I'm looking at dropping my 518 in the Imp and wondered what kind of mpg increase you guys have seen.

The primary concern is to drop the engine rpm because I've become accustomed to the lower engine speeds in newer vehicles. Any mpg increase would be a bonus.

My old beast tips the scales at 5100 lbs empty and runs a 29" tire on a 3.23 rear gear. The 518 would drop the rpm nicely and make for a better driving experience. The 440 should have ample torque to pull the car at the new rpm.

I've had the transmission and Ultrabell sitting around for the better part of 10 years. Might as well do something with them.

I've wondered the same thing, so I'll be watching this one.
Are you going with a lockup or non-lockup converter?

Milage increase I don't know. However in my 422 ci duster 3500#with manual 5 speed od it would get up to 22 23 mpg on the highyway at 70 75 mph. This is with 34 ° maechanical and 12° vacuum for total of 46° Total timing. This all being very conscious of milage! But even when driving spirited and careful of the law 18 19mpg was the norm. Gears are same as yours and tires are very close 275 60 15 BFG drag T/As. Rpms at 70 is 1700 in OD

In reviewing the EPA's fuel economy numbers, it seems like overdrive often added 2 or 3 mpg. Some cars may do even better, while others may not do as well. It is a bit difficult to make comparisons because often the car manufacturers changed the axle ratio when adding overdrive.

I've heard of some folks losing MPG's by swapping an overdrive transmission into an older car. I suspect in these cases the car's carb had been running on the main circuit before the swap. After installing the overdrive, the lower RPM made the carb run on it's idle circuit. If the idle circuit is to rich, fuel mileage may not improve or could even be lowered.

The lock up torque converter becomes more important the slower the engine turns. I had a 1980's car back many years ago that had 2.4? gears in the rear axle. The original 3 speed automatic transmission had a locking converter. When that transmission went bad, I swapped in an older transmission that did NOT have a lock up converter. If I remember correctly, the highway cruising RPM's increased by 300 - 400, and the fuel economy dropped a bit.

dezduster,... what % OD is that 5th gear?

70 Challenger, 340, 3.73 gears, 518 with Lock-up,

Went from 10/13 city/hwy to 12/17.
Definitely improved, best part is the lower RPM's on the hwy, I have the lock-up set at 45mph so I get some around town benefit too.

I had no change but I did it backwards. Went from OD to no Od.

87 Ram D100 reg cab shortie. Swapped in a 5.9L Magnum and a 518. With 3.55 gear and a 28" tire.

I did not drive for mileage. I did much of my driving at a comfortable 120km/hr.

I lost overdrive but continued to drive the truck in 3rd gear. Except now 120 km/hr was too much rpm so I cruised the right lane at 105 which was a little high in the rpm but with the reduced speed I was still getting 18-19mpg.

After losing all confidence in the 518, I swapped in a NV3500 5 speed. Then I was finally had a mileage improvement.

I can't give any prior results, as I stuff an OD into the build from the start, but my Daytona 440/6 pack with a Keisler TKO 600 5spd .064 OD, 4:10 Dana 60, 27" tires, will do 20-22 MPG highway on long cross country cruises at 70-75 MPH, on the street where I have a hard time keeping my foot out of the throttle, it'll suck it down to 8-10 MPG

On the HEMI cars with a Keisler TKO 5 spd .64 OD, 3:54 and 4:10 rear gears, avg highway at 70-80 MPH cruising would be 16-18 MPG, again on the street blasting it, mileage could get sucked down to 5-8 MPG

Played around in the past with various OD's in the .80- .70 range, results were often minimal for the effort, you really need to step into the .64-.50 range for the cost of adding an OD to "pay off"

Adding the GearVendors to my '89 GMC 454 Dually, TH400 w/4.10s, picked up highway MPG by 3-4MPG. 5500 lbs empty, which it rarely is, usual roadgoing weight is more like 6000. I generally drive it casually, but when not towing, it manages to find 70 or so. The lower RPM is pretty nice too.

Exactly. I have a Tremec 6-speed in a 2005 GTO, 400 hp, 3.90 rear, 25" dia tires.

Sixth gear 0.50:1 is super - runs about 1800 rpm at highway speeds.

But 5th gear 0.75:1 pretty much never gets used - it's a waste, not enough reduction. You really need the 0.50:1.

If he doesn't see this thread, ask Stu...he has an OD (4L60E I think) in his Road Runner...and it gets driven a bunch of course..

The Charger got 10.7 with the 440/493, 727 and 3.23 gears. I got 12.7 once with the GV and a 3.91. The OD dropped it to a 3.05 final drive. The GV did help but I agree that a .78 OD is a pretty meager reduction.

I can't speak for mileage improvement, but I get 15 average in my 68 Charger 512cid,a518 w/lock up & 3.23 gear 28" tire.

2-bbl 318 / 904 & 7-1/4" 3.23 rear axle.
lousy, neglected engine tune / very lean @ highway speeds / worn timing chain;
6-7mpg daily / 17mpg highway

4-bbl 318 / A518 & 8-3/4" 3.55 rear axle.
AFR tuned engine / new timingchain / recurved distributor / better ignition ;
11-12mpg daily / 15mpg highway

65mph = 2200 rpm @ 20" engine vacuum
A518 = OD only, no lockup
Fuel = LPG


IMO, the heavier internals and friction of the A518, and the absense of convertor lockup and lack of cr and strong torque in the engine, dampened my highway mileage, compared to the 904 at least.

But, over the years the car has become much more fun to drive with the 4-speed auto.

At one time I had a 518 with OD and LU installed, which was even more fun, where the Lockup pretty much felt as a short 5th gear.

feets, what carb are you using ? have you tailored your distributor curve, including vacuum advance, for mileage now, or will you be doing that once you get the o.d. installed ? are you going to something deeper than 3.23 gears to get the mass rolling easier, then using the o.d. to lower the highway rpm's to acceptable levels ?

My transmission is set up for a lock up converter.

Right now the engine is in pieces.


I plan on dropping a FiTech system on the car and setting the distributor for best torque/mpg.

A proper Imperial is not a performance car.

I have done this twice.

First was with a 69 Barracuda fastback. 318 with 360 1.88 heads; hyd cam w/specs midway between stock 318 and 340 cams. 3.23 axle.
With standard 4 speed: 17-18 mpg highway
With OD4: 24 mpg highway.

Second was 69 Barracuda conv. Slant 6, stock cam, Holley Economaster 1920 1 bbl. 2.76 axle
With standard 4 speed: 19-20 mpg highway
With OD4: 27.5-29 mpg highway.

Now have 2.94 axle (2.76 was just too low with OD). Haven't done a long range highway test yet, but around town stayed about the same at 22-23 mpg.

Robert

I must be doing something wrong, I'm reading that people are claiming mid-20s for gas Mileage and with BB's at that!

My 06 Mustang with 3.73 gears only gets 22mpg on the Hwy.

I am also amazed... my slightly warmed-over '70 Valiant (stock 383 rebuild, .484 purple shaft, 750 vac sec, dual-plane, A833, 3.23 rear) got about 15 mpg on the highway at 65 mph, around 12 mpg mixed city/hwy and my lowest tank was 8.3 mpg since it was all going to the store for a quart of milk 1/4 mile away


And my '76 Valiant with stock /6 and OD A-833 (don't know the rear) would get about 23-24 mpg at a steady highway cruise.

Maybe they're talking Imperial gallons?