Another mileage thread...

[Silver91Hatch]

Well this weekend I drove the 6.9 a LOT. This is the first time I have ran an entire tank of fuel out of it in a day. Its a rollback at you can see in my sig. So highway driving around 60 mph with the no O/D T-19 with 235/85R16's on the rear I calculate I got an average of 18.25 mpg. That includes loading and unloading 3 cars with high idle during loading and unloading. I think that is pretty respectable. Its turning 2200-2500 RPM during highway driving.

On a side note, does anyone know the gear ratio of the F-350's in 86? I haven't checked the rear diff tag yet.

 

[Clayton]

I found my rear axle ratio code on the sticker in the door.

 

[Silver91Hatch]

Someone painted over my VIN sticker partially, and don't have a decoder, I guess I'll crawl under it and check it out.

 

[Clayton]

At 2500 rpm's at 60 mph you would have 4.10
at 2175 rpm's at 60 mph 3.55

 

[Silver91Hatch]

Must be a 4.10, the tire size throws it off a good bit. My speedo is off but according to traffic and other cars on the shows I'm about 8.5 mph off, so factory tire size of a 215/65R16 changed to a 235/85R16 is about 9 mph off, so trip odometer increased by 58.75/50 comes up to 18.3 mpg, with the stock tire size it would have gotten 15.5 mpg. I think when I get new tires I am going back with the 235/85R16's and possibly changing to a 3.55. I really would like an LSD in the rear so that would be the best time to change it.

 

[Silver91Hatch]

Well the door tag says axle code is (45). So by looking on the FAQ's thats a Ford 10.25" with a 4.10 gear, correct? Does anyone know what else other than early F-350's have a 10.25" that will interchange with my truck's gears? I am really wanting 3.55 or 3.73's with a Limited slip diff. Thanks

 

[HammerDown]

At 2500 rpm's at 60 mph you would have 4.10
at 2175 rpm's at 60 mph 3.55

hmmm, that's odd because I have 3.55 rear, stock diameter tires and at 60 mph I'm at 2500 rpm
I'll double check mph/rpm with my GPS tomorrow.

 

[Clayton]

hmmm, that's odd because I have 3.55 rear, stock diameter tires and at 60 mph I'm at 2500 rpm
I'll double check mph/rpm with my GPS tomorrow.

What size are your tires?

 

[surfib]

I averaged 13 miles per gallon with about 2000lbs in the bed and a pop up slide in camper. Low profile. Granted im running 295/75R16s with 3.73 gearing, 4x4, lifted and an e40d.

 

[Silver91Hatch]

Is a 45 Axle code in the door in fact a 4.10 in a 10.25?? Its an 86 F-350 Cab-chassis.

 

[kas83]

hmmm, that's odd because I have 3.55 rear, stock diameter tires and at 60 mph I'm at 2500 rpm
I'll double check mph/rpm with my GPS tomorrow.

C6? X2 on the tire size.

 

[HammerDown]

C6? X2 on the tire size.

235/85r16 with a C6 > again, I'll be checking mph/rpm with a gps today.

For the longest time (17 years) I ran 33" tall narrow 255/85r16 just to drop hwy rpm.

Went back to the oe size tire (31.7) 3 years ago.

 

[kas83]

Your rpm will be higher because of the fact that your torque converter doesn't lockup. That big of a jump is a little out of the ordinary, but not overly so. Pick up a GV, or get an E4OD with overdrive and lockup in there and you'll be gravy.

 

[HammerDown]

OK, I'm back...355 REAR AXLE CODE 39 W/235-85R16 TIRES & C6 TRANY
GPS... +- 50 rpm on my tach
50 mph...2,150 rpm
55 mph...2,250 rpm
60 mph...2,500 rpm
65 mph...2,700 rpm

 

[kas83]

Sounds about right for a C6 non-lockup. With a stick, or lockup converter, you'd be at the RPM stated earlier in the thread at the corresponding speeds. At a minimum, a good, tight, low stall diesel converter would help out. But you'll never get the full drop without the converter locking up.