6.2 gearing and mileage

[77crewcab]

I am planning to put a 6.2/465 in my K5. I have the option of either 3.42 or 3.73 gears both with 31 inch tires. This will have the rpm at 70 at 2595 or 2830. Obviously the 3.42 would be better for the lower cruising rpm, but is there any problem with the rpm being up this high especially with the 3.73s? These were stock gear options correct. Also what sort of mileage would I be looking at with these gear ratios.

 

[77crewcab]

Anyone?

 

[tRustyK5]

Go with some 3.08's and you will probably break 20 mpg...keep it to 65 mph with 3.08's and it should easily do 20+ mpg. With tires that small and an SM 465 it should still do fine around town too.

2500rpm for a diesel ain't no big deal if you can stand to listen to it...but mileage would drop down to mid-high teens. Turning those rpm's i never got better than 16.5 mpg with my K5.

1800 rpm at freeway cruise is the sweet spot for mileage. Every 100 rpm higher than that at the same speed (65-70) loses you almost 1 mpg. This is why all the 6.2's in 1/2 tons got some sort of OD tranny behind them when new. Only the 3/4 ton and one ton's got the TH400 or SM 465 and the mileage on those typically was only slightly better than a small block.

Rene

 

[77crewcab]

Well i definately want to keep it a stick, so I think I will forget about the 700, use the 465 for now and start getting parts for a 4500 swap down the road. With the 3.42 it looks like I could run 65 and be about 60 rpm above 3.08s/70. I am also planning to get some new tires this fall/winter and will be going with 33s, that would still keep me in the 23-2440rpm range with 3.42. Then when swap the NV4500 in I would be right about 1800 at 70.

I think I could live with mid teens until then. I am only getting about 10 now in my gasser burb that is my DD. But once the nv4500 is in I should be getting about 25 or so correct?

 

[tRustyK5]

Yeah for reference I also have an '83 pick-up with a 6.2. It has the 700R-4, 3.42's and 235-75's (28") tires on it. At 65 it turns about 1850 rpm...and consistantly gets 25-26 mpg on the highway. I'd guess the manual tranny might even do slightly better having less losses through the tranny.

With the dual 20 gallon tanks I have a theoretical range of just over 1000 miles. This would mean running each tank down to the last half gallon though, which I'd never do .

Rene

 

[77crewcab]

Cool that gives a good reference to use. With mileage like that, I have something to look forward to. And I can tell the wife, "see honey this gets better mileage than that new truck you want me to buy"

 

[77crewcab]

Looks like nv4500/3.73/33s at 65 is the hot ticket with 1802rpm.

 

[tRustyK5]

Yup, that'd definitely get you decent mileage.

What I'm building now will have the NV4500, 4.56's and 37's. 1950 rpm at 65...should still be good for 23-24mpg on the highway.

Rene

 

[77crewcab]

Did you have any problems putting the 4500 in behind the 6.2? I already have a hydro clutch/boost pedal assembly and will use a hydro bell for the 465. I am hoping that already having the pedals setup will save some headaches when it comes time to swap the 4500 in.

 

[tRustyK5]

i didn't have any trouble, but I did need to buy an aftermarket bell. The factory bell's for the NV4500 all have the external slave on the passenger side where it is prone to damage being hit by the front driveline. The newer factory bells have an internal slave which would work but if the slave goes out it's a lot of extra work to swap compared to an external slave.

The shifter hole needs to be moved on the tranny tunnel. The NV4500 shifter is 5" farther rearward than the SM 465 shifter.

Rene