safe crusing rpms - Diesel Forum - TheDieselStop.com
6.9L IDI Diesels (Not Power Strokes) Technical discussion of topics related to vehicles powered by the 6.9 Liter In-Direct Injection Navistar engines.

 
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post #1 of 7 (permalink) Old 11-28-2010, 10:13 PM Thread Starter
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safe crusing rpms

i have a 86 f250 6.9 4 spd manual c9 3.55 rear end. i want to kno what a safe and efficient rpm range would be for cruising. i will be taking a 1200 mile trip non stop and i want her to make the trip safely without blowing anything but make some good time...any advice?
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post #2 of 7 (permalink) Old 11-28-2010, 10:29 PM
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I have a 86 f250 with the 6.9 and t19 4 speed with 4.10 gears. I usally run 2500 on the highway but this weekend i ran around 3grand and sometimes I pushed her to 3400 down a slight hill. I ran it at 3grand for about 10 miles through Ft Wayne. No problems out of the old girl.

IRAQ WAR VET. 3 YRS SERVICE AND 18 MONTHS IN THE SAND. R.I.P ALL MY ARMY BROTHERS WHO DIDNT MAKE IT BACK. ROCK OF THE MARNE 3ID 3BD HHC 1 15th MECH INF. Old blue. 1986 F250 Xlt larait. Supercab long bed. 4x4 6.9 N/A. 4 speed with granny. No soup bowl. Sunpro 3 gauge pod for water, voltage and oil pressure. Push button glow plug control with Kenworth dash light as glow plug light. AC 60g glow plug conversion. No more burnt up gps. Dana 50 TTB and sterling 10.25 with4.10 gears and limited slip. Custom 5 inch stack from a JohnDeere Tractor and custom push bar/ grill guard. 33 12.50 16.5 Wild Country radail rvts in the rear and brand new BFG All terrains TA KO`s in the front. Shes a beast son. Wish list. New windsheild. New tach and speedo. New paint that matches. Proud BOOB man.
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post #3 of 7 (permalink) Old 11-28-2010, 10:34 PM Thread Starter
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what"s your speed at 25grand? my speedometer doesnt work so im trying to figure out speeds.
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post #4 of 7 (permalink) Old 11-28-2010, 10:55 PM
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Quote:
Originally Posted by mario23 View Post
what"s your speed at 25grand? my speedometer doesnt work so im trying to figure out speeds.

I put a chart on my dash to check rpms in each gear awhile back because I don't have a tach. Anyway I have 3.55 gears and a T-19 4spd, at 2300 I'm doing 61mph so about 66-67 at 2500. I'm running 235/85/16 tires. If you look online you can find calculators for this sort of thing.

Try this one: Randy's Ring & Pinion. RPM Calculator.

1986 F350 Crew Cab 4wd DRW conversion. 1993 7.3 IDI, 5spd Sterling 10.25 4.10 rear, kingpin D60 4.10 front, 12" HD clutch. A Banks waste gated turbo. Midwest rebuilt IP turned up. AC Delco 6.5 GM 12V GP's, working great. Running W80-W85 high performance zero-gel black diesel! 4wd and dually conversion is done. Washer mod on the D60 kingpins eliminated the death wobble. Custom 10" C-channel front bumper with military D-rings front and rear. Pintle hitch, receiver hitch, and goose neck hitch. 7800lbs full of fuel....
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post #5 of 7 (permalink) Old 11-29-2010, 04:20 AM
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The "consensus" such as it is, is that you get the best mileage by keeping your foot out of it (slow acceleration) and by keeping RPMs under about 2k.

You can bounce the truck off the governor all day and not hurt the motor if it's in good shape.

1992 F250 XLT 4x4 Super Cab 7.3 IDI E40D 3.55 ATS 088 4"R.C 360k
1982 MBZ 300SD W126 California model w/Sunroof, no EGR
1997 Audi A8 Quattro 225k Warm/Cold packages
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post #6 of 7 (permalink) Old 11-30-2010, 02:36 AM
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I usally get around 65ish at 2500. Latley though Ive been getting about 61mph at 2500rpm. I also have 33 12.50s.

IRAQ WAR VET. 3 YRS SERVICE AND 18 MONTHS IN THE SAND. R.I.P ALL MY ARMY BROTHERS WHO DIDNT MAKE IT BACK. ROCK OF THE MARNE 3ID 3BD HHC 1 15th MECH INF. Old blue. 1986 F250 Xlt larait. Supercab long bed. 4x4 6.9 N/A. 4 speed with granny. No soup bowl. Sunpro 3 gauge pod for water, voltage and oil pressure. Push button glow plug control with Kenworth dash light as glow plug light. AC 60g glow plug conversion. No more burnt up gps. Dana 50 TTB and sterling 10.25 with4.10 gears and limited slip. Custom 5 inch stack from a JohnDeere Tractor and custom push bar/ grill guard. 33 12.50 16.5 Wild Country radail rvts in the rear and brand new BFG All terrains TA KO`s in the front. Shes a beast son. Wish list. New windsheild. New tach and speedo. New paint that matches. Proud BOOB man.
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post #7 of 7 (permalink) Old 11-30-2010, 07:50 AM
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With the 3.55s and 35s I am under 2k at 65, but barely. I try not to go over 65 generally and 70 ever, and while towing I stayed at 55 with the occasional 60. (it's a light trailer but it's tall... 6'7" interior and axles flipped) I have a bit of speedo variance but it's not bad, probably 4 mph at 65 (calculated by GPS.) Not enough to cause the truck to run funny. When my big goofy mud treads wear off it'll come back towards calibration anyway

1992 F250 XLT 4x4 Super Cab 7.3 IDI E40D 3.55 ATS 088 4"R.C 360k
1982 MBZ 300SD W126 California model w/Sunroof, no EGR
1997 Audi A8 Quattro 225k Warm/Cold packages
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