Everybody knows that a well maintained engine runs better and more efficient than a neglected one.

Diesel Bombers in their high output engines know this better than anyone else.
But what can be done to increase mileage?

How you use your vehicle, normal cruising speed and tire size are the most important influences in milrage.
A simple furmula will help you to get the maximum out of your fuel:

Ideal cruising speed= RPM x tire diameter/ gear ratio x 336

You need: numbers as an example from my truck:
- rpm at highest torque - 2000rpm
- tire diameter - 28,2"
- highest tranny gear - 0.79
- axle gear ratio - 3.73

ideal cruising speed= 2000 x 28.2/(0.79 x 3.73 x 336)
= 56,96mph or 91km/h

As you can see that's exactly the limit because above 55mph wind resistance does increase fuel consumption.
You also can see that tire size does influence fuel consumption.
Bigger isn't always better, or you have to regear.

With this simple formula I increased my mpg from 26mpg up to 29mpg.
Knowing that 57mph is my personal limit to gain maximum mileage.

What's your max mpg?

 

With 30" tires, ideal cruising speed for my truck is 61.5mph or 99km/h.
Best fuel economy to date so far on a trip was 22.0mpg. Averaged 20.8mpg in the mountains in July that was from the overhead though....

 

Would this apply to all vehicles (diesels) given air flow/drag around the vehicle being that they are different body styles? I havent done any hard figure tests but based on the computer read out on the overhead console ( I know not exact mpg's from there) but mine does get alot better around 53mph. Right now hand calculating every tank i am getting a whole wopping 15.9mpg

 

Yes, it works for all cars.
Remember, if you go faster than 55mph you also have to work against the wind resistance.

The ideal crusing speed just reflects the highest torque of your engine with the lowest needed power to keep your truck rolling.

You can aid in fuel savings by the common rules like:
- lowering weight.
- deceasing rolling resistance by proper tire inflation or appropriate tires for roads
- using a bed cover or lowered tail gate.
- dismounting unused roof bars and roof baskets.

I always have a good smile when listening to buddies doing a tune up to save fuel and driving around with ultra aggressive 35" offroad tires and with a load full of stuff in their back.

 

How do you "seat of the pants" find the rpm's at highest torque?

 

I found mine in the factory service manual and in the user manual.
You also can use your Dyno print out. Just have a look at your torque peak and go to the rpm.

last not least- ask your dealer about the rpm at top torque.

My top torque starts, right behind when the turbo starts to kick in. But I'm sure thats not a good method to find out...

 

How about proper tire pressure?

 

I'd like to know where you got your formula because it is misleading. What is the exact defenition of "ideal cruising speed"?

Your formula implies that your vehicle gets its best mileage at 56.96 mph. This is not true. It will get higher mpg at lower and lower speeds.

 

So, try it- it's a compromise between fuel consumption, highest gear and torque.

This formula is taken from the Bosch automotive technical handbook, german issue no.:22

you will also find it in John Lawlor's Auto Math Handbook in combination with Larry Sheppard.

You will find further hints in "On the Physics of drag Racing written by Geoffery T. Fox.

I also recommend Isaac Newtons and his Newton's Laws.

Have a look at the Hp, Tq and BSFC charts of a Diesel and find out that the power band provides the lowest BSFC at Tq peak.

Yes, you can decrease speed down to 50mph- and gain a bit due to the reason that wind resistance starts to develop here. Which is not mentioned in the formulas above. but nobody goes 50mph on a highway- do you?

Ideal crusing speed is a compromise between speed and fuel consumption- nothing else.

You can save a lot-with the truck in the garage...

 

I know I am always amazed to see 21 mpg from my truck weighing 9600 lbs, I believe its the 36 inch tires that do it for me