I thought this chart would be of general interest. I want to get a conversation going about selecting a rear for use with an OD trans. Here is a 904 with both narrow and wide ratios, and the 4 most common rear ratios. I currently get 20 mpg highway with the 2.71 and 904.



The estimated highway mileage assumes a linear ralationship, which is not going to happen,but is good for estimating. The question comes down to, will the higher numerical rear ratio improve in town mileage enough to offset the loss on the highway? Also, how much peppier does a slant powered car feel with a 3.23 over a 2.71?

The current 2.71 makes noise, even though I paid to have it set up. I have to pay (someone else) to do this again, but have the option of changing rear ratios. My leaning now is to get the Gear Vendors OD in there with the bad 2.71 and see if I like it before deciding. I am hoping those with experience out there will try to put into words how they felt about the performance of their slant powered cars with the various rear ratios. Thanks.

Sam

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I am hoping someone who has switched from a 2.71 to a3.23 will comment on how it effected economy and performance. Is there a noticeable difference in the feel with the 3.23?

Sam

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Since I'm the only one that has swapped streetable ratios in my dusters back and forth out of either necessity or to match the powerband of the engine (very important)...my 'swap' actually went from a 3.21 (failed bearing), to a 2.94 (had on hand to bolt right in...ended up with a seized bearing after 3 months and had good lube at fill level), 2.71 was the last bolt in axle I had...that was in my '74, on the Hpak duster it went from a 2.94 to a 3.55, and is now a 4.56...

Unfortunately mopar automatics don't last under my foot, so all comments are a late duster using a manual transmission, and 205/70/14 tires:

2.71/2.76-very slow to get off the line, and high revs needed to motivate the car even with a 3.09 1st gear...even then, it required some clutch manipulation to keep the stocker slant from stalling out when clutch actuated...Mileage in direct was fine, did not have OD to try, but have driven stock Duster with OD and 2.94...2.71 in OD would probably cruise at 80 mph without issue/lugging...no place here to do that safely.

2.94- Better off the line than 2.71, you won't win a heads up speed contest dropping the hammer on these, highway cruising in 3rd and OD OK, stock slant works in OD at 65mph, but lugs a bit at 55....in town they are a good gear to work with...If no OD option I like these overall for daily driving and mileage.

3.21- Easy off the line, in town is very workable, no mileage increase over the 2.94 in city/light-to-light, on highway the rpms are definitely up from the 2.94 in direct/3rd (can be annoying if you have driven with 2.71's all your life), notice a drop of .5 mpg if only using direct, no mpg drop if using OD, but OD now useable at lower mph without stall/lugging.
You can get the .5mpg back if you swap up to a larger tailpipe and/or do some "upgrades"/"adders"

3.55- Very easy off the line, and if you use highway gearing most of your days, the term "fun" or "spirited" becomes the buzz word. Never noticed a change in the light-to light mileage, but found I could use 3rd gear at 40-45 mph without complaint, especially if the car was upgraded (9:1 SCR, oversize exhaust, etc...)...overall gas mileage may suffer due to "sport" driving due to the reaction of the car. When coupled with an OD it will cruise along and return good gas mileage, and it gets better if you match it to a cam and engine that like it at 2300rpm or so (and it gives the car the ability to motivate well enough that I still had the ability accelerate above 120 mph, if I had more road to push the limit a bit more....top speed per the tach that day said 127 and I still had 1500 rpm until redline). You can barely get to OD during the 1/4 mile to use it.
Much easier to motivate cars with fatter tread, than stock. 3.55 and a dialed in engine seems to only give about 18-19 mpg highway best, if you only have direct/3rd gear for your top gear.

4.56- So easy, you can let the clutch out at idle and it will accelerate on it's own from a stop light like it drove itself...then you have to give a little gas in second gear to keep the engine happy. Mileage on the highway with OD (3.31 final ratio) and a built engine in it's sweet spot is about 21 mpg tops...I think a mild motor would be limited since this is a little more than a 3.21 with direct...Acceleration is better than above gearing, rpm recovery is better between shifts during speed test compared to the 3.55, still best to leave in 3rd gear during speed test once there, but redline comes up faster once there and acceleration flattens out a bit at top end.

I have one more 8 1/4" rearend that was a 2.56 rear, when I get back to work on the FDX project again, I will be using 3.91's with the OD to get a nice 2.85 rear ratio (the kinder side of 2.76, but not quite the 2.94's...if I decide on a bigger 15" rim and tire for street like the 225/60R15, then 4.11's and the 2.99 in OD would be the ticket).

