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Old 09-15-2013, 10:18 PM
jl429 jl429 is offline
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Real world diesel MPGs

OK guys, I have been contemplating a 12 valve Cummins swap in my 79 F350 single rear wheel 4x4 expecting to get 18 to 20 mpg. My F350 has a 460 with a NV4500, a NP205 transfer case and 4:10 gears with 33-12.50-16.5 trires. My mileage is 9 MPG loaded, unloaded, uphill, downhill its always 9 MPG. I pull a horse trailer on the weekends so I need the power of the 460 but would like better mileage. I've heard the 12 valve Cummins get 18 to 20 MPG. I had a conversation with a friend of mine who has two Dodge trucks with 12 valve Cummins in them and he has them turned up to around 400 horsepower. He said that he gets 10 to 12 MPG unloaded and 8 MPG loaded. He also said that the people who got 20 MPG had stock trucks with stock tires and high gears in the differentials. My question is to all the people who have made the 12 valve Cummins conversion is: What is the real world mileage for a lifted truck with larger than stock tires with a Cummins engine under the hood? If you are getting 18 to 20 MPG what gears are in the differentials. My truck is about as tall as a highboy, I swapped in a set of highboy front springs so it may have a 4 inch lift. Another question is: I have found a turbo 7.3 ( not powerstroke) and I was wondering if this engine would be better than the Cummins? Someone said that a diesel swap would not get better MPG (maybe 1 to 3 better than the 460) and I would just be using a different fuel. Sorry for the long thread. Thank you for your time and any advice will be appreciated.
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Old 09-15-2013, 10:26 PM
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you would have to put on a massive amount of miles per weekend to ever make the cummins swap worthwhile.it's highly,highly unlikely you would ever save enough on fuel to ever recoup the cost of the swap simply driving the truck on the weekends.those who swap in a cummins don't typically do it for the economy.they do so for performance and you can't save on fuel with a lead foot.

the 7.3l idi will do a whole lot better than just a couple mpg over the 460 engine.i can do better than that with my heavy dually hauling and towing regularly:
1993 ford F250HD DUALLY CONVERSION Gas Mileage (chipper truck) - EcoModder.com


folks with trucks as you mention as stock pickup beds and srw's push mid to upper teens.
keep in mind though,that the idi diesel swap would be drastically cheaper to swap in than a cummins,would still require more work than it's worth imho.i say this because you could probably just sell your 460 truck and buy an idi truck for less and not even have to work at swapping the entire rad supports,fuel systems and wiring.
if idi trucks were hard to find or costly,that would be one thing,but since they are easily found and very affordable,i just don't see the point unless of course your looking for a project for recreation or your truck was handed down by family and your just really hooked on it. otherwise think objectively and just part ways with that you have and move on to a diesel truck if that's what you'd rather have.keeping in mind maintaining a diesel is more costly than a 460.a 460 tune up is what? 100 bucks? the idi's is around a grand.
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Old 09-15-2013, 11:33 PM
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Thanks for the advice. I have owned the truck since 1994 and my dad and I used to work on it together. I really love the old truck and since my dad passed away in 2010, it has a sentimental place in my heart. I want to make it a daily driver thats the reason for thinking about an engine swap. Thanks again for your advice and your time.
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Old 09-15-2013, 11:44 PM
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sorry to hear about your dad.
swapping it to a diesel will really change the whole dynamics of the truck and likely wont represent the old feelings you once had with it with it's massive diesel clatter.perhaps just consider keeping this truck as is and just using it from time to time.i bet she'll remind you of your dad a lot more than it would being massively altered.can keep it in nice shape and the miles down by considering buying a diesel truck.they can be had for just a couple thousand or less.
if you want to change it over to a diesel truck,id start with looking for a complete donor so you have everything you need.just check out the rad support good first and might even get a good radiator.you'll need the dual batteries and preferably new cables.2/0 gauge and with the donor truck,you pretty much just follow your nose.
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Old 09-19-2013, 02:10 PM
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Originally Posted by FORDF250HDXLT View Post
sorry to hear about your dad.
swapping it to a diesel will really change the whole dynamics of the truck and likely wont represent the old feelings you once had with it with it's massive diesel clatter.perhaps just consider keeping this truck as is and just using it from time to time.i bet she'll remind you of your dad a lot more than it would being massively altered.can keep it in nice shape and the miles down by considering buying a diesel truck.they can be had for just a couple thousand or less.
if you want to change it over to a diesel truck,id start with looking for a complete donor so you have everything you need.just check out the rad support good first and might even get a good radiator.you'll need the dual batteries and preferably new cables.2/0 gauge and with the donor truck,you pretty much just follow your nose.
I agree with this,
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Old 09-19-2013, 02:55 PM
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I did the IDI swap on a 69 F250 2wd, and it's worthwhile. I had maybe $1500 invested in the swap, sold the not even broken in 360 for $600, and made the difference by parting the donor truck. I did the swap over a week in a friend's garage.
The mileage before was 12 tops, 7.5 towing. The mileage now is 20-25 empty on the freeway (I got 24.5 last weekend doing 70 heading down I-5), and the lowest I have seen is 14 towing my 20' gooseneck. So, roughly half the fuel consumption. I have put over 20k on this swap, and would highly recommend it for anyone.
Your truck won't get that high of mileage being a 4x4, but driven right there is no reason it shouldn't double your mileage.
The IDI swap is cheaper than the Cummins swap for two reasons-
Nobody respects it. People want the Cummins engines, so they're expensive. The IDI is largely ignored so it's cheap.
The operating RPM is closer to that of a large gas engine. The 6BT makes all it's power down low and has a very low redline, while the IDI makes it's power at a higher rpm and has about another 1000rpm before the redline. This also means that when you're hauling a bit too much weight up a hill and need to downshift, you have some headroom with the IDI while you would be at the redline with the Cummins.
The IDI is also lighter.
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Old 09-19-2013, 06:13 PM
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Yeah, with a 1:4.10 axle you're not going to get the best fuel economy, but with a lift and larger tires, you're probably right where you want to be anyway. You might even consider a lower gear ratio for more pulling power -- 4.10 is ideal for stock size tires pulling a trailer, but might be kind of low for serious towing with a lifted rig.

For perspective, my truck's a 7.3 non-turbo with an E4OD, 3.55 rear end and stock tires. Unloaded and doing 55 mph on the freeway with cruise on, I can reliably get almost 22 MPG. With the cab full of gear and my Alaskan 10-foot camper in the bed and driving in the city (lots of up and down and stop and go and half-to-3/4 throttle acceleration) the fuel economy goes down to nearly 10 MPG. I don't have good data yet on freeway driving economy.
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