Less RPM's at Hwy Speed
 

Is there any way to some how modify the 4th gear or the overdrive gear in the transmission so it would lower the RPM's at hwy speeds?

Why?

Ask any pilot of piston-driven aircraft - the best cruise RPM for range is the speed at which the engine produces peak torque - for the AMC 6-242, that's typically in the range of 2400-3200rpm (we're blessed with a broad, flat torque curve.)

When I quit using fifth gear in my 88, I gained about three mpg on the freeway.

The AW4 is nicely complemented by the 3.55 gearing in the axles, when using tyres sized between stock and 31" inclusive. You'll want to go 3.73 for tyres 32-35", and 4.11 or deeper if you go up further, but there's no real reason to want to lower your cruising crankshaft speed - I'm assuming, since you're talking about fourth/overdrive, that you have an automatic. If you have the automatic with the 6-242, that's the AW4 - and if you're cruising around the range given above for crankshaft speed on the freeway, you're doing fine. The engine will last near forever at that speed with regular oil changes, and decreasing the cruise RPM will actually increase fuel consumption, not decrease it.

Leave it alone. It's just fine.

I've got a 2000 like you it gets going 70 mph at 2100 rpms.

Actually its more like 2300rpm at 70 mph and my engine goes into overdrive at 45 mph.

I have no idea about transmissions that's why I'm asking. I do have an auto trans. I was just wondering why no one talks about modifying their trans. If it's because it a great trans and it can't be changed to make it better, that's awesome. I was just curious.

Regear? Bigger tires?... that'll do it

what are the RPM's of a manual trans at 70mph?

No, there is no practical way to change the gears in the tranny. Anyone looking to change RPM's will regear the axles.
I have a stick shift and am at 2000 RPM at 70. Then again, stock gears were 3.07 in the axles compared to 3.55 in a lot of automatics. Again, it is the axle diff gears that usually change to make the differences. If you want lower RPM on the freeway, get higher gears in the axles. But you don't want to do that. If you are below 2000 RPM at say 70MPH, you will hardly be able to use overdrive because it won't have the power to maintain the speed.

As far as transmissions go, the actual ratio of the overdrive and 5th gears is about the same. I think after around '91, the automatic overdrive is 0.75:1, and all the manuals have a 0.79:1 5th gear.

Great explanation, Thank you

The AW4 is about as solid a slushbox as I've seen - and I've seen many! I used to build the THM350/400, A727, and C6 for drag racers (PowerGlides when they could get them,) and I'd consider the AW4 - mechanically, in terms of durability - to be on par with or just below the THM350. However, it does have the advantage of having an overdrive range.

Changing gearing ratios in a transmission is difficult in the extreme without access to a full machine shop - because you have to machine out a whole new planetary gearset (auto) or mainshaft/countershaft gearset for the range (manual.) Easier to switch transmissions entirely - and that doesn't always net you an improvement anyhow.

Besides, as I said earlier, there's no reason to go with "lower RPM at cruise" - it's a bad idea that's been perpetuated by marketing, which has overridden engineering. As I said before, I gained 3mpg when I stopped using fifth gear - the 3.07 gearing used with the 6-242 and manual gearbox is just a stonking bad idea (3.55 would have been an improvement. 3.73 would probably have been better still.)

The AW4 (and automatics in general) have a benefit in the torque multiplication of the torque converter that the manuals do not - so you can get away with slightly lower cruise crankshaft RPM. Slightly. You don't want to drop it to where you're cruising off-idle, or even under 2000rpm, because you're not going to get a benefit in fuel economy or engine longevity from so doing. So, there's no reason to bother.

This doesn't have anything to do with the strength of the transmission, either. It's simple physics - an engine is most efficient when producing peak torque (plot the volumetric efficiency of the engine against the torque output curve - it's nearly identical!) and "low crankshaft speeds at cruise" are just as bad an idea as buying a truck for horsepower - torque is what you need in any real truck, and torque is what should be advertised as a useful number. Horsepower peaks somewhere north of 5500rpm (it's a mathematical certainty,) while torque will peak down around 2000-4000rpm or so (depending on engine configuration, valve timing events, gas flow dynamics, &c.)

If you want a "tougher" transmission, you're probably going to want to go with something like a 47RE (Chrysler) or an xL80-series (GM) - but that will also require adapting the transmission to the engine, since the 6-242 uses the AMC I6/V8 bellhousing pattern (shared by AMC "Modern Era" six and Gen-II/III V8,) while the 47Rx will use the Chrysler LA-Block and the xL80 uses the GM V8. Not a direct swap (of course, if you're willing to undertake the project, I'd probably see about adapting the GM 6L80 in and making the electronics compatible - four speeds plus double overdrive. Stroke the engine, and you can get some serious power to the ground and have some high cruising speeds.)

But, I'd probably prefer to go with a Muncie SM465 - that ~7:1 first gear range calls me so...

they are about the same actually