Hope that gives you a better general impression of how each gear "feels". The topic is subjective, as a healthy "lou" build is apt to enjoy the better gears and overcome a fatter tread/rolling resistance and still return a decent amount of mileage with the OD gear as long as everything matches the rpm band sweet spot. Conversely the lower compression stock style motor may not motivate at lower rpm as well and may get help from the better gearing light to light but may 'flatten out' at upper rpm when used on the highway...A Ted build would fall in between and benefit from either 2.94's in direct and possibly give better acceleration with the 3.55's (keep up with the modern cars outside), but return good mileage in OD.

2 cents,

-D.idiot

Thanks much DI! That is very helpful. I will read and re-read this mini treatise for awhile. I am open to eventually building another engine of whatever design you would recommend. Goals are drivability, smooth, quiet, decent gas mileage in the low to mid 20's and acceleration that does not feel too lethargic. My 2.71 gears I would describe as a bit lethargic, but not anemic. This engine has an RV cam in it, and pulls like crazy at low rpms.

Where is the stock cam happiest RPM wise on the highway? Maybe the ticket is to leave the 2.71's in for now, and then plan on matching an engine build to whatever gears I ultimately switch to. This is a long term project for sure. I have decided the GV OD is next on the list. Gotta save my pennies.

Sam

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This may be apples to oranges, but anyway...

I used 2.76, 3.23, and 3.55 gears in my 65 Valiant sedan (170, bbd, 252s, A535 Dakota tranny, resonator only, 225/50-15 track and 235/60-15 street) and the 3.55 was the clear victor all around.

It drove fine with the 2.76, helped off the line by the low first gear and relatively lightweight car, but it really came to life with the 3.55s and still cruised nicely with the OD.

If you're set on an OD tranny, I definitely recommend mid-3's in back.

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"A mechanic is Somebody."
- Jim Preston

Thanks again. Those are again helpful experiences to share. What is an A535? What is the final drive ratio in its highest gear?

Sam

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New Process A535, later called NV2500 apparently, has a .79 OD ratio.
The other gears are 3.8, 2.3, 15, 1.
Mine came out of a Dakota 4 cylinder (88 or 89, I can't remember now).

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O==\=/==O

"A mechanic is Somebody."
- Jim Preston

I love this thread! I just picked up an 8.8" Ford, narrowed to fit my '64 Dart and was looking at Gear Vendors as well. My 225 engine is being pumped up (9.2 compression, aussie street cam, 4 barrel, dutra headers) and should develop closer toward the 200Hp mark than original 145. This is going to be a weekend city/highway cruiser, no racing. The guys at Gear Vendors have told me their OD units only work with '66 and newer transmissions as the older 904's have a rear pump in the housing that cannot be removed. So now I'm back to Square 1 on an OD unit that keeps the original pushbuttons. A lack of OD is pushing me away from my intended 3.5, and down to a 3.2 as that should give me enough low end driving the hills of San Francisco and still be able to maintain 65 mph on the highway without driving me crazy.

Group thoughts?..

Just a FYI. If I remember correctly. If you use a manual valve body, the rear pump can be eliminated. It's been many years since I built mine. I know the governor gets removed, but I don't remember for sure about the pump. That said, a manual VB can get tiring on the street, if the PB is on the dash. That's why I mounted mine in a console between the bucket sets.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225

I think the case for the 65-down trans will not fit the Gear Vendors either. Might be easiest to just go with 3.23 rear gear and not use the GV OD. You need quite a lot of trans/driveshaft tunnel enlargement on a pre-67 car to fit that OD, no matter what Gear Vendors says.

Lou

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Home of Slant6-powered fun machines since 1988

Some folks just get a very large hammer and have at it when putting a gv into 1963-66 a bodies


Greg

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http://www.youtube.com/hyperpack

There are some persistent mysteries in life....the GV OD is one of them. Big, heavy, long, and an OD ratio that is not much of an OD. But they seem to stay in business so people must keep buying them.

I could see them in a truck or motorhome, perhaps. Someplace where the size and weight doesn't matter much, and every bit of gearing helps on those long mountain grades. But in a car? Especially a smaller car with a 6? No way.

I don't think I would run any sort of an OD with a rear gear in the 2s, or even say a 3.07/3.08. and with the 3.2x gear it would depend on how much tire I was running out back
Now a 3.55 or higher number? I'd have no problem running an OD
When I had my (last) wrangler, a 4.0 inline 6 with a 5 speed and 3.07s the OD 5th was about useless to me below about 60.