Best gear ratio

[Alan73Charger]

So this winter my top priority other then suspension stuff is the gear ratio in my 73.  Below are the facts as I know them.  Any input on the best direction would be appreciated.

Tire diameter 27 inches.
Rebuilt 318.  Comp Cams 20-221-3 with a 1000-5200 rpm range.
Believed to be the stock torque converter.
Edelbrock dual plane intake and 4br. carb.
Stock exhaust manifolds at the moment but headers are bought and coated and I hope to get them on the car soon.  Straight back dual exhaust.
I believe the current ratio is the factory 2.71, based on the tag on my 10 bolt rear end.

I'm basically torn between a 3.21 or 3.55.  If my calculations are correct at 70mph highway speed I would be right at 3000 rpm with the 3.21, and approx 3300 rpm with the 3.55.  I don't drive the car often but when I do it's street use, not raced.  I would just like some better performance at acceleration. 

Any thoughts or suggestions?

Thanks
Alan

[440]

According to the spicer calculator;

3.21 gear with 27" tire @ 70 mph = 2871RPM
3.55 gear with 27" tire @ 70 mph = 3175RPM

Personally anything over 3000 at cruise is getting up there.

[Laowho]

Yeah, we have 3.23 and 29" tires, and we're still adding OD when the money is there. Difference for us b/t 2200 and 2800 rpm will be much appreciated, but we also prefer highway driving.

[Troy]

Meh, who only drives at 70 mph? 3.23 with OD!

Troy

[Alan73Charger]

Meh, who only drives at 70 mph? 3.23 with OD!

Troy

Hey Troy,

Someone else mentioned the 3.23 and overdrive also.  That's not available to me without changing to an 8 3/4 axle correct? 

[Alan73Charger]

According to the spicer calculator;

3.21 gear with 27" tire @ 70 mph = 2871RPM
3.55 gear with 27" tire @ 70 mph = 3175RPM

Personally anything over 3000 at cruise is getting up there.

Thanks 440.  So I'm about 130rpm high on my estimates.  I agree on the over 3k rpms.  I guess I have to decide if I'm going to see a major enough improvement going from 2.71 to 3.21 to make it worth the cost.  I can get the ring and pinion for under $300 but will have to pay someone to do this work.  I was looking at the Yukon brand, not the cheap stuff.   The jobs a bit above what I would try to tackle myself.  What should I expect to pay and what sort of shop would be best to contact?  A general mechanic or more like a transmission shop?    Thanks again.

[BSB67]

According to the spicer calculator;

3.21 gear with 27" tire @ 70 mph = 2871RPM
3.55 gear with 27" tire @ 70 mph = 3175RPM

Personally anything over 3000 at cruise is getting up there.

Thanks 440.  So I'm about 130rpm high on my estimates.  I agree on the over 3k rpms.  I guess I have to decide if I'm going to see a major enough improvement going from 2.71 to 3.21 to make it worth the cost.  I can get the ring and pinion for under $300 but will have to pay someone to do this work.  I was looking at the Yukon brand, not the cheap stuff.   The jobs a bit above what I would try to tackle myself.  What should I expect to pay and what sort of shop would be best to contact?  A general mechanic or more like a transmission shop?    Thanks again.

I wonder if today anyone can actually figure this out on their own.   You are no more high on your estimate that he is low.  But they are in the ball park for decision making purposes.

[Laowho]

I didn't pull up your cam Alan, but according to the rpm range, and comparing it with our relatively mild cam (1600-5500...our cam is so mild, the TC manufacturer said we needed a 2k stall instead of the 2400rpm that's in it), I'm gonna go out on a limb and suggest you wouldn't wanna go above your 3.21 option (nix the 3.55 consideration). Others will say if I'm wrong. All I can tell you is that I was a little frustrated (tortured for weeks) that we were leaving power on the table, but in the grand scheme of things, our 3.23 differential (and preferred highway driving so the rear was good for us) pretty much determined everything, from the cam on down the line. Took me a while to see the soundness of our set up.

