With that rear-end those rpm's seem high at 65 MPH. Is your speedometer accurate? My 93 dually with a 4.10 rear-end turns about 2350 rpm at 65 mph. I find my best mileage is doing between 55 to 62 with rpm's from about 2000 to 2200. Empty I get about 17 mpg on the interstate, 15 in mixed driving. Pulling my 7500 lb trailer through rolling hills on the interstate going 60 to 65 I got about 10.5 mpg.
1993 F350 Dually XLT, Extended cab, red w/running boards, visor, custom hand painted pinstripping, rhino liner, 4.10 rearend, E4OD Tranny, 7.3 liter NA w/K&N, soon to add Banks Turbo, 2nd owner 88K miles, Banks Turbo now added, w/Trans Command...Totally Cool!!!
If your turning that high of rpms you must have a C6 transmission. You won't get very good milage with the setup unless you go to a gear vendor or US gear overdrive setup. Mine with a C6 runs at least 2 mpg's or more less than my five speed. Same gear ratio's 3.73 in both.
1988 F250 7.3 diesel 4x4 auto C6 now ZF5
1990 F250 7.3 diesel 4x4 auto E4OD. now ZF5
1992 F250 7.3 diesel 4x4 ZF5
1992 7.3 non turbo. I have taken a 1988 E-350 chassis mounted a 1992 7.3 with an allision 4 speed auto. The cab and sheet metal is a 1956 cabover, the rear end gear is a 3.73 and with out an overdrive I feel my rpms are to high 2550 at 65mph. This is my first diesel engine, wanting to get the rpms dowm to get decent mileage any ideas?
Gear Vendor, Get from Camping World, Cheapest price $2450 + ship. No other way to get fuel economy except driving at 2000 rpm or less, 50 mph. [img]/ubbthreads/images/graemlins/bleh.gif[/img]
Sold! 1986 F-350 XLT, Crewcab 4x4, converted to Dually, AT, Axle 39, 3.55 ratio, 7500lbs empty, aluminium wheels, K&N filter, rebuilt IP, and injectors, tach, trans oil temp gauge, chrome headers 2-1/2" exhaust to glasspacks to Magnaflow to 4" single, new headgaskets, new radiator, Mitsubishi starter, Big alternator, Big batteries, Racor R26S water seperator, Folding gooseneck hitch. 97 Taurus electric arm chair front seats, PW, PDL, using Power Service additive.
New Truck. 2005 Dodge 2wd dually, 6 speed, gets 15 mpg with 4000 lb camper and pulls any hill easily!
Personally, If I could do it again, I would probably look towards the US Gear OD. It seems to be a little heavier and built a little stronger. There is nothing wrong with the GV, just not as beefy as the US Gear. Either one of those will drop your RPMS 500 at any given speed. Good luck. Scotty
I think you can order just about any gear size you want from a good supplier. I believe max torque is made around 1850 rpm so I would think that this would be the best place to max out fuel MPGs. I think you are right about changing the gears this is the easiest way to lower your RPMs and if you do them your self they should be as cheap or cheaper than changing tire size. BTW 2150 is about where my truck is going 70.
Also if you get a chance post some pics of your rig. I think people here would enjoy seeing it.
'91 F250 4x4 XLT Lariat ext. cab ZF 5 speed, Ramsey 12k winch, Add-a-leaf in front, Ford Factory/ATS Upgraded Wastgated Turbo (installed @ 252k), 3.5in Hypermax Exhaust (Turbo whistle is awesome) 305,000 miles and counting, next stop? Propane. [color:"blue"]My Pictures[/color]
Dang, my 84 F-250, 4X4, 4 speed, 4.10 ratio differential turns around 2900rpm at 65mph. I try to keep it around 2500rpm which is 55mph. Luckily, I don't need to drive on high speed roads, since we don't have any in Alaska.]
I wish I had an overdrive for the old beast, but maybe someday.
1990 F-350 7.3 Crew Cab Banks Sidewinder Turbo E4OD 4X4,
1984 F-250 6.9 4X4 4Speed Old Style Banks Turbo (gave to daughter, but still in family)
1988 F-250 7.3 4X4, C-6 Auto, Banks Sidewinder Turbo