Rear end Gear Ratios

[340_6pak]

I finally got the Challenger out for a little shake down driving, and I was suprized at the rpm's at 70. A whole 3100. The rear end is a 3.9 and I know that is a little on the far side, and it runs great in town. The highway is another story. So what to do? Does anyone have an overdrive unit, or should I just change out the gears?

The engine is a 340 6pak, 727 trans, and a mix of in town and highway driving gave me 10 mpg. I dont care about the mileage, but the engine at 3100 for a 100 mile trip might be a bit much. 

Oh yeah, the rear is an 8 3/4 and the kick down is working like it should.

[422STROKER]

Sounds ok to me.  What size tires do you have?  I'm running a 3.23 with 26" tall tire and turn the same basic rpm as you.  Just got home from Carlisle PA last weekend, it's 410+ miles each way.

Tom

Gear Vendors, Keisler trans has an overdrive, and the 518 swap can be done though.

[340_6pak]

275 60 15's. Guess I left that out.

Thanks for the input. I just thought it might be nice to have the engine not work quite as hard when highway cruising.

[422STROKER]

Yes the 275/60/15's 28" tire would help mine lower by quite a bit but for you there is the 255 70 15's 29" tire for a little more height but a swap to 355's or 323's will be the cheapest and easiest solution I'd gather.

Here's a cool calc.

http://vexer.com/68rt/speed.html

It shows the 391's and a 28" tire you'd be 3400rpms  355's would be 3100rpms

Tom

[tactransman]

http://www.cuda-challenger.com/cc/index.php?topic=23678.0  Good reading!   

[340_6pak]

That calc is great! It pretty much verifies what I saw on my dash today. Thanks for sharing.

[Moparal]

275/60/15's arent really 28 inches tall. More like 27.5 once you are mounted and on the ground.  I run a 410 dana and 440-6 4 speed.  I just let it go at 3000 or 3200 and dont keep looking at the tac.  Biggest thing is your oil will break down quicker.  My oil pressure and engine temps stay good.  If it blows or something wears out, I will just fix it.   And your right about the 10 mpg at 3100 or so. Mine gets 10 at 2600.  But I mat it alot too.  I like to run it up to around 5k then let off and 4500 then let off. It getting boreing driving a 4 speed muscle car and just hwy cruisin.   My K&N filters lets me know when my carbs start singing to me...I just love it. I never can here my exhaust with a factory system front to tips.

[340_6pak]

I have headers and flowmasters, and I am just starting to get used to the sound. With the dynamat on the floorpans it sounds ok. Scares little kids and the neighbor's dog when I fire it up! 

I like the 6 pak too, but my ear is not tuned enough yet to hear the carbs. The middle one has a bigger than normal jet in it because the engine is bored 60 over. It has a little larger cam in it, and the ac will kill the engine at a stop light. I plan on changing the oil every 3000, and oil pressure is good. The tempp gauage did go to the 1/2 mark today. First time I have ever seen it that high.

[73Chally]

Is your car black?  I was driving down Hwy 66 last week and saw the arse end of what looked like a black 72-74 in the self wash place in town.  I didn't see much of it, bit it was the first black one I have seen around here.

[BIGSHCLUNK]

Sounds ok to me.  What size tires do you have?  I'm running a 3.23 with 26" tall tire and turn the same basic rpm as you.  Just got home from Carlisle PA last weekend, it's 410+ miles each way.

Tom

Gear Vendors, Keisler trans has an overdrive, and the 518 swap can be done though.

Same thoughts here,323's 275 BFG's......... 3100 at 70 sounds ok...

[Fleet 500]

Car Life's test 340 'Cuda with 3.91s went 61.8 mph @ 3200 rpm. With the stock E70x14 tires.

[Bullitt-]

Mancini has some deals on 3:23s for 489 rear
http://chucker54.stores.yahoo.net/choem8489car.html

[tommyg29]

wow...seems to me then that I dont have the dana 3.54 I was told I had. I just took a 130 mile road trip and at 70mph was running about 4200 rpm.
Have 275-60-15 radial TAs in back, so I must have something over 4.00.
And my tires are about 26-3/4" tall. Probably 27 or a little more at speed

[73Chally]

wow...seems to me then that I dont have the dana 3.54 I was told I had. I just took a 130 mile road trip and at 70mph was running about 4200 rpm.
Have 275-60-15 radial TAs in back, so I must have something over 4.00.
And my tires are about 26-3/4" tall. Probably 27 or a little more at speed

You need to check your tach.  Your tires are 28" tall, and based on the numbers you are giving, you would have somewhere around a 5.0:1 ratio.  Check out this RPM calc site:

http://www.csgnetwork.com/multirpmcalc.html

Also, here's a site to calculate tire size:

http://www.1010tires.com/tiresizecalculator.asp

[340_6pak]

Is your car black?  I was driving down Hwy 66 last week and saw the arse end of what looked like a black 72-74 in the self wash place in town.  I didn't see much of it, bit it was the first black one I have seen around here.

The 73 is triple black, but she has not been to a car wash yet.