727 w/overdrive

[Glide]

#s match 73 340 Cuda w/727 auto, 2500 stall converter, 323 gears. stock 15" tires, 4000+ rpm @ 70 mph, 9 mpg. What are my options to lower rpm and get better gas mileage?

Thinking of overdrive transmission. Does anybody use Gear Venders over/underdrive? Do you have to cut the drive shaft? Will taller tires change rpm much?

Will a smaller carburetor help my mileage much? It must be a 750 cfm. Is it my converter? You can literally watch the gauge go from F to E and I don't even drive aggressively. Help!

[Katfish]

You could change over to a 518.
GV might be easier, but may cost more.
Either way, you'll need to shorten the DS.
Other things you mentioned will not make much of a difference.
Upgrading to a 518 made my car so much more driveable.
275/40/18 (26.7" tall) rear tire, with 3.73 gears.

[MoparLeo]

Never was a 15" tire as stock on that car. The rim size is just the size of the hole in the center of the tire,
What matters is the diameter of the tire.
#s matching 340. But you don't say anything about the engine build. That just means the original block was used.
So you need to give information that means something to help you about MPG.
 Engine build specs, tune, tire diameter etc.
Here is a calculator to help you.
http://www.wallaceracing.com/Calculators.htm
http://www.wallaceracing.com/calc-gear-tire-rpm-mph.php

[chargerdon]

i changed out my tranny on my 74 challenger with 360/408 to a 518 and it uses a lock-up torque converter.   A lot of work.  Floor shifter.

Spent $300 on used 518/rh, $125 on rebuild kit to insure the seals and clutches were in good shape (about 4 hours doing the kit), another $220 to have driveshaft shortened and new U joints. Another $50 on the transmission crossmount which i had to fab myself..  Another $30-60 for correct speedo gear.  No mods are needed on the car itself but some webbing had to be cut off the aluminum transmission case.   Still it is a tight fit and biggest problem is with getting the speedometer cable connected as it is right against the Transmission crossmember. 

Results with my 235x60x14 wheels and tires (25" height) at 70 in high rpm is right around 2900 then with overdrive engaged it drops to 2400 and then engaging the lockup tc it drops to 2100.   Fuel mileage in it avgs around 8-9 mpg with local driving and jumps to 17-18 on highway 70mpg.   

Note: with this transmission it shifts hydraulically into 2nd and 3rd gear.  The shift into OD and Lockup is by two wires into the transmission...one for od and one for LU.   Many people put a pressure sensitive switch into the tranny at the transmission pressure point..usually a 50 lb switch and a 10 second timer.   I just installed 2 manual switches.  just have to remember to turn off the LU switch or it'll shudder bad when u come to a stop.  The OD switch hardly matters as it starts in low, shifts normally into second, then normally into third and about 2 secs later into OD.   That engine has so much low end torque it runs fine. 

[7212Mopar]

Assume your 15" wheel with tire is around 26" tall, with 3.23 rear, you should be around 3000 RPM. The factory tach is likely off. Factory fuel gauge is also not very accurate. Best to calculate mileage between two fill ups. I have a 340 stroker, 727 and 3.23 rear. Car runs at around 3000 RPM at 70 mph. RPM readings from an Autometer tach and the Fitech EFI display. Speed is from a Garmin GPS and my dashcam GPS. I get about 8-10 mpg city, 12-14 mpg on the highway. I do pedal to the floor short pull on the freeway once a while to have some fun [when it is safe to do].

At one point I was thinking about the A41 from Silversport Transmission. One member is or soon will be doing an install. I am waiting for him to see how it goes.

[Glide]

Lots of options! Gear Venders still seams like the easiest and least invasive solution. The other options require to many "other" modifications including modifying the transmission tunnel which I don't want to do as I just finished the interior. Thanks for the feedback. That's going to be "next". This week I get Borgeson steering box mod.

[blown motor]

I would say a 750 carb is a bit big. I have a 650 on my 340 with some modifications for more power, cam and headers. No lack of power. 2500 rpm at 60 mph with 255/60-15 and 3.23 gears and 904 transmission.

[tparker]

These cars eat gas, no doubt about that. A smaller carb shouldn't really help with economy since it is thee vacuum that draws the fuel out. A bigger carb just allows more to be supplied, rather than it "sends" more all the time. It just uses what it needs. Though maybe it needs some tuning. Adjusting the jets, rods and/or  the accelerator pump could help a bit if they are oversized for your needs.

Of course things like gearing and tire/rim sizes plays a role. 323s doesn't seem  to crazy. 9 mpg seems a bit low, how is it running? I assume the engine is well tuned. Spark plugs, wires, Points (if it has them), time, etc are all good? 

[Cudajason]

Lots of options! Gear Venders still seams like the easiest and least invasive solution. The other options require to many "other" modifications including modifying the transmission tunnel which I don't want to do as I just finished the interior. Thanks for the feedback. That's going to be "next". This week I get Borgeson steering box mod.

I have not done a gear Vendors, but don't they require a few whacks with the hammer to fit in the trans tunnel.

I second the 518 trans swap, the only mod is to the bolt on section of the trans cross member, no other mods to the car.  Yes, it's a lot of work, as  you are switching transmissions, but it is 100% worth it!

see my lengthy post here.  https://forum.e-bodies.org/engine-transmission-and-rear-end/4/rh-transmission-the-result/12648/   

Page 12 and 13 you can see my trans cross member.

[MoparLeo]

Easy to see if it will make a big difference..
Just  drive your car at a lower speed at the target RPM and see what happens .

[screamindriver]

Let's face it... 9 MPG means something isn't right..Not for the vague description of your drivetrain combo.. You'd be better off finding the problem{s} and correcting those before the Gear Vendors band aide.. With that being said... Figure out what rear tires you want to run before you give them a call they need to know your rear tire diameter to give you the correct gears...If you change afterwards you'll need a different gear to run your signal generator...Yes you'll need the DS cut, and the rear axle angle needs to be spot on per the instructions..But again 9 MPG with 3:23s is just horrid for that combo...

[PLUM72]

Check out this video from CRT transmission on the 518/46RH...
https://youtu.be/yt8DrrYyUV0?si=RiaFcUlWfIGq7Slk