And we're really up in the air about the OD--whether from an A518, a GV bolt-on, a Passon 4-speed A833, or a Tremec 5--just in case you'd have a preference Troy.  

[Alan73Charger]

I didn't pull up your cam Alan, but according to the rpm range, and comparing it with our relatively mild cam (1600-5500...our cam is so mild, the TC manufacturer said we needed a 2k stall instead of the 2400rpm that's in it), I'm gonna go out on a limb and suggest you wouldn't wanna go above your 3.21 option (nix the 3.55 consideration). Others will say if I'm wrong. All I can tell you is that I was a little frustrated (tortured for weeks) that we were leaving power on the table, but in the grand scheme of things, our 3.23 differential (and preferred highway driving so the rear was good for us) pretty much determined everything, from the cam on down the line. Took me a while to see the soundness of our set up.

And we're really up in the air about the OD--whether from an A518, a GV bolt-on, a Passon 4-speed A833, or a Tremec 5--just in case you'd have a preference Troy.  

Thanks Laowho - yea I've pretty much nixed the 3.55 thought.  Did you end up changing the converter?  If so what did you choose? 

[Laowho]

Nah. It's brand new with the rebuild. Did take it out to flash it tho--flashed right about peak torque--while I was still tryin to figure out how to "get more out of it." Think we all get bitten with that. (Local builder wanted to put a 3k in.) No, it works great for a driver and the throttle response is more where we want it (entrance ramps). Dunno where the kick down is spaced for, wh/ I guess could be another adjustment to fiddle with, but we like the way it "behaves." Shifts anywhere from 1800rpm to about 2500, and flashes WOT at about 3800. All good.

[Alan73Charger]

Nah. It's brand new with the rebuild. Did take it out to flash it tho--flashed right about peak torque--while I was still tryin to figure out how to "get more out of it." Think we all get bitten with that. (Local builder wanted to put a 3k in.) No, it works great for a driver and the throttle response is more where we want it (entrance ramps). Dunno where the kick down is spaced for, wh/ I guess could be another adjustment to fiddle with, but we like the way it "behaves." Shifts anywhere from 1800rpm to about 2500, and flashes WOT at about 3800. All good.

Goals!

[Laowho]

Definitely got some goals. Some rear suspension updates and subframe connectors/torque boxes will have to go in before we sound treat the cabin, and just began reupholstering and added 3 point seat belts. Heater box needs to be refurbished and gauge cluster recalibrated with new circuit board, so will do the sound stuff with the dash out and havin access to the firewall. And while everything's out gotta strip the lacquer paint job off, weld some spots, look under the vinyl and then paint. One little thing winds up bein a complete tear down.  Somewhere in there we'll get OD.

[68CoronetRT]

I'll throw in my .02.

Running a 28" tall tire, 3.91 gears cruises at 3k rpm at about 65, adding a GV and swapping back to an auto here shortly.

Doing a 3500 stall and manual valve body incase you were wondering.

[Troy]

Meh, who only drives at 70 mph? 3.23 with OD!

Troy

Hey Troy,

Someone else mentioned the 3.23 and overdrive also.  That's not available to me without changing to an 8 3/4 axle correct? 

I answered before I read all the details!

Most of my cars have had 8 3/4 rears (a couple with Danas) so when I reference the numbers just use that as a reference for whatever is in the same range.

Honestly, if you had power, I think a 3.55ish gear with an OD trans or 3.23ish without OD would be very fun on the street. Most all of my cars have manual transmissions so I don't think much about converters. I tried 3.91s and they were a blast stoplight to stoplight but I was limited to 61 mph on the highway to keep the outboard carbs on the Six Pack from opening. 61!!! I yanked them out within a week. On that same car, 2.76 gears made it way to hard on the clutch trying to get moving. This wasn't a problem with my other Charger with a 440/auto or my 318/auto cars. I've settled on 3.23s and I can cruise around 73 mph reasonably comfortably. My 318 cars have always been great highway cruisers with the stock 2.76 gears. However, acceleration is nothing special. At least a 383 or 440 will still make it somewhat exciting.

