Hi
I am just geeting everything together ready to carry out the A518 overdrive transmission swap and replace the A727 in my 1979 Dodge B200 Van (318cu), gearbox now ready and have found somewhere to get the prop shortened but stuck with regard to the rear axle ratio.
1. How do i find out what gearig I have in there?
2. With the overdrive fitted any ideas what gearing will give best fuel consumption.
3. Been offered a "Richmound Hi Pro ring and pinion set Mopar lsd 8 3/4 . 4.88 from a 1968 R/T 440 Dodge charger 1.875 dia Pinion 10 spline casting 489 - Is this any good for my swap????
Over in the UK so hope my terminolgy is the same!
Thanks
Mark

I think the 4.88 is way too much for your van. I wouldn't do more than a 3.73 ratio. There may be a metal tag on the differential cover.

The higher the ration number, the shorter the gears.

I.e. 4.88:1 will give you more torque but horrible with fuel economy. Ultimately, you'll have higher RPM's at lower speeds. Great for towing though.

2.05:1 (for instance) will give you higher speeds lower RPM's. Downside is longer to get up to speed.

Since you're putting in an overdrive transmission I'd suggest somewhere in the 3.3:1 to 3.8:1 ratios. I don't know what exactly Dodge has for their ratios but that would be a good starting point.

I've got an 88 G20 Chevrolet, has 3.73 gears and the 700R4 overdrive. At 60 MPH (100 KM/H) I run about 2000 RPM's. I had an El Camino with 2.05 gears with a TH400 3 speed auto transmission. at 60 MPH would run about 2400 RPM's, had there been overdrive it'd probably sit about 1300-1600 RPM's, darn close to idle.

What it comes down to depends on your driving style and your level of commute. If you're in the city all the time maximum speed about 45 MPH I'd suggest 3.8-4.8 range for quicker get up and go during those busy left turns without a signal light (right turn in your case?) If you're normally in the countryside and hit highway speeds more often go with taller gears (2.0-3.5) Personally I like the better of both worlds with my 3.73's plenty of torque for Vegas traffic but can still maintain high speeds and keep the RPM's lower; less wear on the motor.

If all else fails, trial and error. See what you like most.

You should have a sticker on the bottom of the hood (bonnet), usually over the battery that lists all of the equipment the van was built with, including the rear axle gear ratio. Your 1979 Dodge van was not built from the factory with an 8 3/4 rear axle, but it is possible someone swapped one in. Most likely is your van has the 8 3/8 (more commonly called the 8 1/4) rear axle. The 8 3/4 rear ring and pinion set will not work in an 8 3/8 rear axle.

If you are installing overdrive, I also recommend a rear axle gear ratio in the range of 3.5 to 3.9. Fortunately there are many many aftermarket gearsets for the 8 3/8 rear axle. Unfortunately it will likely cost you a large sum to ship a set to the UK. See here: http://www.ringpinion.com/DiffWizard.aspx

Yes..your van will either have the 8-3/8 or 9-1/4 rear (8-3/4 was last used by Dodge in 1974) or like Reed said someone could have swapped in an 8-3/4 rear axle.

Since you're using the A518 OD trans, you can get away with using a 3.92 or 4.10 gear (if you plan on towing).


While your at it, go with a Sure-Grip (posi-traction) gearset.

Firstly thanks to everyone who has replied, not had anything to do with diffs before so bear with me if I ask some stupid questions!. Think I may have confused the issue with the diff i was offered, it was simply someone offered it to me if it was of any use - have no idea what is fitted to van at present. The sticker under the hood has been sprayed over by a previous owner so that was really helpful of them! So looks like I need 3.50 diff, are there any other models that I may be able to source a second hand one from i.e ramcharger etc, there are a few american vehicles over here being dismantled so may be able to find something, your right Reed shipping for USA to UK is now impossible since the USPS got rid of the slow boat service. Think it will be best to actually get the diff in my position before attempting anything otherwise may be off road some time!
Thanks again to you all
Mark

When the rear axle was originally built, a small metal tag was attached to one of the bolts with the rear axle gear stamped into it. It is possible that this tag is still in place. Crawl under the rear of the van with a bras bristle brush and some sort of solvent and see if you can't find the tag.

If you have an 8 3/8 rear axle, you are in luck. Chrysler used that rear axle in many many vehicles through the 1990s, including Jeeps. I know Jeeps were sold in Europe, so try and find a 90s era Jeep with the same axle and the right gear ratio. I believe Jeeps used a version of the 8 3/8 rear axle that is compatible with the older 8 3/8 axle through 1997. Do a bit of research and count the splines on the axle shafts.

Hi Reed
Had a look under the van at the axle and found what looked like "two tags"?????, they are in place of the washers that the bolts go through in two positions and are a square shape with a semi cirle on one end. One has "6110" stamped on it the other has "25" stamped on it, can't see anything else there ns part of the 25 was covered by the bolt but thats what I think it says.
Is that any help?
Mark

There are often two tags. One is some sort of internal Dodge reference number, but the other should be the gear ratio. Was the tag that read "25" clean? Go back and look at it very closely and bring a rag to scrub it a bit. I bet it is either "3.23" or "3.55."

Any chance of some pictures?

Hi Reed
Srubbed both tags and definitely on 6110 on one and 25 on other, heres a pic of the diff housing and it has 10 bolts on it.


you can see at the 4 o'clock position what I am assuming is the tag with the semicircular top to it under the bolt head.
Regards
Mark