulling it than I did with my previous trailer which was an Award 23 weighing about 3300 lbs. The Award was advertised as being especially aerodynamic. Once I pulled a Nash 16 "square ol' box" several hundred miles and got awful gas mileage even at 55mph. Anything over about 40 mph and I was pulling a full size vacuum behind it! It wouldn't have mattered much if I was pulling a 16' trailer or a 26' trailer if it were similarly shaped.

For the record I got about the same mileage pulling the Award 23 with my V6 Tacoma as I got with the Tundra.
__________________
So Long!
Bob Thompson is offline   Reply With Quote
Old 11-06-2007, 11:24 PM   #14
2 Rivet Member
 
geirws's Avatar
 
Join Date: Aug 2003
Posts: 56
i have a 77 argosy motor home. 454 engine. It has a vaccum gauge installed as original equipment. It has a green zone which is good gas mileage, a yellow zone that's fair mileage and red zone that's poor mileage. I drive at around 60 mph. 2800 to 3000 rpm. I get 9.5 to 10 mpg. weather i'm driving across the praires or over the mountains. I gear down to second gear going over steep hills as the owners manual suggests and drive at 45 mph. That keeps the vaccum gauge in the high yellow zone to mid red zone. Also going into a head wind the vaccum gauge reads slightly higher. So if you want to get the best mileage maybe you might consider installing a vaccum gauge.
__________________

__________________
geirws is offline   Reply With Quote
Reply


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
servicing gas part of refrigerator nickcrowhurst Refrigerators 3 02-21-2006 08:46 AM
What's your Gas (or Diesel) Mileage? 85MH325 Tow Vehicles 29 04-04-2004 07:40 AM
Gas Furnace ketchumwt Furnaces, Heaters, Fireplaces & Air Conditioning 4 12-04-2003 01:46 PM
Engine will not start - LP Gas detector? AccessMaster Airstream Motorhome Forums 6 08-31-2003 09:38 AM
looking for F250 gas mileage examples 2PNPOD Our Community 12 04-19-2003 10:18 PM


Virginia Campgrounds

Reviews provided by




Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -6. The time now is 05:17 AM.


Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, Jelsoft Enterprises Ltd.

Airstream is a registered trademark of Airstream Inc. All rights reserved. Airstream trademark used under license to Social Knowledge LLC.