Which Rear End Ratio with Overdrive

[Chatt69chgr]

For a more or less stock 1969 440 what would be the optimum rear end ratio to use with a 0.70 overdrive?  Assume you are running at 70mph and have 28 inch tall tires.  I'm thinking it more or less will depend on how low you can run your engine rpm and still develop any hp/torque------this would be for interstate cruising.  I played with some of the calculators and it seems that if you ran a 3.54 then you would end up with engine rpm at something like 2100 at 70mph in overdrive.  Wouldn't that be too low rpm?   

[68X426]

It depends on the cam and where it's power range is located. 2100 sounds low, you'd probably be wanting to always downshift. Especially if you ever see even one hill in your travels.

Consider that with 4.10s the rpm increases to 2450 (70 mph, .70 od, 28 inches). Most cams would have 2500 in the lowest end of their range. And 2500 would feel really nice and sure help with hills and acceleration, with small effect on gas mileage.

[thedodgeboys]

I have 4.10's in my car with a .5 6th gear and 27 inch tall tires and it works out great. (I think 5th is close to .75 I don't recall) my point is I would lean twords the 3.92 if you'r plan is lots of 70 mph cursing that would give you a good street gear around town and let you open her up on the highway. 

[Cooter]

2.5:1 in Overdrive in order to be at your targeted range.

[Chatt69chgr]

Cam is a Engle K56. 

[68X426]

I have 4.10's in my car with a .5 6th gear and 27 inch tall tires and it works out great. (I think 5th is close to .75 I don't recall) my point is I would lean towards the 3.92 if your plan is lots of 70 mph cursing that would give you a good street gear around town and let you open her up on the highway. 

Your situation I think demonstrates my point. At least it sets the discussion for what chatt69chgr needs to evaluate.

5th gear for the dodgeboys: 4.10, 70, 27, and .75 yields 2700 rpm.
6th gear for the dodgeboys: 4.10, 70, 27, and .50 yields 1780 rpm.

If your cam has a power range as low as 1780 (is that even possible in a decent performance cam?) then you have a responsive motor. Seems to me that a downshift is in order to accelerate, whether it's an incline or a slow Brand X you're passing. A downshift every time for me is not responsiveness.

chatt69chgr, the question as I see it is responsiveness of the total package. I personally don't like a lot of downshifting, and surprise, I have lots of hills to climb and slow Brand Xs to pass. So is your cam responsive at low rpm? You need to do some homework and find the crossover point of the low end of that cam and the gear ratio that works with it for your type of driving and your expectations.

In general, not always, the sweet spot for responsiveness is 2400-3000 rpm and cruising is lower (2000-2500). Remember you can change the tire height in the future.

[flyinlow]

I have a 446 with a Lunati 60303 cam (1800-6200). 28" tires 3.55 , .69 OD with a lock up converter. 2000 rpm at 70mph. Do alot of road driving. Works well. Seams to run a little better at 80 mph . ( yea the cops will buy that)  If I switch gears would look at a 3.73 or 3.91. A little more acceleration without losing much milage or noise. If I want to pass a car I just turn off the converter, normally dont drop out of OD. With half throttle with the converter unlocked the engine will run up to maybe 2800-3000 rpm durring the acceleration then drop back down as you let back off the throttle and the lock-up re engages. Perfection would be a 4.56 axle and a .50 OD and one notch milder cam.   Do you think I can fit a Gear Venders behind My A518 trans?

[68X426]

Don't see why not. The experts have to weigh in on this topic.

Here's a link to Gear Vendors and 518: http://www.gearvendors.com/hrdodge4s.html

Link to an article with 518 info at the 2nd page: http://www.carcraft.com/newlook/crc_act/116_0302_trans/index1.html