Does slowing down make a difference? If so how much?

[JERRYKAM]

Folks,



I don't know if the price of diesel has caught your attention but I for one have begun to consider what changes I can make to save a few bucks. At 10mpg and a 80 gal tank, a fill up definitely grabs my attention. Of the available options the only one that seems to make sense, and may provide a savings is to slow down. I typically tow between 65-70mph (19,500 total weight) and get a pretty consistent 10-11 mpg under ideal driving conditions (level highway no headwind).



Has anyone tried slowing down to see what if any improvements in MPG can be realized?

 

[jtwcummins]

I drove a 97 CTD Auto 4. 1 CC 4x4 for 11 yrs. The only real impact on fuel economy was slowing down. Unloaded at 70MPH was 16-17 MPG. At alittle under 65 got 21-22 MPG. Towing with added weight of trailer and everthing else was 6500 lbs. At 55 got 15+mpg all the time. If speed increased MPG fell like a rock.



My 08 gets almost the same if I pay attention to my right foot. I can get 18-19 MPG keeping just below 70mph. Also, got 14+ towing same trailer. Problem with the new truck is it wants to GO. I can easily get 12-13 mpg driving around here, when I watch the foot get 17+mpg



The only thing I ever found that improved fuel economy was slow down and control the right foot.



Use the boost gauge like a vac gauge on a gasser. As C. D. says take offs are important and keeping the boost gauge as low as possible really helps. The 08 is very hard to drive slow on the hiway so I use cruise almost all the time. Slow down and enjoy the view and good tunes.

 

[DennyDay]

I'm finding out that not only slowing down helps, easy start ups make a difference -- -- watching the boost guage, and keeping it below 10 psig makes a big difference. One of these days I'll try it with the trailer.



Denny

 

[bigdaddydieseldan]

If you keep your truck under 2000 rpms it will help improve your mileage somewhat too #@$%!#@$%!#@$%!Oo. Oo. Oo.

 

[cwsoules]

I usually tow at 62 mph and usually get 11 mpg or better. As the hills get steeper it will drop to 10 mpg. Around Denver I expect 9. 3 or better mpg. A couple of years ago I was in a hurry crossing the country towing and saw my flat land mpg drop to 10 mpg.

 

[Grizzly]

When towing I start out slow and easy. I pretend that there's an egg between my right foot and the accelerator. I hold my speed to between 60 and 65 mph.

 

[starkmr]

Slowing down and avoiding jackrabbit starts can get you up to 5mpg. It's boring as hell until you hit the fuel station.

 

[icman95]

Pulled my 5er to SD from CT last summer. Best MPG I got was 10. 5 in the slower states. Illinois truck speed is 55, for example. In SD where traffic is 75 mph+ I got 8. 3. Slowing down helps a great deal!



Also, here at home just last month I tried an experiment. Using the truck to go to work I shifted at no more than 2000 rpm and did the speed limit, and no higher. It got me 8/10 mpg better over the course of one tank. I'm sure if I had continued with more tanks I could have tweaked it a bit more. Since it's motorcycle season, I've been taking that lately. 8/10 might not seem like much, but for all intents and purposes, it's a full 1 mpg better just for driving smooth and slow. After some time, that really adds up.



Remember, fuel consumption isn't linear. It's exponential. That is:



A 10% increase in speed does not mean a 10% increase in fuel. The fuel usage is actually much more than that. I forgot the exact number, but to travel at, say 72, rather than 65 takes something like 20-25% more fuel.

 

[HPetrat]

I have read the posts with great interest, as like every one here, moderate take-offs and keeping your speed down (RPM around or below 2000) gives the best mileage overall.



Jtwcummins, however, gave the best response of all. Like most marine applications, your boost can be directly matched to a fuel rate (no varing gear changes & discarding minor fluctuations for wind and current).

Our trucks are very similar especially when running at a steady state such as highway O. D. driving.

Icman is also correct, fuel consumption is exponential.



On my '98 12V towing the 9600lb TT, every 1psi increase from 18-26psi boost equates to a 1litre/hr increase in fuel consumption. The truck will burn 17L/hr @ 18psi and 26L/hr at 26psi boost, at 36psi boost it is closer to 42L/hr.



HP

 

[jelag]

For 8 years I had access to a computerized dyno for testing vehicles... I guess that I have over 400-500 hours doing so... that was 30 years ago... but the math still applies...

The math... . and its nothing more than an engineering formula goes something like this... for every 2-3 mph over 55 you lose 2% in fuel economy because of the following fixed numbers... .

Weight, wind resistance, rolling resistance & acceleration to speed. As the speed increases the wind resistance quadruples every time you add 10 mph in speed...

My wife, driving the same route, with the same vehicle always gets 15-18% better fuel economy than I do... she accelerates slower, watches traffic and coasts to a stop not needing the brake... and drives slower than I do...

Today on the way to the office with one of our dually's I was on a 4 lane in town highway with the speed limit at 35 mph... (2 lanes each way) a guy in a 2nd generation dodge was behind me... and decided that my 35 mph wasn't fast enough for him... . he jumps around me, blowing all this smoke... . I was at the time letting up on the throttle as 2 blocks up the light was red..... he has to blast around me and slam on his brakes to stop for the light... I'm coasting with the exhaust brake on... down to 28 mph, the light turns green and now I'm passing him... as he's fully stopped...

Just my thoughts on this...

