Rear End Gears

[65 Dartman]

I was planning on going with a 65 833 in my 65 Dart; however, leg problems are making me go to an automatic. I have a completely rebuilt A500 which I’ll be using. First gear ratio is 2.74 with the OD ratio being .79 (I think). So what readily available 8.75 rear end gears would you use? I’m leaning towards 3.55s based on how they work in my 93 Dakota with the OD and LU A500. How about 3.23’s. Any and all comments appreciated

 

[toolmanmike]

I was planning on going with a 65 833 in my 65 Dart; however, leg problems are making me go to an automatic. I have a completely rebuilt A500 which I’ll be using. First gear ratio is 2.74 with the OD ratio being .79 (I think). So what readily available 8.75 rear end gears would you use? I’m leaning towards 3.55s based on how they work in my 93 Dakota with the A500. How about 3.23’s. Any and all comments appreciated

With a overdrive and lock up transmission I would gear up a ratio. It also depends on your engine, especially the cam, and rear tire size. @AJ/FormS, can you help with the combination?

 

[Professor Fate]

 

[65 Dartman]

With a overdrive and lock up transmission I would gear up a ratio. It also depends on your engine, especially the cam, and rear tire size. @AJ/FormS, can you help with the combination?

I guess I need to add a bit more info! It ‘s my conversion wagon. Engine is a 66 273 Formula S built to pretty much stock specs. Cam is from Racer Brown nothing exotic. Trans will be the A500 with OD and LU with a stock style converter spiced up just a bit by Dynamic Converters. Rear end will be a LBP 8.75 that currently has a 742 case 3.23 Sure Grip in it that should have worked nicely with the 65 833 that WAS going in. Rear end is sitting on a homemade stand is ready to be installed so right now gear swaps would be easy if needed. Tires are gonna be 14 inchers since that’s the size wheels I have - probably no bigger than 215-60’s or 70’s

 

[toolmanmike]

mag I guess I need to add a bit more info! It ‘s my conversion wagon. Engine is a 66 273 Formula S built to pretty much stock specs. Cam is from Racer Brown nothing exotic. Trans will be the A500 with OD and LU with a stock style converter spiced up just a bit by Dynamic Converters. Rear end will be a LBP 8.75 that currently has a 742 case 3.23 Sure Grip in it that should have worked nicely with the 65 833 that WAS going in. Rear end is sitting on a homemade stand is ready to be installed so right now gear swaps would be easy if needed.

I think a 3:23 would work fine. I think the a500 has a low 1st. gear to get you out of the hole. That combo should get darned near 20 mpg/hwy without the lock up and overdrive. Mid 20's with. The higher the rear gear the peppy it will be off the line. You could probably run a 3:73 or a 3:91 and still get good mileage in OD and lock up.

 

[grimreaper]

Calculate GEAR, RPM, MPH, TIRE DIAMETER

These calculators at Wallace Racing are pretty handy to compare different scenarios.

 

[Bad Sport]

Rear end will be a LBP 8.75 that currently has a 742 case 3.23 Sure Grip

If you got it, I'd run it.

 

[65dartcharger]

3.23 or 3.55 would be my idea.

 

[65 Dartman]

If you got it, I'd run it.

3.23 or 3.55 would be my idea.

3.23s are in the housing right now with the rear end ready to install. If I tear it down I could use a set of 355s I bought off another member awhile back. Unfortunately both sides of the disc brake assemblies gotta be pulled so the axles can be pulled to pull the pumpkin.

 

[toolmanmike]

3.23s are in the housing right now with the rear end ready to install. If I tear it down I could use a set of 355s I bought off another member awhile back. Unfortunately both sides of the disc brake assemblies gotta be pulled so the axles can be pulled to pull the pumpkin.

Stay with the 3:23's they will be fine.

 

[MileHighDart]

Your A500 actually has a .69 overdrive not .79
I just recently did the A500 swap. I,m running 3:23 gears and 225/60/15 rear tires, which are around 25-1/2" tall.
Seems to have plenty of low end, with the factory high stall converter that came with the tranny, and the slightly lower first gear, it will smoke that right rear tire no problem.
But I am running a 5.9 magnum.
With this setup the overdrive is great. 60 mph is barely 2000rpm. 75mph is like 2500
I think it's going to be great cause I do a Lot of highway driving.
But,
I'm thinking of switching to 3:55's as well, already have a new set on the shelf.
3:23 to 3:55 is a pretty small change so it shouldn't change my highway rpm much.

I'd think that running a 273, you might want to go ahead with the 3:55's, since you already have them, might get you off the line a little quicker.

 

[RustyRatRod]

I'd run a 3.91 so quick it'd make your head spin. Your final drive ratio will still be 3.08.

