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7.3L to 6.7L comparison

HMS_Beagle
Explorer
Explorer
I replaced my '99 7.3L with a '15 6.7L recently. The two trucks are as identical as you can make from the order book: Supercab F350 dually 2WD Lariat 3.73 gears. For several years I have made the drive from home to Lake Tahoe, making this trip perhaps 40 times with the 7.3L, so I am quite familiar with the grades, fuel mileage expected, etc. The road climbs from sea level to 7200 ft Donner Summit and then drops into the 5900 ft Truckee valley, 165 miles each way. I set the cruise control on 63 when traffic allows. Hauling a 4300 lb Bigfoot 10.4E.

With the 7.3L, there were 13 grades which required a downshift to 4th. Practically this also meant dropping the speed to 55 due to the engine roar from the big gap between 4th and 5th. After crossing the flat Central Valley I would have about 15 mpg on the meter, maybe 16 if there was a tailwind. By the summit, this would have dropped to 12, sometimes 11.9, then picks up a little going down the other side. On the downgrade into Truckee, I had to downshift out of overdrive and ride the brakes a bit to keep the speed under control.

Today was the first opportunity with the 6.7L (also, the first trip with the air suspension - but that is another topic!). The gearing in top gear is pretty much identical to the 7.3L, but with 6 speeds spaced better in the transmission. It managed the whole climb in 6th on cruise control without apparent effort, no downshifting, no noise, no fuss. From the gages, it was working a bit, EGT peaked at 910 deg, oil temp at 242 (oil is designed to run hot in these engines), boost about 30 PSI on the steepest grades. Quite relaxing by comparison to the 7.3L. There was plenty of power to spare to pull back up to speed if I got caught behind a truck.

I had about 16 mpg on the meter when I hit the grade, it was down to 13.2 at the summit. Both trucks are about 1 mpg optimistic when checked by hand. On the downgrade into Truckee, the exhaust brake kept the speed at the cruise control set point though it did have to downshift to 5th to do it. No brakes required.

These new Superdutys are sure complicated and expensive - but they sure are nice!
Bigfoot 10.4E, 2015 F350 6.7L DRW 2WD, Autoflex Ultra Air Ride rear suspension, Hellwig Bigwig sway bars front and rear
46 REPLIES 46

SoCalDesertRid1
Explorer
Explorer
I would certainly hope that the 6.7 truck, with about 350 lb-ft more torque and about 200 more hp than the 7.3, would climb the hills effortlessly in high gear with a truck camper! :B

I'm guessing you must have had the ZF 6 speed manual in your '99. The 4R100 auto only has 4 gears.

ZF S5-47 diesel close ratio 5 speed manual gear ratios-
Rev- 4.66
1st- 5.08
2nd- 2.60
3rd- 1.53
4th- 1.00
5th- 0.77

ZF S6-650 6 speed manual gear ratios-
Rev- 5.23
1st- 5.79
2nd- 3.30
3rd- 2.10
4th- 1.31
5th- 1.00
6th- 0.72

E4OD/4R100 4 speed auto gear ratios-
Rev- 2.88
1st- 2.71
2nd- 1.54
3rd- 1.00
4th- 0.71

5R110W Torqshift 5 speed auto gear ratios-
Rev- 2.88
1st- 3.11
2nd- 2.20
3rd- 1.54
4th- 1.09/1.00
5th- 0.71

6R140 Torqshift 6 speed auto gear ratios-
Rev- 3.28
1st- 3.97
2nd- 2.32
3rd- 1.52
4th- 1.15
5th- 0.86
6th- 0.67

1999 7.3 Powerstroke-
500 lb-ft @ 1600 rpm
235 hp @ 2700 rpm

2015 6.7 Powerstroke-
860 lb-ft @ 1600 rpm
440 hp @ 2800 rpm

Amazing how much power they're getting out of the new engines.
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92F350 CrewCab 4x4 351/C6 285 BFG AT 4.56 & LockRite rear

Gary3
Explorer
Explorer
Much cleaner and smoother engine much more power also.
Gary  Lance  1191 solar Gen.