I aquired a 83 4x4 k2500 6.2 with the sm465 trans. I have a couple of questons and i am not to familiar with these so here goes; 1. With the 4 speed manual, can i run a taller tire to get more speed without loosing all fuel mileage. my idea is a 4" lift with 36" tires. My goal is to stay around 20 mpg but still drive to work everyday, 110 miles round trip at about 65, is that possible? 2. Where can i hook up a tach to watch the rpms?
I turned the pump up a little today and really didnt notice a difference, but at 55 she sounds like its screaming. any imput would be great..

 

Sorry I cannot be of much help as I am sort of new to 6.2s myself. I rebuilt 6.5s for the USMC and those engines used the hole were you would put a distributor in on a gas engine for the tachs. I know on my 6.2 that same hole holds a vaccum pump. It is on the back passenger side on the engine valley.

I've been trying to figure out where to put a tach myself. No tach and a 4in fuel gauge doesn't make much sense to me. Crazy 80's engineers

 

You should be able to run taller tires and not hurt your fuel economy. My 83 has 35's and when I put them on it I noticed no difference in acceleration and get in the mid 20's on fuel mileage. I don't know how you would easily put a tach on it.

 

I have a 6.2L 83 1 ton dually w/4:10 gears and put on the 8:10 lug conversion w/37" big rig tires. No change at all in mileage on my 84 mile RT commute. My controlling factor is my foot. Dropping from 65 to 60 mph I gain 1 mile per gallon. I did gain a 1/2 mile per gallon by replacing my stock thermostat (180 degrees) with an aftermarket 195 degree. My diesel instructor suggested it since you get a more efficient burn and capture more BTUs in the chamber during expansion. Apparently it's all about air and BTU management.

 

I believe the tach mounts on the alternator. It's a special tach meaning you can't use one for a gas engine.

 

2. im pretty sure there is a spot for it in the ecm wiring harness, if not the 6.5 does it through the alternator, i would poke arounda at some threads.

and it prolly sounds like its screaming because it is technically a 3 spd. 55 is alot

my dump the same tranny and i dont usually go over 50 because it screams

 

Taller tires will drop your rpm's some, but it's also going to drop your fuel mileage.

Why?

Heavier tires, increased rolling resistance and more tread on the road. All this is also going to conspire to make your truck a "dog", resulting in your foot deeper into the throttle most of the time, which brings mpg down.....

Your other issue is going to be your 4" lift.

More aerodynamic drag, less mpg, less speed.

You're going the wrong way all around for mpg and speed.

Lifted 4x4's are not built for speed or MPG. The dynamics are just all wrong no matter how you look at it.

What you want to do is lower it (or stock height), keep stock tire sizes and install an over drive of some sort (ie: gear vendors, etc) or drop your rear end ratio.

Keep in mind, if any of these modifications take you too far outside your engine's torque peak, expect mpg's to drop again.

Everyone thinks the key to more speed and mpg is to drop the engine rpm. Not true. All you do is end up running the engine outside it's peak efficiency that's it's designed for. That's why guys who swap on bigger tires find their truck is a dog and gets poor fuel mileage and eventually swap in lower gears to get the RPM's back up to where they're supposed to be.

What you need to do is keep the engine spinning around it's 2000-2200 torque peak (we're talking high speeds here), raise the tire rpm and not increase the rolling resistance or aerodynamic drag.

Quickest and easiest way to that goal is swapping axle gears.....but even that is a trade off.

Higher gears mean less brute acceleration and towing capacity, resulting in your foot deeper in the throttle, resulting in............

 

x2

 

In every other case I would agree with great white but the 6.2 tends to be the exception to the rule. This is mostly due to where the 6.2 makes its torque. It been my experience that in this scenario you should gain some MPG.

 

concerning your question about adding a tach to a 6.2l my guess is you would have to find an alternator from a 6.5l with a tach hook up and being that all sorts of crazy crank to alternator pulley arrangements were made you would have that to figure that out or steal the crank pulley off the same truck you took the alternator off of. Anyway I believe Dakota digital makes a electronic device that converts input from the alternator to an output that any over the counter v8 tach can read.