Proper engine speed for best mileage?

[Frijoli]

What is the engine speed range specifically for the 6.5 TD, or any diesel for that matter? I'm trying to add the best tire/axle combo.

Thanks all!
Clay

[DmaxMaverick]

Welcome to the forums!

That range can vary according to the vehicle and engine configuration. There are too many variables. Generally, the 6.2/6.5 engines like to run 1800 to 1900 for efficiency, but gearing, load and platform may effect that. Your intended use has to be a factor, as well. You wouldn't want to gear too high for empty mileage if you plan on loading or towing with it.

Provide all the info you can about the vehicle and intended use for a more detailed reply.

It may be better to post this question in the forum specific to the engine you have. Try searching the forums, also.

[Frijoli]

Thanks for the welcome!

I guess I was a little vague. What I was trying to say was, what is the proper engine speed for the best engine efficiency. I'm specifically interested in fuel mileage. All engines have a natural range of efficiency and regardless of load or otherwise, that is where they want to be operated. I am trying to determine what tire, transmission, and axle combinations are best for each situation. My understanding of this is that fully loaded vehicle are most efficient at just above the maximum torque peak, and unloaded vehicles are most effiecient at the lowest engine speed in the power band.
Did that make sense? I'm having difficulty wording this.
Any way my point is that gasoline engines are what I am familiar with, and I understand the limitations of them. What I don't know is where diesels like to be.

I specificaly want a good combination for Highway, unloaded mileage.

Thanks for your help.

[DmaxMaverick]

Still too vague. What vehicle? What engine? Turbo or no? What year? Etc.... Details....Details....

Torque peek efficiency is the key with diesels. Not so with gassers. A gas engine may be more "efficient" at their torque peek, but unloaded, they are not economical there. Usually half of the torque peek RPM or there abouts is much more economical. Where is the torque peek of a 6.0L Vortec? 4,000-5,000 something? What kind of mileage do you think you'd get screaming down the highway at those RPM's? With the EPA's demands, you'd only have a 2 speed tranny if that was the case.

The 6.2/6.5, turbo or not, on the other hand, will do best unloaded near the low end of the torque plateau, and more near the torque peek loaded. The intake plenum type also plays a part, as it can move your torque map around. Lugging an engine defeats the torque rise efficiency, such as you would have trying to run a gasser at 50% of torque peek under load.

I'd really like to answer your questions directly, but I wouldn't be doing you a service that way. There are just too many variables. Someone may give you a pat answer, but your results won't be ideal.

There are several threads in the other forums here that are directed at economy. I suggest you decide which category best suits your needs, and gleen all you can from them. You'll find there are many different opinions on the subject, and very little tested fact.

Try THIS THREAD (http://forum.thedieselpage.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=1;t=007818) (click on colored text) to start with. Maybe you'll find your answers there.
I bumped that thread to the top of the 6.5 forum.

[ 04-20-2005, 03:35 PM: Message edited by: DmaxMaverick ]

[Frijoli]

Well it's tuff to answer with more etail. I don't have a vehicle. I initially asked about the TD6.5 because I was looking at one with a 4.1 gear and five speed.
I wanted to know the best unloaded vehicle, engine speed, on a 3/4ton 94 chevy pickup, so I could pick a tire size to get the truck to max effiecny at 70mph or so. I wanted to make sure I wasn't needing to look at different gear.
I want as much fuel mileage as I can get in overdrive.

I'm not trying to be difficult. Incidentally, I can't view that thread. It says it's private?

Thanks, Clay

[DmaxMaverick]

If you are serious about buying that truck, I suggest you purchase a membership to access the member's areas. It will be money well spent if you wrench on your own vehicles, or to keep your mechanic honest. Too much good info to pass up. The BB Forum is just a small part of The Diesel Page. The member's areas cover just about everything GM Diesel. It's not often something new comes up any more.

[93c2500]

I have a 93 3/4 ton 6.5 with the 4.10 and 5 speed, and I cruise unloaded at about 2100 rpm, will average about 20 mpg on a half rural half suburban commute. Currently running 235/85 tires instead of the 245/75 tires, they are a little taller and narrower. I do have a fiberglass shell over the bed that may change the aerodynamics a bit.

I do think I would have been better off with the 3.73 axle and then tow in direct drive instead of the 4.10 and trying to tow in overdrive. My load is only about 3500 lbs though.