What Is Concidered A Good Highway Gear Ratio?

I have the OD unit, Iron Duke engine. Just looking to get some highway mileage and still do some mild off roading ?

 

leave it the way it is or buy another jeep..

cheapest and only real option it to find 4:27 axles assuming you have 5:38's
juice may not be worth the squeeze.

 

just got a D30 and it needs re geared, D44 rear is 5.38. with the OD it would bring 5.38 down to 4.0 something

 

The Novak Guide to Gearing & Gearing Math for Jeeps

 

X2 on 4.27 gears being a good all-around compomise. High enough for road use, low enough for trails. An overdrive and your 5.38s are likely fine too, as long as the overdrive is in good shape. Especially with the overdrive, make sure your gear oil level is full in the transmission!
-Donny

 

sound like ill be regearing the D30 to 5.38 then after cleaning and inspecting the over drive

 

X2

 

5.38 only gets you to 4.10 with an overdrive. If you are really looking for a highway gear, that may not be quite enough for you.

 

4.88 with an overdrive is pretty good on the highway. A 4 cylinder may not be up to the task though.

 

Iron Duke can handle taller Final Drive Ratio than a 134 Willys so...
Really good hiway gears with an Iron Duke engine and 25% OD ....
I would run 4.88 axle ratios.
The compound FDR would be 3.67.
You could run all day at 60 - 65 MPH (depending on exact tire diameter) and get max fuel economy from the Iron Duke.

5.38 are also fine but noise and fuel economy will suffer up about 10%

Obviously 5.38 is generally best when off road and 4.88 is roughly 10% faster.

 

How fast do you want to go? I suspect you won't want to be in the middle lanes at 70 or above. A top speed of 65 is pretty reasonable for a stockish '60 CJ-5. What diameter tires? With 31s, that's 2800 RPM at 65 with a Warn overdrive, which will be fine for your engine if it's in good condition.

I drove my '73 with 4.27 axles, no overdrive and 30-31ish tires all over the southwest without a thought to whether it was spinning too fast. You may be disappointed by your 151 if you slow it down too much. Power is the product of force and speed... torque times RPM is horsepower. Lower the engine speed too much and the power drops as both the engine slows and the torque falls off. You have a lot of frontal are to push, and wind resistance goes like velocity squared, so drag from wind resistance will be a significant obstacle.

Accept the engine sound, insulate the floor, run a quiet exhaust, and don't worry.

 

Ken, could you expand on this a bit? Do the 5.38 gears make more noise and drivetrain drag than, say 4.88s? Or are you thinking the higher engine speed as the cause?

 

I'm considering that the complete assembled drive train needs to be envisioned as a synchronized system.
First, the jeep model and it's planned use-age typically mandates the ideal engine selection.

Like all engines the Iron Duke has a specific crankshaft velocity that produces maximum torque output.
RPM at Max torque output happens to be the most efficient RPM for any given engine.
Therefore maximum fuel economy will be obtained if cruising at that specific RPM.
I'm not sure about Iron Duke dyno specs but it's likely to be about 2400 RPM.

This RPM of maximum efficiency (maximum torque output) only holds true if the engine can provide sufficient HP at that travel speed.
And I estimate (guess from no experience) that the Iron Duke can provide sufficient HP to travel at 65 MPH under most conditions excepting abnormally steep grades.

After engine selection one needs to consider the ideal FDR .
In this instance the FDR is compounded due to optional OD unit.
If the Iron Duke puts out maximum torque at 2400 RPM then a compound FDR of 3.67 should be near ideal to produce sufficient speed at ideal sustained crankshaft velocity.

Iron Duke with O.D. should easily handle the slower 5.38 differential ratios.
But the RPM's must increase above ideal in order to effect the same hiway speed.
Make sense ?

 

Yep

 

Right I certainly feel that 65 MPH is a good stable speed for most well maintained early cj's.
With Iron Duke and O.D. I certainly would not gear the differentials any higher than 4.88.

The engine must be able to handle the speed.
The HP output of the F-134 for example simply cannot maintain 65 MPH on steep interstate grades regardless of exact RPM.
So 5.38's are certainly best for those low HP engines

 

I plan on stock skinny tires around 29", the engine was just rebuilt, bored, decked and balanced. comes out to a 155 according to the shop that built it. (Hemmings Motor News stated: The four was rated at a modest 87 horsepower at 4,400 RPM, although the torque peak of 128-lbs.ft. was reached at a relatively low 2,400 RPM, for quicker takeoffs from rest.)

 

Peak HP at 4400 RPM, peak torque at 2400, 29" tire.

With 5.38s you'd be at 3040 / 4050 with/without 0.75 overdrive at 65 mph.
4.88s: 2750 / 3675
4.27s: 2400 / 3400
3.73s: 2100 / 2800

To me, this looks like 4.27s would be optimal for the highway. Get on a long hill, and you could drop down out of overdrive and not be winding it out too much. To me, 4.88s or 5.38s would also be ok, and I'm not sure that the effort and expense of regearing a second axle would be worth it. The 5.38s will be best for the trail.

 

Slap the Dana 30 in without the front drive shaft and see how you like it. That is the cheapest way to know for the highway.

 

My 2 cents ............it's all about the Horsepower and when your down on power , mechanical advantage / gears ratios are your best friend.........the effects of aerodynamics and Drag is also a concern when down on power in a vehicle like a Jeep that probably has a similar Drag coefficient of a large barn Door...........

 

I had a 3b in high school that I built with a 151, t15 and a 20 with 3.73 Dana 30 and 44 that worked really well with 31" tires on it. It also got about 25 miles to the gallon. The only downside was steep uphill grades but no different then any other 4 cylinder Jeep.