Unbelievable Fuel Mileage - REAL

[EKlem]

Instead of changing ratio's you could just up your tire size.



Correct me if i'm wrong, but aren't most vehicles most aerodynamically efficient at 47 mph? In other words, above this point the vehicle has to overcome both drag and headwind?

Increasing your tire size does effectively change your gearing but you have to be careful about increasing your rotating mass too much. As long as you keep a reasonably streatable tire on there, bigger rubber should help.



As far as the 47 mph thing goes, that sounds to me like a combination of the engine power versus fuel consumption curve and aerodynamics but I could be wrong. A vehicle's aerodynamics are best at 0mph. Each vehicle is different but the drag equation goes something like a+bx+cx^2+dx^3 etc where x is speed. As the power of the term increases, the coefficient decreases. At slower speeds, the earlier terms are more important(higher coefficients) but at higher speeds, the later terms(higher powers) become more important. This is the reason that the difference between 60 and 70 mph is so great. The dominant term at these speeds is usually a squared or cubed term. Obviously, driving at really slow speed where your aerodynamic drag is low doesn't make any sense because you are burning more fuel to keep the engine running than to propel the vehicle. Fast speeds are not efficient either because of issues with aerodynamic drag. 47 seems like a reasonable overall estimate for the most efficient speed for all vehicles produced but it will vary a lot from vehicle to vehicle. I apologize if I am misinterpreting what the 47mph stands for.

 

[KBennett]

I was really impressed by how efficient my truck is at idle. We made a 600 mile round trip recently. During that trip there were a few times we had to leave the truck idling for 20-30 minutes to keep the air on for the dogs. I thought it would use a lot of fuel but my hand calculated mileage was 19 MPG with the speed kept under 70 MPH.

 

[KBennett]

Here is a link to MPG Secrets from Cummins that may be able to answer the speed questions:

Cummins WhitePaper

 

[wlowe]

I just completed a trip from California to Washington state and back. Was empty going up, best MPG was 22. 5 at 65 to 70 MPH. Pulled a 5000 lb. trailer coming back, 65 to 70 MPH, averaged 17 MPG. I love this truck!!!



This is the best mileage I've seen since purchasing this truck. Recently installed a 4" exhaust, BHAF, and amsoiled the engine, don't know which one did it but something helped.

 

[Stake Man]

In 105,000 miles, I've never seen better that 15. 3mpg. Towing, empty, loaded, bobtailing, speeding, or going 55. Each tank I average around 14. 5-14. 7.



But in the other Cummins powered truck we have(Freightliner FL112-ISM400) I have a 230,000 mile average of 9. 3mpg. I have even gotten 10+ on trips to San Diego and back(400 mile round trip)



I gave up trying to figure it out.

 

[keimmmo]

I have observed that both my trucks get the best fuel economy at 48 mph... that is empty and running high gear on fairly level ground. A load or hills make a difference.



Steve Keim

 

[COBRAJET]

My Dad use to have a 93 Cummins with a 5 speed and 3. 54's. It was a two wheel drive that he drove all around East Texas and up to Iowa 10-12 times a year. He passed away about a year and a half ago. The truck had 440K on it and would still get 22-23 all day at 70mph. He could routinely get 25-26 out of it at 50-60 on the country roads. He mentioned several times getting close to 30 with the right conditions. I was so impressed I bought a 96 PSD... for the CREWCAB and averaged 18 for 230K miles with an auto, 3. 55's and 4 wheels drive. I have been in diesels ever since... . 800K miles and have been amazed at the mileage they can get if you figure out how to drive one. The truck below has seen close to 20 several times at 60-65 on country roads in no big hurry and routinely gets 18 at 70-75 with little wind. All hand calculated with fuel to the neck. It has only 8K on it and will get better. City driving, jack rabbit starts, poor tire pressure, extensive idling, big tires, lift kits, accelerating up hills to maintain speed... ie using cruise versus gaining speed down and losing going up, POORLY calibrated diesel pumps... . truck stops are the worse and a non steady foot ruin all hopes of good/obtainable fuel mileage.



I know of several fuel stations here in Texas that are so off it is not even funny. I found another one in Canton Monday. Fuel light had just come on with the 35 gallon tank and the truck held 35. 6 gallons before it shut itself off... . had to be 8-10 % off. I told them inside and they basically told me to pound sand. I do not think the truck stops pay near as much attention to pump calibration as do the gas stations or it is not enforced. I buy a ton of diesel as do 10-12 guys I have working for me and we have a black list of about 15 stations in Texas that we know are at least 5-10% off. That will kill your mileage as well.

 

[Flinty]

I agree with the truck stop problem. In a hurry I'll buy feul at a high flow pump and lose 2mpg. Our excavator has a 100 gallon slip tank that takes 103-110 gallons regularly. He has buying power though and they work with him. BTW, feul here is 3. 30/gl. (N. W. Montana)