 

[Chief USA]

I get the best mileage cruising in the highest gear allowable at about 1250 rpm, zero boost. As mentioned above coasting to a stop whenever possible with the exhaust brake and making very smooth, gradual accelleration. I get the best fuel mileage pooping along at about 45 mph indicated which is probabl a bit over 50 with the size tires I am running. From my days in aviation, I believe the formula for wind resistance is that when airspeed is doubled, the drag is squared. For most trucks and SUV's 45 mph is about the fastest efficient airspeed and for each mph over that, mileage as mentioned above is reduced by about 2%. If I drive 70 to 75 mph indicated, my fuel economy drops to about 12 to 14 mpg with 12 or there abouts being the norm. When I drive 45 indicated and take my time, I average about 22 mpg locally and have gotten as much as 26 mpg on one really good 60 mile trip with no stops and perfect conditions.

 

[CUMMINZ]

We are planning a 3 week trip from NC to CO. , UT. , and NM. and back in a while. I have just added a milage box. Not going to reveal the brand till I get back. So far my overhead has almost changed 4mpg. They claim 2-4. We plan to record every fillup and miles driven. I plan to set the cruise when I can at 65 and no higher. This 5000 miles should be very revealing.



I have done all I can to the engine so far. Modified my factory intake to cold air, added MBRP 5" to tip, K&N high flow and added free rolling 19. 5s. We have unpacked and repacted both truck and RV to lighten where we can.

 

[BILLVO]

I have slowed down as well and it has made a 3mpg increase. I drive 65 miles one way to work and used to drive 75 on average and now drive 67 on average and went from 15-16 to 17-19 overall. that is driving empty with my 05 with smarty jr on economy and by staying at or under 1800 rpms.



BUT... ..... while towing I find I need to keep it at or above 2000 rpms to get any milage. If I try and stay at 1800 rpms it lugs really bad and fuel milage drops like a rock and the smoke rolls with smarty jr on. I have towed and tried to stay at 1800 and get 10-12 mpg but same load and conditions and at 2000-2100 rpms I get 14-15 mpg while towing. 3. 73 gears with auto in tow haul.

 

[RVTRKN]

I towed my 5ver from So. Cal. to Pahrump NV. , at 58 MPH and set cruise when I could, and I got 13 mpg on the overhead (best so far). In California the speed limit is 55 for auto's with trailers, so I drove it slightly higher as not to attract attention and go faster than the speed limit. I then went to Parker AZ at 65 mostly (speed limit) and the overhead was at 11 MPG and then returned to So. Cal. On the return to So. Cal. my MPG never got above 10 mpg because of a severe head wind, driving at 58MPH.

 

[Pavemen]

I get 19mpg empty mixed driving but most miles are highway. Stay at 70-75mph most of the time on the highway.

Towing 12k (my modded 1500 on a 20+5 deckover 14k goose) was 11. 5mph and that was from central CA to Moab and back via Vegas. I stayed at 70mph most of the trip and around 50mph on the steep hills.

I noticed that my MPG goes up when I leave my exhaust brake off when empty. I think I like the brake sound so much I keep getting on and off the throttle to hear it!

I have a Palm based smartphone and use a prorgram called pFuel to track mpg and take notes, etc. I track "empty", "light" and "heavy", the latter two being for hauling/towing situations where I can run a 75% or more of a tank with that load. It tracks my trip or odometer, fuel consumption, pricing and even deals with partial tank fills. Gives per tank mpg, overall mpg, cost per miles, etc.

 

[raksasa]

Has anyone tried slowing down to see what if any improvements in MPG can be realized?

I made a round trip drive, Memphis - Little Rock - Memphis on Memorial Day. I have never seen so many trucks driving EXACTLY 65mph on the interstate. I think the slowing down idea is catching on with the big trucking operators as way to save significant amounts of fuel. We even do it in our airplanes now, it save tons of gas (literally).

I stayed with the trucks and let all the cars pass me.

 

[JLandry]

This might be of interest to everyone:



Secrets of Better Fuel Economy by Cummins



Best regards,



John L.

 

[JDiepstra]

Folks,



I don't know if the price of diesel has caught your attention but I for one have begun to consider what changes I can make to save a few bucks. At 10mpg and a 80 gal tank, a fill up definitely grabs my attention. Of the available options the only one that seems to make sense, and may provide a savings is to slow down. I typically tow between 65-70mph (19,500 total weight) and get a pretty consistent 10-11 mpg under ideal driving conditions (level highway no headwind).



Has anyone tried slowing down to see what if any improvements in MPG can be realized?

Is this a serious question?

 

[Bob V]

I have found that slowing down on the hills make the most difference, loaded or empty.

 

[DMcGalliard]

Factors behind fuel use

No one has discussed the factors that are the major users of fuel, so here are my comments: Fuel use is largely based on speed(as everyone has noted) and the coefficient of friction. When computing power use the speed is cubed(sxsxs). So every mile/hour increase has a big impact on fuel use. The other important factor is the coefficient of friction. For a streamlined car it is about 0. 3, it is hard to get below 0. 3, ideal is to be 0. 0, then you would need no fuel. On a fifth wheel it is probably 0. 5 to 0. 7. These two numbers are multiplied together as the mayor components of fuel use. There other factors that add, drive train losses, rolling resistance, etc. None make the power use less. My two cents worth.

Dave Mc

The factor I failed to mention is slope of the hill you are pulling, flat would be 0. 0 and I am not sure if it is based on actual slope as 6% or what as it takes a lot of fuel to pull a 15K fifth wheel up a 6% at 50 miles per hour where the speed and coefficient friction would not be much of a factor.