Honestly though, without doing a bunch to the motor I wouldn't even bother with the rear axle. Yes, gears could really "wake up" the car but I don't think you're going to see a world of difference. For the same cost you could probably get some decent heads on that thing.

Troy

[c00nhunterjoe]

Going from a 2.76 to a 3.23 is a waste of money in my opinion. You can figure about 700- $1000 to take it to a shop parts and labor. (Gears, bearings, seals, oil). I dont feel that 318 is going to feel any different and a gear swap doesnt make a car "faster" despite popular belief. You trap speed will not change. Spend the 1000 dollars on the engine and it will both feel and be faster.

[440]

According to the spicer calculator;

3.21 gear with 27" tire @ 70 mph = 2871RPM
3.55 gear with 27" tire @ 70 mph = 3175RPM

Personally anything over 3000 at cruise is getting up there.

Thanks 440.  So I'm about 130rpm high on my estimates.  I agree on the over 3k rpms.  I guess I have to decide if I'm going to see a major enough improvement going from 2.71 to 3.21 to make it worth the cost.  I can get the ring and pinion for under $300 but will have to pay someone to do this work.  I was looking at the Yukon brand, not the cheap stuff.   The jobs a bit above what I would try to tackle myself.  What should I expect to pay and what sort of shop would be best to contact?  A general mechanic or more like a transmission shop?    Thanks again.

I wonder if today anyone can actually figure this out on their own.   You are no more high on your estimate that he is low.  But they are in the ball park for decision making purposes.

Of couse measuring your actual tire circumference and calculating roll out and working backwards would be the most accurate method, the generic calculators are just that, ball park figures to compare.

Gears and converter can make or break a car...

[BSB67]

I don't think roll out has anything to do with it.  The math is the same whether you use an on line calculator, or just punch a few numbers into your calculator.  The torque converter slip is an estimate.   My point is there is not anything special that makes one calculator better than the next.

[440]

No, but measuring the actual circumference of your tires and calculating the distance they travel per RPM/mile is more accurate then a "one size fits all" tire size figure.

[BSB67]

No, but measuring the actual circumference of your tires and calculating the distance they travel per RPM/mile is more accurate then a "one size fits all" tire size figure.

of course

[flyinlow]

 

3.55 with 518 O/D .... 2000 rpm @ 70 mph in O/D and lock up. 

Interstate 70 today... tomorrow the Autobahn!

Really Officer, my cam's rpm range is 2200-6200 . I have to go at least 80mph or it will damage the engine.   

[forphorty]

The change to a 3.21 or 3.55 will be a boost. You might be pleased with the change if you have reasonable expectations. You have to consider your long term performance goals. I know you said you have no plans to race it. Even so, think about the  cars you have ridden in  and consider what their drag strip performance might be.  The typical musclecar of the sixties ran in the 14 to 15 second range.  A few( Hemis, Six Packs, etc.) would go 13s. Decide the rate of acceleration you will be happy with. Keep in mind a new v6 Mustang will run in the 14s. Just guessing,  I would expect a 3800-4000 lb (?) gen 3 Charger with 318, XE 256, headers, stock converter and 271 gears  would be about a 16 flat car, maybe a little faster. Gear change might put you  in the mid 15s.  A little engine in a big car is going to need cam, converter and gears to run really strong. And heads. 318 heads are fine for the two barrel, single  exhaust equipped engines they came on. They are awful for performance. Having said that, I have a stock 318 with a mild cam, Edelbrock ld4b and headers that will be going in a 72 Duster I have.  I plan to eventually replace it with  360, but I'm poor and it may be a while. Anyways... I think the gears will be a noticeable  improvement. My long winded point is  just to be careful not to spend too much on a basic platform that might not meet your performance goals. My B body cars have big blocks. 400 in a 73 RR and a 440 in a 68 Satellite. A stock 440 with cam, intake and headers is an easy 13 second car. Add converter and slicks,12s.  With 3.23 gears. My two cents: drive and enjoy your car while saving for a bigger engine.