First Post - Axle Ratio question

[canadaler]

Hi there, hope somebody can help me out with this one. I'm looking to purchase a used full size Dodge, (hopefully extended body style) which will see a lot of highway driving. I've seen some other posts re: axle ratios and people seem to regularly talk about the (I assume) diameter of the differential, rather than actual ratios. I'd like something with as low a ratio as possible for the best milage (no significant towing involved.)

I'm trying to find out what the available rear ends were in the Dodge (1990's vintage) and what was "standard" (if there is such a thing.) I assume the smaller diameter differential means a lower ratio....something like 3:50 (or so) and larger (9.25"?) means something like a 4:10.

Was there a "standard" rear end matched with the 318 and 360 motors and how do I tell which ratio is in a van that I'm looking at?

Thanks in advance, hope someone can point me in the right direction. I used to have a '74 B100 "hand me down" and am looking forward to getting a van again to haul my bike to the races.

Oh yeah one more thing...(sorry this is getting long)... what kind of milage should I expect from the 318 and 360...thanks again.

[hurricane2]

The diameters refer to the diameter of the ring gear, and actually have nothing to do with the gear ratio. Dodge vans came with 7 1/4", 8 1/4", 8 3/4" (up to 73), 9 1/4", and 9 3/4" (Dana 60). Most 1/2 ton vans (100, 150, or 1500 depending on year), will have the smaller 7 1/4 or 8 1/4), whereas the 3/4 (200, 250 or 2500) will probably have the 9 1/4. Dana 60s aren't all that common. The larger te ring gear, the stronger the rear end.

That being said, what you are concerned with is the gear ratio. Most vans you will find will have a 2.73, 3.23, or 3.55 gear ratio. If the van has the towing package, it's a good bet it has 3.55 gears, otherwise they were an option the original buyer would have had to ask for. The lower the gear ratio number, the more highway friendly they are, but the slower the takeoff from a stoplight will be.

I would expect around 14 mpg with a fuel injected 318 (5.2), and about 10-12 with a 360 (5.9). Carburated vans will be worse.

Hope this helps.

[canadaler]

Absoluetly....thanks very much for the info. I'll be checking for stickers under the hood and tags on the differential to see what's there. I'm sort of partial to the 318 myself my dad had a Plymouth Fury with a 360 which always ran like crap....mind you that was back in the carburetor days when the first thing I used to do was pull off the automatic choke and go manual....another story for another time...thanks again.

[vandude]

Some of us are still stuck in the old carburetor days, and we still swap the electric for a manual choke.

[canadaler]

This is interesting, I was looking at a diesel Ford (sorry for swearing) a while back and it seems to me that 3:55 was the LOWEST ratio available with several of them being 4:10's.

Anyhow, I'm going to take a look at a couple of them this weekend and see if I can find the highway hauler I'm looking for.

[hurricane2]

That would make sense, diesels make power at low rpms, and the numerically higher the gear ratio, the quicker it puts the power down.