Transmission Swap

[Skrilex]

At the cost of taking away from the bottom end acceleration in direct proportion to what you're adding to top speed. And of course the cost of replacing tires.

Not enough return on investment in my opinion. The 37's are fine. Cruising speed 55ish is fine.

I'm not anti-mod or anti- tires and a lift. Everything is a set of compromises. The stock configuration on the M998 is an adequate set of compromises for me.

There's all kinds of stuff we could do to make them a better interstate road trip vehicle. Thing is, if I wanted an interstate road trip vehicle I wouldn't use this platform as the basis.

Back on the topic of the 4spd vs the 3spd - the 4spd is better all around. But it's also considerably more expensive to get one with it already in place (for good reason), and installing one in place of the 3spd isn't a trivial undertaking.

For the guy who uses it off road, around town, and an occasional trip a couple of hours away the 3spd should be fine.

But it’s an easy mod that’s reversible and looks cool, and drops highway rpm a few hundred. I installed 39’s on dodge wheels and shaved almost 200 lbs of unsprung rotating mass in the process, and you know what that means!

 

[98G]

But it’s an easy mod that’s reversible and looks cool, and drops highway rpm a few hundred. I installed 39’s on dodge wheels and shaved almost 200 lbs of unsprung rotating mass in the process, and you know what that means!

It means you spent maybe $1000.

You gained just under 10% top speed, or working it the other way you dropped your rpms by 10% at any given speed.

Practically speaking, you increased your hwy cruising speed from 55ish to about 60.

Any losses to bottom end acceleration are probably cancelled out by the decrease in rotating mass. I speculate that this also makes breaking a halfshaft less likely.

And cosmetics... here's a pic of my stock M998 next to a civvy H1 on 40's. I like big tires. I went to the trouble and expense to put my 5ton on HEMTT rims and 53" tires.

But for the hmmwv I'd rather take the $$ and use it for a winch...

And I think the OP would be happy with a 3spd. And i agree that if you be just have



to have that extra top speed, a tire swap is more feasible than a transmission swap.

 

[TOBASH]

I will be Devil's advocate here...

Another solution is the Diesel truck version of the 700R4 transmission. No computer required but driveshaft lengths will most likely be affected, (or so I'm told. I have not measured things out yet). No electrics BUT there are transmission cables and such that require customization.

I like the military tires just the way they are for now, but when they run out... I will consider a rubber upgrade with 40's. Downside is crawling will be affected. Larger tires give a larger torque arm and will increase stain on axles and CV joints and such.

Another option is to open the pumpkins and change the gearing. Expen$ive.

Larger tires may also cause damage if you are running run-flats and get a flat. There will be a bigger mismatch.

If you're planning to wheel the vehicle, TH400 trannies are plentiful, fairly robust, have HUGE aftermarket support, and are preferred by many on the trails, (but not all).

Best,

T

 

[TOBASH]

Let's crunch the numbers:

8 miles is 9-18 minutes with a 3 speed and 7-14 minutes with a 4 speed.

Occasional fishing and hunting in town is nothing, BUT family vacations 500+ miles away will make 55MPH annoying.

Three speeds are bullet proof as they are mechanical. 4 speeds with their electrics can be frustrating IF something goes sideways, so I have read here on the BB.

My suggestion is take a month to read and search the forums and don't be in a rush to purchase. Read everything, even stuff you think you're not interested in.

Best,

T

 

[Skrilex]

I will be Devil's advocate here...

Another solution is the Diesel truck version of the 700R4 transmission. No computer required but driveshaft lengths will most likely be affected, (or so I'm told. I have not measured things out yet). No electrics BUT there are transmission cables and such that require customization.

I like the military tires just the way they are for now, but when they run out... I will consider a rubber upgrade with 40's. Downside is crawling will be affected. Larger tires give a larger torque arm and will increase stain on axles and CV joints and such.

Another option is to open the pumpkins and change the gearing. Expen$ive.

Larger tires may also cause damage if you are running run-flats and get a flat. There will be a bigger mismatch.

If you're planning to wheel the vehicle, TH400 trannies are plentiful, fairly robust, have HUGE aftermarket support, and are preferred by many on the trails, (but not all).

Best,

T

I thought we only had the two ratios to choose from? 2.54 and 2.73. Are there others? Not that I care I wanted the 2.54’s.

 

[TOBASH]

I thought we only had the two ratios to choose from? 2.54 and 2.73. Are there others? Not that I care I wanted the 2.54’s.

I defer to those with more knowledge regarding rebuild information about the exact ratios, but there is more than 1 ratio option IIRC

 

[TOBASH]

It means you spent maybe $1000.

You gained just under 10% top speed, or working it the other way you dropped your rpms by 10% at any given speed.

Practically speaking, you increased your hwy cruising speed from 55ish to about 60.

Any losses to bottom end acceleration are probably cancelled out by the decrease in rotating mass. I speculate that this also makes breaking a halfshaft less likely.

And cosmetics... here's a pic of my stock M998 next to a civvy H1 on 40's. I like big tires. I went to the trouble and expense to put my 5ton on HEMTT rims and 53" tires.

But for the hmmwv I'd rather take the $$ and use it for a winch...

And I think the OP would be happy with a 3spd. And i agree that if you be just have

View attachment 744782

to have that extra top speed, a tire swap is more feasible than a transmission swap.

Civilian models have additional horsepower engine options IIRC. Easier to push bigger tires.

 

[Action]

I thought we only had the two ratios to choose from? 2.54 and 2.73. Are there others? Not that I care I wanted the 2.54’s.

2.56
2.73
3.08

2.56 better for cruising but slow off thd line
I have 3.08 and ot takes off quick. And cruises fine at 65. But that is slso 6.5 turbo with 4l80e.

 

[Wire Fox]

My suggestion is take a month to read and search the forums and don't be in a rush to purchase. Read everything, even stuff you think you're not interested in.

Sage advise. I thought I read enough and was ready. I got impatient, and I'm about $1000 more spent than I needed to be because I was impatient (buying broken, ineffective components that I thought was an easy adaptation). Take your time, do it right.

2.56
2.73
3.08

2.56 better for cruising but slow off the line
I have 3.08 and it takes off quick. And cruises fine at 65. But that is also 6.5 turbo with 4l80e.

I need to take a look and see what ratio I have. I drove a HMMWV belonging to a fellow on the SE side of town and it was absolutely slow. I was taking it easy since it didn't belong to me, but still felt like I was driving an old bus. Driving mine, the thing feels like a rocket by comparison. I can put the pedal down and easily merge into fast traffic. But, his was also much heavier (non-armored slantback with turret vs unladen cargo truck) and his is using a 6.2L and mine is using a non-detuned 6.5L. Best comparison for driving feel? Accelerated faster than an old 2.2L S10, and right about the same as an unloaded Ford E-350 box van.