did we ever talked about rear gear ratios?

i must admit, i'm not great with computers, so probably it has been posted before, so be it, i'll repost it again,
i have 3.55 ratio on my rear axle, i would like it to be more economical on gas, so i thought i would change it to 2.76, i would like to gain some better top speed and economy( and yes i'm also planning to put front disc brakes )..my reasons for that, i'll be using it like a westfalia, to travel light with the family on mainly hightways, and around town.
wanted to know if anybody have done it and if it was worth it and how did they do it?.....thanks for any inputs ..corrosive

i went from 355 to a 273 ratio and saved about 300 rpm at highway speeds. when i changed to the 360 from the 318 it even slowed it down more (camshaft) but it's still very good. it is no rocket ship off the line (automatic trans) but i can cruise nicely and have lots to pass someone with if needed.

There are several options to consider when trying to lower rpm's at highway speed. I just went through this myself so I'll put out what I experienced.

I went into it trying to lower my rpm's enough to improve drive-ability on the highway. I live out in the sticks pretty much so you've got to get on the expressway to get anyway, 65 is pretty much the minimum for safe travel around here. Any slower and people will be passing you on both sides.

My van has a 273v8 and a 3/spd manual, it came with a 3.55 rear and had 185/70/14 rims and tires on it when I got it. It would turn >3k rpms at 50mph with that setup. Since I have the original 45 year old 273v8 that was way too much for my comfort so I had to lower it.

I ended up changing the rear gear to a 2.73 and it helped quite a bit BUT it changes the way you have to drive the manual trans. I have to ride the clutch a little taking off on grades and I have to string the gears out to about 2500-3000rpm to shift from 2nd to 3rd or it can chug pretty hard under some conditions (going uphill for example).

I also changed my tires, since I didn't want to buy new 15" rims I was pretty limited unless I wanted to spend big $ on MT Indy Profile tires or something. I ended up with Cooper Cobra 225/70/14's which is about as big as you can go these days with an off the shelf tire.

Between the two changes my speedo is now off (slow) by about 5mph but I now run about 2500-2700 rpm at 65mph, I can do 65/70 pretty comfortably now without worrying about something coming unglued. I don't know for certain but I would expect an auto trans to run lower rpm's than a manual since the manuals tend to be geared so low.

If you already have 15" rims you can try and bigger tire in the back and see how that does before you drop the $ on a rear chunk. For what its worth I got the tires for $500 out the door (shop around) and the chunk on the local craigslist for $125 bucks. Just put out a "WTB 2.75 gear chunk for 8 3/4" Mopar rear" add.

Also, while I got the rear gear cheap I had to pay for installation since I don't have anywhere to pull the axles. I think it cost me about $500 by the time I paid labor and some new wheel bearings/seals.

all that being said here's some pictures:
tires:
185/60 on left, 225/70 on right:





my $35 steering shaft mounted tach:

bet those tires really do test the old clutch out ,dave

u have to pull diff cover to tell what ure running?dave

kykayaker37 wrote:u have to pull diff cover to tell what ure running?dave

The gear ratio is listed on the VIN plate mounted beside the drivers seat (on the wheel well hump). It's also stamped on the edge of the ring gear but you have to pull the chunk to see it since there's no cover on the 8 3/4" rears.

The rear gear stamp is a great way to verify ratios when you are looking at used ones though.

cool ty ill look this weekend u saved me alot of work,dave

What you want is to be at the lowest RPM that your engine develops enough torque to pull your beastie around.
Modern 'vettes can chug at 1400 at 60 in 6th gear.
They are strong engines developed for this and the drivetrain is optimized for this too. It's how they can get modern MPG.
But, it's also an optimized, slippery shape. Do not expect to be able to push a van along like this.
I'd say that a typical 318 would like to be around 2400 RPM at 60. Maybe a little higher since it is pushing a barn door!
There are various sites that allow you to play with tire size, gear ratios and RPM for a given speed. If you plug in the various Mopar gears (2.76, 2.93, 3.23, 3.31, 3.55, 3.73, 3.91, 4.10, 4.56, 4.89, 5.17, 5.57) and different tires sizes, you should be able to come up with a combo for your ride. If you go with a 2.76 and a tall tire with a tired V8 or /6, you will probably end up with gearing that is too tall, meaning that it will not pull well from a stop and it will not have enough power to do 65 MPH because it's RPM is too low to develop enough torque.

i just went and see(to be shure) the tag and tire size that i got, so A745 tranny with 3.55 ratio and 225 60r15 tires...the tires are due to be change, so i could start with this, btw the average speed around here is in miles 75 (120 kmh),so if i could go 70 i would be happy, i also know this not Lotus handling..any sites to recommend?..corrosive

ok i've found one gear ratio calculator.

