Difference in trans = better milage

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MoparPat

MoparPat

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Hello, I don't know if this will be considered a performance issue but I have a 1971 340 Dart with a 904 trans 8 3/4 diff 2.76 ratio with small block pattern lugs. I drive it on the freeway doing 70 mph at about 3k rpm's. If I was to put in a 727 auto, would that up the mph some. I was told that the 727 auto has a higher final gear drive that makes the car drive faster with less rpm's. Thanks for all your help on this matter
 
MoparPat -
The final drive gear ratio in both the 904 and 727 trannys is 1.00, so no difference in RPM at 70MPH in 'Drive'. However, the 904 has less HP loss due to lighter internal components.
The standard gear ratios for both the 904 and 727 are as follows:
904 trans
1st: 2.70
2nd: 1.54
3rd: 1.00

727 trans
1st: 2.45
2nd: 1.45
3rd: 1.00

The only way to lower your 70MPH rpms is to go to a higher rear gear or install an overdrive trans.
 
Well Holy Crap. I might as well keep the 904 in there. I already have 2.76 highway flyers, guess I won't be cruising to far away from home, gotta get a trailer I guess
 
Install a MSD yet? They hide well under the battery and are good for a few miles per gallon.

How many are you getting?
 
Actually I'm using the chrysler electronic ignition, using the msd will make it better mpg, car runs real smooth at 3000 rpm's, don't know what the mpg are, never took it on a big trip. I am going to take to the spring fling in April at Woodly Park in Van Huys CA, thats about 48 miles from my house
 
LOL, that's a big trip? Wellll, OK then.:thumbrig:

Soorry man, I ran my Duster everyday at 50 miles one way to work.

The MSD's multi sparking ignition helps burn every last bit of fuel. This turns out torque and HP so now you can go father on the same amount of gas. The stock igniton does not.
 
I know, I should not be running so much rpm's with those gears. I'm running BF Goodrich 14 inch small bolt pattern 60 series tires

I will look for a good MSD unit for the car
 
An MSD or Mallory 6 box will do you right.
 
rumblefish360 said:
LOL, that's a big trip? Wellll, OK then.:thumbrig:

Soorry man, I ran my Duster everyday at 50 miles one way to work.

The MSD's multi sparking ignition helps burn every last bit of fuel. This turns out torque and HP so now you can go father on the same amount of gas. The stock igniton does not.
Click to expand...

I'm saying I have not driven it any where yet, it's that new, planning on going to alot of car shows this year, was just curious about rpm's and gears...etc...
 
I will have to get on-line at home with that info, I have to sets, one with cooper cobra's on stock ralley 14" rims, again can't remember the size, will get back on an put them both sizes on here
 
ocdart said:
MoparPat -
The final drive gear ratio in both the 904 and 727 trannys is 1.00, so no difference in RPM at 70MPH in 'Drive'. However, the 904 has less HP loss due to lighter internal components.
The standard gear ratios for both the 904 and 727 are as follows:
904 trans
1st: 2.70
2nd: 1.54
3rd: 1.00

727 trans
1st: 2.45
2nd: 1.45
3rd: 1.00

The only way to lower your 70MPH rpms is to go to a higher rear gear or install an overdrive trans.
Click to expand...

Early 904's had the same ratios as 727's. They started using 2.71 low gear sets, I believe, in the early '80's (Forgot exactly when.) But you have to be careful because that's when they started going to lock-up converters in both, and some parts, like the converter, will not interchange. So most, if not all, 2.71 low gear 904's will have lock-up converters. The gears can be swapped into the old cases. I just don't know the little things that need to be checked/changed to do so.

But a later, lock-up converter 904 should give a little better mileage while giving a little better off-the-line accelleration also because of the 2.71 gear!
 
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