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4.7? 5.2 (318)? 5.7?

684 Views 12 Replies 4 Participants Last post by  GeorgiaMoparMan
I am planning a build for my 56 Dodge C3 pickup, going onto the chassis from my 2001 Dakota. My SLT Dak has the 4.7V8 with a 5-spd manual, 3:55 RWD, and if I drive it at 60 mph the revs are just under 2,000 rpm. When drove steady at that speed I have gotten around 20mpg. I found that by noting the gas used per 100 miles, not as indicated by the way-off overhead console. My plan for the 56 is to use a remanufactured 4.7HO with a 545RFE tranny, which the 6th gear ratio will get me 70 mph at near 2,000 rpm, the sweet spot for decent highway fuel mileage.

I've read that the 4.7 is not known for fuel mileage. So, is the 5.2/318 or 5.7 Hemi really any better or worse? If I can get around 300 hp from the 4.7HO with the 545RFE behind it, it seems the best choice to get good power and reasonable economy for daily driving. I doubt the 5.2 or 5.7 with the 545RFE trans would produce any better economy.
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I feel the 4.7 is the best compromise for usable power and reasonable fuel mileage. It seems the sweet spot is 1900 to 2100 rpm to get into the 20mpg range. A few times I've driven over 100 miles on back roads at a steady 60 mph, 5th gear, at just under 2000 rpm and the overhead console indicating 24mpg, which I know is really 20mpg.

Is your 24mpg indicated on the overhead console? Or do like i do: fill the tank up, then drive 100 miles and fill the tank up again, divide the 100 miles by the number of gallons to fill. For 24mpg that would be just over four gallons of gas. I know the overhead console on my Dak is way optimistic, by about four miles per gallon. On my Dak, the overhead console showing 21 mpg means its only about 17 mpg.

If anyone knows a way to calibrate the overhead console, let me know. I used to have a 2006 Ford Explorer RWD for a work vehicle. It had the 300hp 5L V8 with a 6-spd automatic, and I think it had 3:55 rear axle ratio, possibly lower. At 70mph on stready throttle, flat roads, it would get 24mpg with the engine humming along at 1600 rpm.
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Heh, my last time driving through Montana was back in 92, dad's V6 Camry back home with my brother. I got on the highway and set the cruise at 85 and got blasted by everyone else doing well over 90. Screw it, set the cruise at 105 and just over four hours later we covered 400 miles. I wanted to get stopped back then just so I'd get a warning saying I was doing 105. Five over the posted limit, actually seven over, is what I drive all the time here in WI and the cops don't even look my way. I know the speedo in my Dak is very close to correct as I also confirmed it with a Garmin GPS.

30 years ago it was standard that any speedo was showing at least 10% higher than actual speed. These days most speedos are electronic and much more accurate. That officer must have been having a bad day.

For my 56 pickup I'll get a remanufactured 4.7HO long block. The remanufactured 4.7s have the coolant, sludge, oiling issues resolved, steel valve seats pressed in, improved cam chains tensioners, and improved lifters to address the lifter fallout issues. With that all addressed the 4.7HO is a great engine for an easy 200,000 miles. I have 203,000 miles on my 2001 Dak with the original engine. No oil leaks, burns a bit of coolant, always starts and runs steady.
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