5.9 vs. 8.3 fuel economy?

RomaniGypsy · 09-30-2017, 09:38 PM

Seems many people say that the 5.9 is bad for power on hills and in mountains... "if you're doing mountains or a lot of highway driving, you want an 8.3"... well, in regular passenger vehicles, a larger engine typically means worse fuel economy.

In a full size bus, roughly what fuel economy difference can you expect between the two engines? (I'm willing to sacrifice top-end highway speed and I'm willing to deal with going 25 up mountain hills if for the rest of my trip I can get good fuel economy. My main mount is a 2014 Prius v... tells you how I feel about giving my money to OPEC.)

Brad_SwiftFur · 09-30-2017, 09:59 PM

Honestly probably about the same economy. In fact, the 8.3 might do a little better since it's not working as hard as the 5.9. Diesels by their very nature only use "enough fuel to do the job", as opposed to running at a certain fuel/air mixture as gas engines do.

Dead pirate · 10-01-2017, 07:58 AM

The tranny has a lot to do with it too. A four speed vs. A five and all that....

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2kool4skool · 10-01-2017, 08:27 AM

11mpg avg. over 3 years and 10,000 miles. 5.9/545, 3.54 diff, 20,000 lbs. All terrain 55-60 mph.

turf · 10-01-2017, 09:23 AM

seems i'm always on the low end of mileage threads.... everyone gets better mileage than me.

7.5 mpg, cummins 5.9, ~70mph, at545, 4.7 rear, 21k lbs. ive put 20k miles on it.

go with a an 8.3 or dtt466, they get better mileage than i do.

PNW_Steve · 10-01-2017, 09:54 AM

Quote:

Originally Posted by turf

seems i'm always on the low end of mileage threads.... everyone gets better mileage than me.

7.5 mpg, cummins 5.9, ~70mph, at545, 4.7 rear, 21k lbs. ive put 20k miles on it.

go with a an 8.3 or dtt466, they get better mileage than i do.

I have to agree with Turf.

I have only put a couple thousand miles on my 8.3, md3060,5.38 drive trane and have gotten 9.7-11mpg.

Brad_SwiftFur · 10-01-2017, 10:40 AM

You'll probably find the factor that affects your fuel economy more than anything else is how fast you drive. Keep it 55-60 and I would expect somewhere around 10 MPG in a full size bus, regardless of engine size, transmission, and gearing. Push it up to 65 and I would expect mileage to drop to the 8-9 range. Cruise at 70 and watch it drop to the 7-8 range. Bump it up to 75 and expect 6-7 MPG. It takes a lot of fuel to push air out of the way.

Yes, engine size, transmission and gearing *Does* affect mileage, I won't argue that. I drive a Volvo road tractor and average 6 MPG (I generally run 70 on the highways). I have found that if I put the cruise on 62, it will average over 8 MPG.

I drove my bus home from WV at 55 - its top speed. It has a T444 engine and though I have yet to calculate it, I feel like it averaged around 10 MPG.

turf · 10-01-2017, 10:47 AM

i was pretty curious of mileage and started using an app called "fuel monitor". its handy after a fill up to enter the receipt into the app.

i think for free it tracks one vehicle. i have it on an ipad for the bus and my phone for my daily driver.

RomaniGypsy · 10-01-2017, 11:09 AM

I don't worry much about speed. I doubt I would ever try to push the limits of a school bus. When I had my Class C, I rarely put it up over 55. If I had to get somewhere quickly, it went to 60, and I only hit 65 once when I wasn't watching the speedometer. The point of having the bus will be to travel all over, hopefully leisurely. If I MUST drive faster than 55, I didn't plan well. (That said, I really don't want a bus that tops out at 55 mph. Surely such a bus would get worse fuel economy at 55 than one that could do 75 but is being driven at 55.)

mc11237 · 03-25-2020, 09:43 AM

Does an 8.3L GMC Gas bus even exist? I feel like the ad poster I'm looking at doesnt have any idea what they have....

WIbluebird · 03-25-2020, 10:09 AM

Quote:

Originally Posted by mc11237

Does an 8.3L GMC Gas bus even exist? I feel like the ad poster I'm looking at doesnt have any idea what they have....

GM made an 8.1 liter V8 gasoline engine that was in some of their cutaway buses in the early 2000s iirc.

To my knowledge an 8.3 Cummins ISC has never been fitted in a conventional school bus.

mc11237 · 03-25-2020, 10:12 AM

Quote:

Originally Posted by WIbluebird

GM made an 8.1 liter V8 gasoline engine that was in some of their cutaway buses in the early 2000s iirc.

To my knowledge an 8.3 Cummins ISC has never been fitted in a conventional school bus.

If it is the 8.1 gas any opinions on it? Could it tow 10k pounds across country? I know its an open ended question.

WIbluebird · 03-25-2020, 10:14 AM

Quote:

Originally Posted by mc11237

If it is the 8.1 gas any opinions on it? Could it tow 10k pounds across country? I know its an open ended question.

They are desirable in the 3/4 ton Silverado & Sierra trucks, in that application they can tow 10k just fine. Not sure about a full size bus though.

Nice thing about the gas motors is its easier to find mechanics to work on them and parts are cheaper. But they are much worse gas mileage and if you have a small tank you could be stopping for fuel every 150-200 miles at worst.

mc11237 · 03-25-2020, 11:37 AM

Quote:

Originally Posted by WIbluebird

They are desirable in the 3/4 ton Silverado & Sierra trucks, in that application they can tow 10k just fine. Not sure about a full size bus though.

Nice thing about the gas motors is its easier to find mechanics to work on them and parts are cheaper. But they are much worse gas mileage and if you have a small tank you could be stopping for fuel every 150-200 miles at worst.

More and more reason why gas shouldn't be a consideration for me. Good points- thank you.

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