Here's a good one that is also pretty.
http://vexer.com/automotive-tools/speed-rpm-calculator

With your tires 3.55s and in 4th (Direct, same as third in your trans), 70 MPH =3289 RPM.
With the same stuff except 3.23 gears instead it's 2992 RPM.
With the same stuff except 3.91 gears instead it's 3622 RPM.

You can find other sites with more basic info and more gear ranges. I can't give it ALL to you, you have to do some digging yourself!

BTW, you will notice about 300 RPM difference in each at around 3000 RPM. Also notice that the gear change is about 10% each time too. Funny how that works!

http://www.moparregister.com/articles/calc/ try this,dave

Wow great sites, i will pla..work on it...thanks alot guys, i think i got everything to make a decision, i will calculate the loads( dead and alive)....corrosive

looking at all that, i think 2.76 is too hight for our vans(good for cars),slowflapper is right saying it change the way it shift, i'm looking for a compromise between fuel effency and torque, i'll go for 3,23 with 225 75 15 tirs which will give me about 2800 rpm at 74 mph(118kmh), i think its the best ratio for what i'm planning to do... thanks all...corrosive

2corrosive4u wrote:looking at all that, i think 2.76 is too hight for our vans(good for cars),slowflapper is right saying it change the way it shift, i'm looking for a compromise between fuel effency and torque, i'll go for 3,23 with 225 75 15 tirs which will give me about 2800 rpm at 74 mph(118kmh), i think its the best ratio for what i'm planning to do... thanks all...corrosive

sounds good.

I was initially going with a 2.94 rear end and the shop I had my van at started screwing things up pretty badly. I had to rush and find another rear and the 2.73 was all I could get on short notice.

I'd still like to run my van at 70mph but Im not comfortable with those kinds of rpms, when I get the new motor in I'll feel better about it though

I have 3.55 with a 318 auto and i most likley step down to a 3.23 one of these winters.

i find the shifting of the tree on the three a real labor, so imagine when when you have to rev the engine really hight

anyone know where to get thoses 3.23 ring and pinion?..

2corrosive4u wrote:i find the shifting of the tree on the three a real labor, so imagine when when you have to rev the engine really hight

I'd rather have an auto myself but I wont complain, at least its a van.

2corrosive4u wrote:anyone know where to get thoses 3.23 ring and pinion?..

Put a "Want To Buy" add on the auto parts section of Craigslist in your area and people will respond to you. I paid $125 for my whole chunk and the guy lived 10 miles away.

here's a link, just pick the part of Quebec you are in.

thats the biggest problem with chryslers 8 3/4 differentials. not enough choice. richmond gear is just starting to make some choices avaliable but at a cost. the ratio you want will have to come from any car and not a van or truck. if the car had an 8 3/4 it has better choices and they fit no problem so don't limit your search to a-series vans and 1/2 ton trucks. your best bet is a search through a lot of rear gears and all you can do is count the number of turns the pinion makes to turn the axles 1 complete turn. that will be the only way. but a tip for you, the last 3 numbers of the case denote the best carrier. 489 is the best, then 742, and 741. 489 cases have a bigger front pinion bearing and are most likely to have a hidden posi unit. good luck on the search.

IF you do go new, I'd recommend Dr. Diff. He's the guy for Mopar axle stuff!
http://www.doctordiff.com/
It's a nothing website, but he's in the Mopar mags and on the forums. He's got a very Good reputation.

will look for new, but since not in a rush, will also for used.thanks for the links guys, will shop around..corrosive

slowflapper wrote:

2corrosive4u wrote:i find the shifting of the tree on the three a real labor, so imagine when when you have to rev the engine really hight

I'd rather have an auto myself but I wont complain, at least its a van.

my wife just cant shift that three on the tree and when we came back from picking the van we went trought Montreal at night , it was very dangerous cause slow shifting, just not a porshe....

useful info about gear and pinion installation;https://www.youtube.com/watch?v=dAqAqODmcj4&feature=colike