I drove an 88 with a 351 and C6 for many years, and it is a solid heavy duty transmission. If I was building a toy or heavy duty work truck that would not normally see the interstate my first choice would be a C6.

However, the C6 is a pig. It requires a fair amount of power to operate (read it will rob you of useful power) compared to the M5OD and you will have fewer forward gears, including no overdrive, so highway driving will be hard on the engine and hard on fuel economy. I don't know the gear ratios off the top of my head, but it will have the same ish performance in 3rd as the M5 has in 4th.

In your case performance and fuel economy will probably suffer all around as the more of the available power will be used just to turn the equipment.

While far from ideal I would think your current setup would work reasonably well so long as you don't tow anything or engage in any serious off road driving.

If you are worried about the transmission failing, well the C6 is tougher, but the M5OD is fine if you don't abuse it. Probably similar rebuild costs.

If you want your performance back the most cost effective solution is to go back to stock tires.

If you want your cake and to eat it too, than swap in a 351 with a C6 or ZF5 combination and enjoy. Or you know, buy a truck that already has what you want in it.

 

What is your end game with this truck? You have two different threads on this subject. Are you just tired if shifting gears? Does the truck not have enough oomph for you? Short of going with better gears and keeping the overdrive, of going with a bigger displacement engine you aren't going to gain much for performance

 

 

Fix your engine.
Mine (identical engine/trans/gear) has no problem holding 70 in 5th up a light grade,
and 4th will climb any normal hill at 60.

Now, towing, yes, that's where hills cause some slowdown...

t

 

ok gotcha. Well a c6 is actually going to suck more power than the 5 speed so putting that in will make your situation worse.
First check for codes even if the engine light is not lit.
second, do a tune up and maybe bump the timing a few degrees.
third, 33s are heavy and have crap rolling resistance. You have basically 2 options that will help. Go to 31s at most, or regear to a minimum of 4.10s.

 

 

A healthy 4.9 should be able to push that setup around. Will it be amazing, no. But it should be able to do 60 up a minor incline. Hence we suspect that the engine is in need of attention. The powertrain was designed to deliver enough power to move the truck plus a load, so even as they lose power over the years they still have enough to move the truck reasonably well. Even a well cared care vehicle loses power and efficiency over the years. But your right the engine isn't the problem.

What you have done is added a significant amount of load from the mass of the taller tires, added wind resistance by raising the vehicles height which gets worse the faster you try to move the truck, and created a gearing disadvantage with the taller tires. Your engine simply cannot provide the power needed to over come the extra load. Whether from age, tune, or control problem we don't know.

Re gearing will help over come the extra load. More power from the engine bay whether by refreshing/rebuilding the 4.9 or swapping in a larger engine would help. More power and more gearing advantage would be ideal. Either option is relatively economical if you can do the work yourself.

The better solution in my mind is to remove the tires and lift, stow in your garage, put this truck back on stock equipment and sell it to someone who worships the 4.9. Use the proceeds to buy a truck with a 351 and put the lift and tires on the new to you truck.

 

I would personally give up the taller tires or buy a different vehicle. I'm not trying to be a wiseass - with wanting to be able to tow a decent load and accelerate faster, I think that's the best solution. Practically speaking, I think by the time you reach your goals with that particular truck, you'd have already spent enough time or money that you could have bought a different vehicle which would be factory-equipped to do what you want. Given the slim pickings locally, I had to travel 1200 miles to pick up my current truck, but the extra hassle was worth it (and the road trip back was a good excuse for a family vacation). I'd say it comes down to if the fun is in buying parts and working on the vehicle or using the vehicle itself.

 

Ah, ok- I forgot, I'm on 31's, and it's surprisingly good for as heavy as it is.

How's your mileage?

I was thinking of you today as I was towing things around- mine, on 31's, WITH 1500 lbs of trailer on behind, will hold 65 on the level,
and even up a slight incline. Even over a (admittedly small) pass, I held 4th at 50+ with the trailer. Foot flat on the floor, but 50- 55 all
the way up.

I bet you could find some more power out of yours...
t