Axle ratio question

[Bmartin]

This is a question for down the road.

 

I've found an 8.25 rear axle out of a 93 Cherokee.  I believe it hasthe 3.55 ratio.

 

My long term plan is to replace the existing engine in my P9 (200ci) with at least a 230 or even a 260 and add some hot rodding parts.  I would like to add a 3 speed overdrive transmission to that engine and then replace the rear axle with something with a taller ratio and modern drums.  My goal is to add A/C to the car and still have good power on the highway, 65 MPH up the hills would be nice.

 

Would the 3.55 ratio work well with that combo?  It just popped up and I have not had time to research the answer yet.  Thanks.

[Fastback]

I have a 3.23 diff in my '38 (218) sitting behind the standard 3 speed box. The lower ratio works very well for what I need the car to do. It cruses well at 60mph. Locally there have been some who comment that this lower ratio overloads the engine resulting in engine failure. Of the 8 or so cars that I know of none have had (or admitted to) this problem. At 3.23 ratio generally the guys have pulled the O/d out as it becomes redundant.

Fastback

NZ

[Plymouthy Adams]

try using the term "higher ratio"  saying lower would indicate that the 3.9 was way to high for you to begin with..and yes..todays roads with long low grades etc and paved unrutted surfaces with mud and such pretty much make going to a higher gear a no brainer

[Don Coatney]

This topic has been discussed many times. I would look up the answer for you but I don't have the time.

[Tim Keith]

I think 323:1 would work better with a 251 or 265.  If you mostly deal with flat terrain and have light loads I think a 218 could work. Depends upon your expections of driveability.  I'd not take out an O/D if I had good working one.   I don't think you would overload the 218 without it first giving some complaints - you'd have to keep hunting for the right gear.  I think the 200-R4 auto might adapt well to this scenario using a higher numerical ratio - but I don't really know about the 200R4 other than its a relatively efficient auto.  Many slant six cars had a 323:1 and the torque output at lower RPM isn't that different from a 230.  Quite a few of those slant six cars had 2:91 or even lower.  I'd only hash this over if you plan to do a lot of highway cruising.  You can't have it all with a flathead, you have to make some tradeoffs somewhere.

[Fastback]

For me I would say 90% of my driving is flat roads, some mild climbs and almost never any hard climbs. Given the chatter here (NZ) around these diffs straining the engine I just down change that bit earlier and let the rev's pull the car up the incline instead of letting it chug its way up. I am also still running 16 inch rims with tall radial tires. One of the mild climbs is close to home, with that I simply take a run up at it and then almost let the momentum carry me up.   Cops haven't caught me ..... yet.

 

If I lived where there was some real long steep climbs I not sure the would use the 3.23 diff.

 

Fastback

NZ

 

PS apologies if I have the low V's high ratio around the wrong way  Monday morning and no coffee yet ! 

[Bmartin]

First, yes I was being lazy and abusing my forum privileges.  I should have searched for the answer.  Its a good deal and I don't want to miss it. 

 

From some of the answers, I need to determine the final gear ratio of the overdrive tranny as well as the 3 gears, then calculate the rpm's.  From that, decide what ratio I need.  I'll work on it.

 

Ideally, I would like to be able to run 65 MPH, up a steep grade, with the A/C on.  That is probably a lot to ask for Flathead, but its at least a goal.  I don't mind if its not a hot rod off the line.  Its a cruiser not a racer. 

[Lumpy]

How high, or how low first gear is would be a consideration. My first gear is fairly high, so I wouldn't gear my rear (!!!) higher for that reason. If my first gear felt lower than it is, I'd do it.

 

I'm not saying that Lumpy would not pull a higher first gear "okay", but I wouldn't want it any higher. But another person might drive Lumpy and think it had a plenty low first gear, so just a matter of personal opinion.

 

Or to put it another way, some guys might not mind having to slip the clutch quite a bit in first, some would. I'd rather have a low first gear, and an overdrive, than visa-versa.

 

ken

[DJ194950]

Most OD's are around .70 reduction, including the Mopar units that fit into your car with lilttle or no mods.

1st gear in a stock 3 sp. is plenty low for new driving conditions. Forget about slipping the clutch with the torque of a flat 6 motor, no need. You don't start out in 1st. OD engaged!

With AC engaged-uphills, I'd suggest only a 3.73 rear gear. Without  an AC on up a hill  then 3.55 Seems to be great for most.

Assuming a motor in top condition only for these ideas.

 

Doug

[Robert Horne]

 I'd rather have a low first gear, and an overdrive, than visa-versa.

 

ken

 

Ken, this is how my present setup is with my Ranger 5 speed/3.73 rear gear, 218 engine.

My first gear is great on my steep driveway.     Second is good for starting on level highway,

and 5th is great for over 40mph.       Virginia has a lot of up and down highways, so I have

to shift to 4th often with only 80hp.     Fun to drive, for sure.....

[Lumpy]

The Ranger 5 speed does sound like a good conversion. But I sure love the three on the tree....wonder if one could make a 5 on the tree. ? Retaining my three on the tree is the main reason I've never seriously considered going to another type of transmission. Sounds like the Ranger tranny is the way to go if that is not a consideration.

 

Certainly, on level ground, you could gear the rear end quite high. On level ground I can start off in second gear. On level ground, I can pretty much let the clutch out with my foot off the gas pedal. However, you never know where there will be a steep driveway or hill, or some situation where you don't want to flog the clutch to death. Pull some prize hulk out of a field? I don't know. Maybe for some that/those situation(s) never arise(s). ? I know I don't want a first gear that feels like second.

 

k.

[1941Rick]

If you want to run 65mph up hill with a/c on, you have the wrong engine.....bump it up 100 cu in. 318.

I have 218 , 3 spd O/D, 3.9 gears. Strong 218 as well......steep hills I go out of O/D and run up at 55mph. 

Remember the higher the ratio the more power it will require to move the car. Thats why they came out with 4.0 and 3.9 gears.

As for hurting the engine...I would worry about the clutch first.