 

[toolmanmike]

I'd run a 3.91 so quick it's make your head spin. Your final drive ratio will still be 3.08.

That's how all the new cars run like a scalded dog but get good highway mileage. He could always start with the 3:23's and trade the 3:55's for a set of 3:91's. Both mine have 3:23's and then I added tall 15" tires.
did I do that for? The looks. Killed my performance. Both get close to 20mpg/hwy. though.

 

[65 Dartman]

If you got it, I'd run it.

That’s the plan- run what I have on hand as I’m retired and extra car funds are slim at best.

Your A500 actually has a .69 overdrive not .79

Thanks! I wasn’t sure what the OD ratio was in the A500. Couldn’t find the info in My 93 FSM.

I just recently did the A500 swap. I,m running 3:23 gears and 225/60/15 rear tires, which are around 25-1/2" tall. But I am running a 5.9 magnum.
With this setup the overdrive is great. 60 mph is barely 2000rpm. 75mph is like 2500

Since I’ve got to use what I have, wheels are 14 inch Magnum 500. So tires are likely to be no larger than 205 or 215 70 or 60 14’s.. In my 93 Dakota, tires are stock size 15 inches, rear end gears are 3.55. At 70 MPH in OD and LU RPM is but 2200. Takeoff is decent considering the motor is a 3.9

I'd think that running a 273, you might want to go ahead with the 3:55's, since you already have them, might get you off the line a little quicker.

Thanks for your comments!

I'd run a 3.91 so quick it'd make your head spin. Your final drive ratio will still be 3.08.

Using what I have, it’s either 3.23s or 3.55s unless someone wants to trade me 3.91 WITH Sure Grip for my 3.23s WITH Sure Grip.

 

[RustyRatRod]

Using what I have, it’s either 3.23s or 3.55s unless someone wants to trade me 3.91 WITH Sure Grip for my 3.23s WITH Sure Grip.

No way would I run a 3.23 with an OD. GHEY

 

[plumkrazee70]

I have the A500 in my 70 Dart, not running yet, but I opted for 4.10's. I wanted a really fun street car but still have a decent cruise rpm at 65. My tire size is 245/45/17. I'm running a rebuilt 340, with a little hotter cam.

 

[AJ/FormS]

Any and all comments appreciated

I think it depends on your engine and performance requirement.
I have run a combo that put my cruising rpm at less than 65-1600; which is really comfortable. But your engine will have to be built to support that.
But then, it will be pretty sucked out to about 30 mph. To combat that, you need a bigger engine.
So; round and round it goes. Pretty soon you get to thinking about a stroker, or a V10.
So I no longer have that trans.gears combo.
I found that cruising in the 2200 to 2400 @65 was a reasonable compromise.

In your case, that would require a rear gear of 3.91s to 4.10s with the .69od and loc-up. So with a 273 I would go 4.10s, and with a 360 I'd go 3.91s.
But if the 360 is warmed up, it might like the 4.10 better.

In your case, you don't have a big range of tire sizes available, and will probably end up in the 24 to 26 inch range, so about 8% from the smallest to the largest. After 3.55s each next bigger gear size is about 5%.
So my numbers reflect the taller tires. If you have really small tires then the gear range should likewise be reduced one size.
That is to say;
4.10s will cruise 65=2377 with 26s, and 2584 with 24s
3.91s will cruise 65=2267 with 26s, and 2464 with 24s
3.73s cruise 65=2163 with 26s, & 2351 with 24s
3.55s cruise 65=2058 with 26s, & 2237 with24s
As to first gear performance;
a starter gear of 9/1 is usually plenty, with a smaller engine liking a lil more and a bigger a lil less.
In your case, 9/2.74= calls for a minimum of 3.23s in the back. So you ain't got much to worry about there!
The other thing is that much over 12/1 is not really all that great, so
12/2.74=~4.30s is getting to be on the big side, so 3.55/3.73s go right up the middle.Which is why I included them above.
3.73 x 2.74=10.30, plenty of starter gear to spin the tires in first gear and what's not to like about 65=~2240 I've been cruising there since 2004.
And finally the kickdown into first at 30mph will get you 4300 rpm@8%slip with 26s..... already too high for a factory cammed engine, to be much good. 3.91s or 4.10s would only make it worse.
3.55x 2.74=9.73. a 360 will have no problem breaking the tires loose with this. But a low-stalled 2.73 is gonna struggle.
Kickdown at 30mph is now 4074@8% slip, still high for a stock-cammed SBM, excluding the 340 of course,lol.

I'm not saying you should run 3.55s or 3.73s.
I am saying here are the numbers, choose for yourself.

