I have a dodge project going. 5.9/auto. Rolled the dice on a rh46. Turned up snake eyes. Before I spend more on an od over a 904 or 727, just wondering what the impact would be on mgp. With a 5.9 selection it's going to be thirsty either way. So with other factors equal, anyone have hard numbers? Less rpms would be easier on the engine, but at 2000 to 3000 miles of driving a year, what's the return on od cost? Thanks

A little more info on the project would help. If it is a car with decent aero and steep gears od should add several mpg. If gears are longer and of puts it in lug not so much.

If you count fuel cost only probably will not pay. If quiet cruising and longer range are important to you may be a go.

It doesn't make economical sense if you run the numbers.
The satisfaction factor is off the charts. I did 2 separate drives this last weekend of over 2hrs each at 70mph, no way in the world would I have considered it before the OD swap.

I spent $2500 on my swap, went from 727 to 518 with LU.
70 Challenger with 340 and 3.73 gears.

Gas mileage improved from 12 to 16 on the hwy.
My 06 GT Mustang with 3.73s and 5spd auto only gets 19 on the hwy.

I would not bank on more than a 3 mpg increase. Its possible to get more but depends a lot on your engine build and rear end gearing.

Your mileage may vary...really.

I am in the process of going to overdrive and that was highly motivated by my lack of desire to buzz down the highway at 3400rpm. I didn't want to drive the car with the deep gearing, but I didn't want to give up the acceleration either.

I bet my mileage per year quadruples once the car hits the road again. The mpg numbers will hardly ever work out by themselves. Something else has to be included to determine if it's worth your time. For me, it'll be more fun to race and I will definitely hop in it for more drives, which makes me happier!

It's not just what you save on fuel, you will see a return on your overdrive investment when, or if you decide to sell your car. As long as there is no cutting involved, you should be able to recover a good $1000.00 back.

I would take a nicely done automatic overdrive conversion over a 3 speed auto dinosaur any day.

Does anybody have real #'s for the performance difference a 46RH will cost over a 727? How much extra HP does it use?

The other factor is you save a lot of wear on your engine when you install an OD. I had lots of trouble sorting mine, but now I'm glad I have one.

aNUTter factor ....if your motor does not have the "stones" or the vehicle has turrrible aero to pull that shorter gearing ..... you probably will get worse MPG.

I have done a bunch of those swaps and see typically 2-3 mpg. I have never seen a car lose MPG going to a higher final drive. I plan to do an experiment in my dakota at some time and really push the limit as in 2.45 gears AND OD for a final drive of 1.69.

Your math will vary depending on if you are buying a trans either way, you can get a 727 with a few mild upgrades for about $750 and a 518 for about $1200 so only about $450 difference (my prices, shops in your area will vary in price and quality). If you are making a drive shaft for something than the 518 shaft will be slightly shorter and should be a couple bucks cheaper and a shorter shaft don't mind the RPM as much. The only performance difference is a a little friction from the extra clutches that are not engaged, a few pounds of rotating mass and a couple pounds of non-rotating, probably no more than 1 or 2 tenths, couple in the fact you can run a little lower gear and there should be no real performance difference. Also if you are doing the work than you may or may not have some extra time into it, if you are paying someone to do it you may have some extra dollars into that.

I upgraded my Duster to a 44RH and it cost me about $800-$900 including the cost of the core. Rebuilt the unit, upgraded some parts and purchased a MP lockup converter.

MPG gains were not outstanding (I do have a heavy foot) BUT I'm now able to pass everything on the highway without pounding the motor to death and the cabin noise is greatly reduced.

Highway cruise is now 2100 vs 3500 RPM

The cost of upgrading from a 727 transmission to an overdrive unit is well worth the investment. The additional costs of converting to overdrive will pay for themselves in fuel savings over time. In addition to fuel savings, drivability of your Mopar will greatly increase. The reduced noise level from your engine revving makes long-distance driving much more comfortable. And, sustained driving at a lower RPMs puts less wear on your engine.

My Cuda has a Vortech supercharged 408 attached to a non-lockup a 518 transmission. It was one of the best upgrades I did to the vehicle. Because of the transmission, I drive the vehicle a lot more and I put regular highway miles on the car. On the highway, I get about 17 miles to the gallon. Years ago, I had a 1986 Mustang with a Vortech supercharged 351 and a five speed. At best, I could only get about 18.5 miles per gallon. My Mustang and of the Cuda weigh about the same and they both share the same fuel injection system. I would imagine that if I had a lockup a 518, mileage would be a little better.

Based on my experience and the enjoyment I get out of driving my Mopar anywhere and everywhere, my vote is that you spend the extra money converting overdrive. In fact, the conversion to overdrive made more of a difference to gas mileage than switching to fuel injection.

I daily drive a 73 Dart with 318 & A518 OD.
Going from 904 and 7-1/4" axle to 518 and 8-3/4" rear end didn't improve my mpgs, but it certainly made the car way more fun to drive.
I've got 3.55 gears in the rearaxle.

I say tuning your engine to operate more efficient in the most used rpmrange will net better mpgs easier.

A lot of variables, what gear are you running, tire size, average cruise speed. Most engines that we deal with are built and cammed to pull on the road at 2000 rpm or better, get them below that and you are labouring it all the time eating more fuel. I am putting a O/D in my 77 short box truck with 3.91 gears and 28" tire, most of my driving at 55-60 mph so it should fine. My daily drive is a 92 W150 - 318, the O/D and lock-up quit working early Dec., my fuel mileage went from avg. of 12 to 13.5, turns out it better with out, not going to fix.

Thanks for your opinions. A 518 was installed yesterday. Some small items to tweak and should be ready for a test cruise.