A gearing question for a street only scrambler.

jmarkel

Scrambled for Life
City
Fairfax Station
State
VA
#1
Hi all,

looking for a little advise regarding my scrambler and gearing. I am running the stock T5, dana 300, and original 2.73 gearing. I'm now running 255/75-17 tires which makes them 32" but only 10" wide. The engine is rebuilt 4.2 running a 4.0L head, borla header, clifford intake, HEI ignition, and Howell TBI.

I live in Northern Virginia so the jeep will be used for mostly flat driving, with occasional runs out into VA, WVA, PA, and NC mountains. With this setup, the jeep runs right at 2000 RPM in 5th gear and 75mph. It has absolutely no difficulty getting there and maintaining that with my foot only 1/3 on the throttle. I have yet to get it out on mountain passes but the inclines I've experienced so far have not required any downshifting to maintain speed Means 1733 RPM for 65mph with no issues either. Considering stock 235/75-15 tires, it would run approximately 400 RPM faster across the board if I had stockers on...but it would also be running a stock engine, which as I recall, performed ok in similar situations.

Anyway, I don't believe I am lugging this engine geared this way. And it doesn't appear to need more power/revs to get to my desired speed in any situation. However, I've read on so many forums about changing out the gears to 3.73 for this setup which would raise my rpm's to 2700 at 75mph or 2340 at 65 in 5th gear. I"m not sure why I would do that, but this seems to be a theme...up the gear ratio as you get bigger tires...however, I have now provided significantly more power and torque to move them. Again, this is a 95% street scrambler...the only time this will be offroad is on mild groomed gravel roads, the beach, scenic offroading.

Can you lend me your thoughts regarding the gearing here? Am I missing something or any reason why I should bother regearing this?
 

walkerhoundvm

Just trying to stay upright
LIFETIME
CJ-8.com Member
City
Cave Creek
State
AZ
#2
I have now provided significantly more power and torque to move them.
You're right about this - but not in a good way. It puts added stress on every component from your transmission on down. You'll burn through that T5 faster, your driveshafts faster, maybe even your yokes - but something's gotta give and it ain't always the tires leaving tread on the pavement.

I had 31s and 2.73s and while I could hardly notice it struggling, I went through 3 transmissions in as many years. Since my rebuild, I jumped up to 3.73s and 33" tires and blew my drieshaft straps in less than 2500 miles. At this point I wish I'd gone higher than 3.73, but am trying to manage with what I've got.

I would wholeheartedly recommend you jump up to 3.73s with your tire size. In addition to the gearing changes, you might need a new speedo gear housing if it hasn't been changed from stock, and a new gear. The 2.73s had their own unique housing for the 24(?) tooth gear.
 

jmarkel

Scrambled for Life
City
Fairfax Station
State
VA
#3
I had 31s and 2.73s and while I could hardly notice it struggling, I went through 3 transmissions in as many years. Since my rebuild, I jumped up to 3.73s and 33" tires and blew my drieshaft straps in less than 2500 miles. At this point I wish I'd gone higher than 3.73, but am trying to manage with what I've got.
I purchased this jeep with 39k miles in 2001. At 43k, I put the 4.0L head on it, borla, and a TFI upgrade, weber carb, and 31's. I ran it this way until about 70K when i put the TBI injection on it. at 90k, i rebuilt the engine when I did a complete rest-mod on the jeep because it had a lot of blowby, but didn't have any power issues. I have not experienced any issues with anything after all those miles...and that included two cross country trips to Colorado, countless trips into the PA and WVA mountains, and many long highway excursions to Ohio, NY, NJ. I feel that 3.73 are simply too much...what about 3.31 to split it?
 
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walkerhoundvm

Just trying to stay upright
LIFETIME
CJ-8.com Member
City
Cave Creek
State
AZ
#4
If I had stayed with 31s, that's what I considered doing. I think you'll be surprised at how well it does at that gearing; it may seem like the engine is going to fly out of the bay given how it sounds now, but it'll be fine. Just be sure you won't want to jump up in tire size down the road.

I'm assuming you checked the gearing somehow yourself, without assuming the stamp was correct? Spin the tires, count the driveshaft revolutions, etc.?
 