 

[TrailBeast]

There is so little difference between 3.55 and 3.21 I wouldn't even bother switching them.
I think running 14 inch tires will feel like you have 3.55's anyway.
You really can't go wrong no matter which gears you use.

 

[65 Dartman]

I think it depends on your engine and performance requirement . . . .

Engine is a 66 273/235 already assembled. Tire burning days/stoplight to stoplight days are over. I’m just a cruiser now with most trips on the interstate.

But your engine will have to be built to support that.

It’s basically a stock Commando build with a slightly upgraded cam from Racer Brown, with dual exhaust of course.

I’m not saying you should run 3.55s or 3.73s. I am saying here are the numbers, choose for yourself.

Thanks for all the info which now I’ve got to digest.

 

[65 Dartman]

There is so little difference between 3.55 and 3.21 I wouldn't even bother switching them.
I think running 14 inch tires will feel like you have 3.55's anyway.
You really can't go wrong no matter which gears you use.

Based on how my Dakota drives with 3.55’s and 2.35-75x 15 tires, I find that even on some small size hills, the trans drops out of LU and OD pretty easy. I’m a bit concerned that with 3.23s and 14 in tires it’ll be all the time.

 

[TrailBeast]

Based on how my Dakota drives with 3.55’s and 2.35-75x 15 tires, I find that even on some small size hills, the trans drops out of LU and OD pretty easy. I’m a bit concerned that with 3.23s and 14 in tires it’ll be all the time.

It drops out so easy because the trans is computer and vacuum controlled.
You probably won't use a computer in your swap?

Mine is set up so it automatically shifts in and out of OD and lockup determined by speed and not load or vacuum.
Mine is also adjustable, so I can set those speeds.
I can pull a 15% grade in OD and lockup at 75 without any problems at all and not much throttle either.
You can't really base it on how your computer controlled trans in the Dakota operates.

 

[grimreaper]

If the 3.23s are in t I'd run those and see how it feels. If it's good to go, you got lucky. If it's a little tiny bit off you can always switch to the 3:55 at any time and if it is way off, then you know you will need to get into the 3.91 or 4.10 range. All of the facts and figures are great but there's nothing better than the seat of the pants and the computer directly above our shoulders to determine which gear ratio we want.

 

[65 Dartman]

It drops out so easy because the trans is computer and vacuum controlled.
You probably won't use a computer in your swap?

Mine is set up so it automatically shifts in and out of OD and lockup determined by speed and not load or vacuum.
Mine is also adjustable, so I can set those speeds.
I can pull a 15% grade in OD and lockup at 75 without any problems at all and not much throttle either.
You can't really base it on how your computer controlled trans in the Dakota operates.

Ah, i didn’t consider the Dak OD and LU being computer and vacuum controlled. I remember seeing a post by someone - probably you? - since you just mentioned being able to manually control your A500. Since I was going to use the 833, I didn’t really pay that much attention to it. I’ll have to do a search and follow up on it.

 

[Clelan]

When I first installed the A500 in my Duster I had 3.21 gears in an 8 1/4 with a 25.5" tire. It drove really well but to get the cruise RPM up to where I wanted it to be I was highway driving about 80 mph wherever I went. That's just where my engine wanted to be. I now have 4.10s in an 8 3/4 with 28" tires and it cruises right at about 60ish MPH. Now my combo likes about 2200 RPM cruise. Your 273 may want a little more.
I'd use the lowest (numerically highest) ratio you have available to you and try it. Only then can you really decide what you will be happy with. Once you know, save some money, sell some parts and get the gears you really want! Good luck with it! OD was a game changer for me and my car!

Cley

 

[AJ/FormS]

OD was a game changer for me and my car!

me too


Just to add, you can usually determine your lowest cruising speed by the vacuum gauge. To do this; slowly rev up your engine in neutral watching the gauge.
If you have a performance cam, you should see the vacuum increase with rpm, up to about 1700 to 2200. In there somewhere, it should plateau, and no longer increase. After 2400 sometime, it might begin to decrease.
Th lowest rpm that the vacuum plateaus is the rpm where your engine first begins to be efficient. That would then be your lowest recommended cruising rpm.
You can cruise lower, but your economy will not be there, as the pistons may still be pushing mixture back up into the intake past the late closing intake
and the headers may be pulling mixture straight across the tops of the pistons on the overlap cycle.
This method depends on the ignition timing to be correct, or at least close to correct. Otherwise the the vacuum will be lower than it should and the peak less pronounced.,

If you have a stock lo-po cam; the idle will be messed up and reading high. and will stay high then begin to fall,lol . So the slower you run it on the hiway the better.