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?

+ 5.5 mpg going from regular 833 to an 833od on a small block.

There's no sense grumbling about putting overdrive in and not getting any fuel mileage returns because you're doing 75, 80mph. You want fuel mileage? Try 55 to 60mph, it works and that's all there is to it.

Who's ridculous idea it was to put 3.91's and 4.10's in these cars should be slapped upside the head. 400hp First-Gen Vipers with 3.07's beat the pants off a stock muscle car with 4.10's.

The 3.07 was coupled to a trans with a deeper 1st gear and a much taller OD.
Not a fair comparison.

At 400-ish HP, first gear doesn't win a race: Second, third and fourth does.

Is a fair comparison with competent Drivers.

Hey, just for fun...CHECK OUT THE TOPIC THAT THIS THREAD IS ABOUT.
It is about mileage, not racing.

"It's about mileage not racing"






Then, what did you post this for? I took this comment as some sort of excuse why a Viper is faster.


I merely made an example of performance and mileage of a newer car that can be equaled in an old car by throwing a 727 and 3.91's in the dumpster is all I was saying.

Called a win/win is it not? You've done it, right? Must you argue?

Nevermind. Another misunderstanding.


Dammit, I knew I should have used an '85 5.0 Mustang with a 5 speed as an example.

One of the areas where new cars do well is the gearing. The lower/deeper transmission ratio gets the car moving quicker while the overdrives allow lower rpm cruising at speed. The new cars don't need deep axle gearing to get moving from the line. Our 2015 Challenger R/T had a 3.07 diff. The 8 speed auto has a 1st gear ratio of 4.7 to 1. Final drove through the OD 8th gear is just over 2 to 1. I'm guessing that the Viper has similar gearing.
I would LOVE to use an 8 speed in my car along with a 3.23 gear. I used to run a 3.91 with the Gear Vendors OD. I now run a 3.55 in the car since I sold the GV. The 3.91s and 4.10s gave snappy response though.

I've stopped asking questions here as much because 75% of respondents don't read the whole original post and give irrelevant or already-done advice, or ask other questions... I spend more time re-explaining than I do learning anything.

In the W350 with the 727, it would never get better than about 12-13 no matter how or where you drove it. When I added the 518 with lock up and going to TBI, it has got 18 going up the 2 big mountain passes here. I'd suspect flat land at 65 could get it up to 20. That's with 4.10 in the front/rear also.

It wasn't just an od swap, but my trail jeep, 78 wagoneer I was getting 16mpg highway after put a 700r4 in With 4.10s on 33s. I thought that was pretty good.
Same deal with my 74 duster, it wasnt just an od swap. I put in a Re ringed mild smogger 400 and a viper t56, ford 8.8 with 3.73s and 25.5" tires I was getting 20 mpg highway. Mpg has dwindled away though after 10,000 miles.the re ring was just to get the car all setup. Part of me thinks 4.10s would make 6th gear a little more useable.
In the end, they both cost me about the same to do. I can't say it was worth it just for mpg but I drove them both farther and more often after the over drive. If you can afford to waste the cash to do it properly, then yeah you should do it.

With the 4L60E and my 3:73 rear end gears i got near 20MPG cursing on dead flat roads at 65. Most of the time its' more like 15MPG though.

Hey Stu, did you get your trans through Kiesler or the guys that took over that company? I'd like to hear from you about the swap.

Weren't teh late Feather Dusters and Dart Lites rated at 36mpg highway with the /6 and A833OD?

That would be a considerable jump above what my 72 Swinger with a /6 and A904 got, about double. I know there was a bit more than just the trans swap invovled, but even half that improvement would put me int eh mid-upper 20's.

I got the trans conversion via Kiesler many moons ago. The whole install was not too bad BUT the controller box for the trans was no good from the start which took forever to get them to send a replacement box as they would not believe it. Also the trans leaked from the get go so i had to have it fixed by a transmission shop. Which i had to pay to have removed after the install and fixed and re-installed. In the end i got it all squared away but i sure hope the new owners of the company do a better job.

Stu

hmmm I'd be cuious to know this as well

Those cars had a 2.94 axle. The deep 3.09 1st gear made it easier to get moving with such tall axle gears.
Also, the Duster was a shorter car by 3 inches, saving weight.

It is a far different frontal area vehicle,
But on a 5400 lb 1995 Ram two wheel drive Ram Magnum 5.9 V8 46RH 3.21 diff gears
With several aero tweaks
(Tonneau, lower air dam, vortex finders, low Cr tires)
60 mph in 0.69 lockedup tcc overdrive gear gives
22 MPG on level blacktop pavement interstate cruise
And
18 MPG in 1.00 third gear lockedup tcc 60 mph over the same stretch of I40 in NC
( but on different days so temperature and wind varied a little)

The feather also had a very lean carb and only got driven 55 mph in the test. Modern cars would probably get 5 more mpg if they only had to do 55 mph.