Jeeperdd

CJ-8 Member
CJ-8.com Member
City
Gordonsville
State
VA
#5
I run 3.54 in my daily scrambler and love it. It has a 258 Howell injection, T5, and 32" tires. I don't recall the rpm, but i don't believe its above 2000 at cruising speeds. I can set the cruise control on the interstate and it never has a problem pulling hills
 

jmarkel

Scrambled for Life
City
Fairfax Station
State
VA
#7
For calculation sake, the stock gearing (2.73) and stock tires (29") give the exact same rpm range of converting the gearing to (3.31) running (32") tires. I will never go larger than 32" tires on my jeep. I also see a chart floating around that shows gear ratios against RPM that shows optimal selections in green, orange, red for fuel economy, daily driving, and towing. These recommendations are approximately 400RPM higher than the factory set my jeep up to get to the economy stage, and 600RPM to get to the daily driver stage.
 

spankrjs

Scrambler Junkie
LIFETIME
CJ-8.com Member
City
Biloxi
State
MS
#8
If you are happy with it, leave it alone. Some of this is personal choice/performance expectations/driving environment.

I have 3.31's, 31" tires on my green Scrambler. It has a 4.2 bored out 40 over, RV cam, and Mopar MPI with a T5. Before adding AC, I was happy with my set up, Now, I want a little more gear when the AC is on. But a slight jump to 3.54 from 3.31 is a big chunk of change for "a little bit more". And, i really only want that "little bit more" when in 5th gear, AC on, going up grades.

My red Scrambler runs a 4.0 with Mopar MPI, AX15ish 5 speed, 4.10's and 33" tires. I am spinning around 2700 RPM at 75mph. On this Jeep, I need it. It is heavier then stock (roll cage/winch/rear bumper tire carrier), drive it cross country (big hills/long grades), and it is typically loaded down with camping stuff/spare parts/tools.

On the red one, when it had the 4.2, the 4.10's were perfect. With the 4.0, something like a 4.27 would be perfect, but again, hard to justify the money for a gear swap for that little bit, IF that gear ratio is even available for my axles. 95% of the time i am happy with the 4.10's, sometime I want a bit more. Also, this Jeep is used off-road, so the lower gears help out in this environment. No way I would go off-road with 2.73's :twocents:

I have drove many stock Jeep's with stock tires, too. 2.73, stock tires, automatic transmission seems OK. 2.73, 31" tires, and T4 is way too fast out the hole, not enough ass at high speeds/grades. 3.31's, T5's, stock to 31" tires seems OK. 2.73, T4, stock tires seems OK, if a bit fast.

Like I said, if you are happy with your Jeep's performance, leave it alone :twocents:
 

Kane

CJ-8 Member
CJ-8.com Member
City
Berlin
State
ct
#9
One thing to consider also when selecting gearing for highway rpm is the timing advance. In order to be working at optimal efficiency the distributor timing on the older Jeeps need to have the mechanical advance fully out so when you are driving the vacuum advance is working to give you the last few degrees of timing based off of engine load. When the mechanical advance isn't fully out then they two will sort of fight each other and you will lose gas mileage and can also get pinging. I always try to shoot for 2200-2300 rpm full mechanical advance and then have my gearing match that at my target highway speed.
 

jmarkel

Scrambled for Life
City
Fairfax Station
State
VA
#10
Thanks for all the advise. I feel like I am going to swap my gears, but likely swap them to retain the original factory ratios based on tire size. So, that means, a 3.31 set of gears. Unfortunately, my research says that with the carrier break, that 3.31 gears won't fit in here. Any thoughts on that? Anyone know if they sell thicker versions of 3.31's so I don't need to swap the carrier? I'd prefer not shimming things.
 
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spankrjs

Scrambler Junkie
LIFETIME
CJ-8.com Member
City
Biloxi
State
MS
#11
Thanks for all the advise. I feel like I am going to swap my gears, but likely swap them to retain the original factory ratios based on tire size. So, that means, a 3.31 set of gears. Unfortunately, my research says that with the carrier break, that 3.31 gears won't fit in here. Any thoughts on that? Anyone know if they sell thicker versions of 3.31's so I don't need to swap the carrier? I'd prefer not shimming things.
You should be able to find a pair of stock axles with 3.31 gears, in good shape, cheaper then what it would cost to set up new gears :shrug::twocents:
 
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