mileage - a stupid question, perfect for me

[jay r]

i have long searched for the perfect diesel engine - for so long that i fear i may fit the definition of insanity (repeating the same actions while expecting different results). the driving reason for this was to have low-end torque (which can be accomplished with gearing on a gas motor) and more drivability since a diesel is more efficient than a gas motor. inspired by the rumored 4BT swap yielding 20 mpg (a second check mark in the insanity column) i've searched long and hard for something in the budget (below wife threshold).

those of you with gas motors, how much fuel do you have to buy? i want to be able to enjoy the truck (eventually) and if i continually choose to drive something else simply because of fuel cost, then i've failed.

i was getting about 10 mpg with the stock 230 prior to teardown. is an overdrive transmission and a gm tcase with a 350 going to improve this, i would expect the best that i could expect would be in the 16 mpg range.

thanks,
jay

[sermis]

350 chev and stock drive train with 11.00 / 16's around 7 - 7.5
I don't drive it much anymore.

[wolf.dose]

I own my M715 since 1980. I used it about 85,000 miles +. First I used a Hanomag 65 HP Diesel, which was cheep in Germany. With This engine I had a consumption of about 15.6 mpg on long distances, sometimes ever better. On short distances (two or three miles only, of corse the consummation was higher due to the oil you have to warm up (up to 9 mpg)). About 5 years ago I changed to a 6.2 litres Detroit Diesel (commonly known as the 6.2 litres of the CUCV-trucks). The consummation is about 11.8 mpg. But driving performance is worth the fuel. From 15 to 60 mph its like automatic trance in direct gear.
About 20 years ago they also offered a Hercules conversion in the States, but I have no experience with that.
Wolf

[Barrman]

My 396 big Block/NV4500 combination gives me 12-13 highway at warp speed (54 mph). I get 10-11 at the same speed loaded down and pulling another M715 on a trailer. About 8-10 in town.

[firefightinsam]

I've got an m715 with a 4BT NV4500 and 4.56 axle ratio on 1100R 16 Michelins, I get a legit 20 M.P.G. If I keep my foot out of the throttle (55 MPH or less I have attained 23).

[Binford]

What I'd like to know is how do any of you know when your tank is "full?" Do you put so much gasoline in that it actually starts going up the filler neck?

[sermis]

I fill mine until the hose cuts off. At 7.5mpg it will be down enough when I get home not to over expand.

[Binford]

Okay, but I can put the nozzle almost halfway in the neck!

So you fill it until gas is actually in the filler neck then, huh?

I've got my home equity loan application in already, so I'm hoping to top my tank off some time today.

[MarknessMonster]

I fuel my truck's filler neck until I see gasoline. Then I grasp the side of the cargo box and shake the truck. This gets the air bubbles out and leaves the filler neck void of fuel.

Not to completely change the subject, but here's a question that has often interested me. When folks speak of fuel economy and list their specifications, one often includes the axle ratio. I generally assume the purpose for listing the axle ratio information is to promote the reader to make an assumption that the owner is indicating the truck runs at lower RPM's. All other items being equal, whether a truck has 5.89 or 4.56 axle gear ratios, the truck should consume the same amount of fuel if the engine RPM's are consistent. Obviously, one version would be driving the highway at 45mph, while the other would be driving 60mph. Is this a fair assessment of the way axle gear ratios affect fuel economy, or would there be more specific issues, such as the way the gear teeth encounter more resistance in a 5.89 set?

Another form of the question would be to ask if the fuel economy would be equal in two M715 trucks traveling 60mph and 2500 engine RPM's, whether one had a 4 speed T-98 w/ a 4.56 axle gear ratio and the other had a 5 speed NV4500 w/ a 5.89 axle gear ratio?

[rpgdeity]

Another form of the question would be to ask if the fuel economy would be equal in two M715 trucks traveling 60mph and 2500 engine RPM's, whether one had a 4 speed T-98 w/ a 4.56 axle gear ratio and the other had a 5 speed NV4500 w/ a 5.89 axle gear ratio?

I know your trying to compare apples to apples, but the above comparison can't quite be made. The final gear ratio of the two trucks would be different. One truck would have 4.56 axle ratio, and the other would have about a 4.30 ratio. To keep both at 60 MPH, the truck with the 4.56 ratio would have to be doing about 2700 RPM's to keep up. Therefore the one doing 2500 should be getting better mileage.

I kept the speed constant, because then the engine would be "seeing" the same "load" or resistance. If you mess with the gearing so as one vehicle is doing 2500 RPMS at 45, and the other is doing 2500 RPMS at 60, the one doing 45 is working much less. The power required to push a body against aerodynamic resistance (i.e. "drag force") is a function of velocity CUBED. Meaning, if you were to double your MPH on the freeway, it would require EIGHT times as much horsepower. As you go faster, the power required climbs really fast.

Now, what I've always wondered is when comparing engines, lower RPMS doesn't always mean better mileage, because aren't different engines more efficient at different RPMS? Based on the HP and TQ curve for an engine, the max TQ is the point of highest efficiency (lowest fuel consumption), correct? My only evidence to this is a graph of HP, TQ, and Fuel Consumption for the 4BT engine. According to the data, the sweet spot for a 4BT is between 1500 and 1700 RPMS (Which is also the point of max TQ). Any lower or higher will increase your fuel consumption. I sure wish I could find data like this for other engines to see if I'm right.