I recently purchased a 1990 2500 which came equipped with a 3 spd auto. It's a military issue Cheyenne, but not a CUCV or M1008.

Primarily I'll be using this as a 'mule' for 8 months of the year- hauling firewood, stone, mulch etc, so the 3spd does just fine for this. But this is also going to be my 'get to hunting camp' truck in the fall and winter as well. So there will be some infrequent but long highway drives. This is where the 3 spd ain't so good.

I'm really leary of taking this truck at highway speeds. There is no tach in the truck, but at 55mph she's really winding. A couple friends (including an old mechanic) said don't worry about it and DON'T swap the 3spd for either a 3 or 4spd O/D tranny. They said I won't hurt a thing running that truck at 65mph on the highway.

They may be right- but dang I've still got doubts. I don't want to be penny wise and pound foolish by running this engine at high rpm's and blow her when a tranny swap might be the thing to do.

Any advice from you guys? If anybody says 'swap it', what tranny would you recommend? (and I suspect 4:56 gears are in the truck, too).

Sorry for the long post- thanks in advance. (BTW- truck has 38,000 original miles- if that makes any difference)

You'll be fine. Im assuming you have a TH400 transmission, that is a great transmission. As far as pushing the 6.2 to its limit, I don't see any problems, although its ideal top speeds are probably around 55-65 mph. The truck is a tank, built for heavy duty use and definately low end power. However you should be fine to cruise on the highway from time to time, taking into exception of top speed limit (55-65) probably not ideal to go much faster than that.

I would check the gear ratio and tire size. Compute the RPM (or hook up a tach) and determine how high the engine is turning at hiway speeds. These engines don't like high RPM's and can destroy themselves from sustained high speeds. If your truck turning high RPM's, you would be money ahead to change gearing to avoid engine damage.

I've changed a few engines over the years from customers using second gear for engine braking while towing heavy trailers down steep mountain grades. One said he always pulled it into second gear when descending grades, even above 50 mph. He bought two long blocks before realizing the engine "didn't like high RPM's".

I have a 84, M1010 w/ 6.2l /th400/np208/4.56's. It came with the metric eqiv. of 31's and 55 -60 mph was about all it could take. 35's seem to really help, but still could reduce the gearing some to get more MPG's. I like the gearing, and the truck was not build to run up and down the interstate, so I will leave the 35's. At least now 55 mph does not stress the motor.

Work a trade + cash with a wrecking yard for 3.73 axles.

OD trans is going to mean swapping/adding/modding other parts also.

C.R., Your setup sounds good. It seems to work for your application. I, too, have the same basic drive train in my P.U., except mine has 4:10's and 245 X 75 tires. It, too, doesn't like freeways. I drive it on the local country (read: REDNECK!) roads around home. I live in the foothills of the Cascade range of Oregon. It likes this type of environment and does what I want it to do as long as I mind the RPM's.

Work a trade + cash with a wrecking yard for 3.73 axles.

OD trans is going to mean swapping/adding/modding other parts also.
Whoa. Heck I thought a 3spd OD or 4spd tranny would be a fairly straight forward swap. Maybe have to change out the gear selector on the column, possibly flywheel and clutch and be good to go. What all would be involved?

What about a manual tranny swap? If I'm gonna have to swap/add/or otherwise mod stuff- I'd much rather have a standard trans when I'm done than an auto.

P.S.- I drove a loaded bed of firewood to my friends house about 60miles from me on Memorial Day. I couldn't avoid the interstate for about 7 miles.. Talk about some peeved travelers when I was doing 55 in a 70mph congested highway!

The 6.2 is a governed engine it will only go as fast as the gov is set. now engine breaking is a whole nother story 2nd gear at 50 what was he thinking.Gearing down it is possible to exceed the rated RPM as the governer is not controling the RPMs the speed and weight of the truck is. Just use some sence when using the engine to slow down.I think max RPM is about 3200 for this engine

When I bought my M1010, I drove it from VA. beach to Raleigh N.C., about 250 miles, at 50 MPH. :eek: I understand about the pissed off drivers....... I would really consider keeping the TH400. It is one tough tranny, and is HIGHLY reguarded for its strength. Get some larger tires, and just drive a bit slower. Having dealt with several 700r4's and a few other OD trannies, I went looking for a non- OD tranny. OD is great when it works right, but the vast majority of OD trannies dont hold up to off road use and " mule " runs. I have had a couple built for those purposes, and they always seem to go south after awhile. Stay with the 400. Other than having to drive slightly slower there is no advantage to swapping IMO. These trucks are about as tough as Chevy has ever built and thats exactly why I bought one. I wanted a truck, not a pretty grocery getter. :lol:

You could put in an NV4500 quite easily. Just need all the parts from a 91-94 truck that had one. Need bellhousing, flywheel, clutch, master & slave, fork, pedal(s), and probably driveshafts.

You can re-use your t-case, and likely re-use the same ECM and harness. (TH400 has only electric kickdown... no lockup or other electronics)
You can probably deal on good used axles (economy-minded ratio) for less than having gears installed, or less than a manual trans swap (when you figure on all the peripheral parts required).

Heck... the last time I did my rear brakes I swapped the rear axle instead and got better (used) brakes and better ratio in one shot (cheaper than new brakes too!) :grd:

The 700R4 is a lighter trans than the 400. That's why GM built the 4L80E, even after they told us, "We'll never build a heavy-duty overdrive version of the 400". I quote this from the Hydramatic Service Team Meeting, held in March of 1987.

The size of the internal components between the 400 and the 700 tell the difference in torque-carrying capability. The 400 is much heavier all through the list of internals. The 400 will handle twice the torque (or more) than the 700.

Aftermarket parts for the 700 will improve this greatly but, until they have been subjected to the "Hot Rod Diesel Guys" best efforts, I withhold my judgement.