I'd believe the Feather Duster EPA numbers, if the drive train was in an Omni.

I am so glad this thread still exists, but "dmerc" has proven what can be done with overdrive in an old car:

25 mpg 408 small block

If I remember right, a follow up post he hit 27mpg

Whoa, now you sound like Jimmy Carter...

It was proven the national 55MPH speed limit did NOT use less fuel, that was why the Government changed the sheeples "mindset" to "saves lives"....

Lets not ever bring this HORRIBLE ( 55 mph ) thing back.. I had to live ALL of the years it was in existence while I was in outside sales driving....

Driving at a slower speed absolutely can save you fuel, at least on every vehicle with active gas mileage calculating that I've driven it did. Is there not more wind resistance against a car the faster it moves? If it uses more power to sustain a cars movement at a higher speed, it'll use more gas. The only factor that messes with this a bit is where your engines optimum cruising RPM is. For example, if you have a big cam with a lot of overlap and low gearing, you may be lugging the hell out of your motor at 55mph which isn't very efficient.

FYI, for those criticizing the Gear Vendors OD units .78 OD, I completely agree that I wish they made a unit with a numerically lower OD ratio, but remember with many muscle cars/hot rods running higher overlap cams and non lockup converters, it may not be a good thing to drop the RPM's so low.

As far as 400 hp viper smoking muscle cars despite the gearing, or any modern car for that matter, it has NOTHING to do with 4:10's being a poor choice for racing: modern fuel injected motors have very broad power curves, are making more power from a given HP rating than a muscle car with the same rating and the power to weight ratio as well as aerodynamics probably destroys most muscle cars you'd put it up against. I can guarantee you that if you took that viper and set it up with 4:10's, if it hooked, it would be faster through the 1/4 mile than the viper with 3:07's.

Simply put, unless you are talking about a turbo car which benefits from the load, the more gear you have the faster the car will accellerate up until the point where you run out of gear. That's just the way mechanical advantage works.

Motors that make more broad power curves may not benefit as much though, as a numerically lower gear will give them more average time in places where they are making good power versus a less technologically advanced small block that really only starts to build steam in the the high RPM- the idea is you need to get there quickly.

Look at the difference between some of these late model motors versus an old school small block. To make 500hp, my 340 needs a huge overlap cam that shifts the power band way up and won't make for the greatest street manners at low RPM's. 500hp late model motors are doing it with cams that barely make a chop in the idle and the engine still makes great power/torque off idle.

You really can't compare ideal rear gearing or OD gearing for a muscle car motor to a late model, fuel injected one.

As far as the O/P's discussion goes, I have alway seen about a 2 maybe 3 tops MPG increase on cars I have installed OD's in, but there are a ton of variables here. With our older cars, typically being MODDED cars, the biggest variable is the cam profile/converter. A car running a big cam with lots of overlap and a high stall speed to make efficient use of that cam off idle may not benefit from an OD at all, depending on what rear gearing you have. You don't want to be cruising the car so far below your cam's power band or at a range where you are burning up your converter.

I curse on dead flat roads too, when I'm stuck behind a motor home going too slow and there are no passing zones

I never said we should go back to driving 55, just stated the fact that it was tested at 55 instead of the higher speeds of today. It is a very simple fact of physics that it takes less energy to move the air around at slower speeds. My super duper mpg 318 cuda would get over 35 mpg easily at a 55 mph cruise speed, it dropped to around 29 when I tried to more closely simulate the actual, current EPA test.

I have found that some vehicles do get better mileage at 55. Going faster does result in increased wind resistance.
NO, I do NOT advocate the return of a 55 mph speed limit.

as was stated previously, even with an overdrive installed, be careful you don't lug the COMBINATION at "x"mph where you will be cruising at. that's why i asked feets if he was contemplating a rear gear switch to help the imp at take off, while enjoying the benefit of the od at the targeted cruise speed. the total combination and tune of the entire package, plus the weight and air speed drag need to be considered to get the most out of what you have. using his imperial as an example, it might get better mpg @ 70mph turning 2700rpm than 1900rpm at the same mph using the overdrive in both examples, yet different rear gear sets, just by optimizing the total combination and putting the engine in it's proper operating range for best mpg. just because you are turning a significantly lower rpm at speed doesn't necessarily result in an increased mpg if the engine is somewhat struggling to maintain "x" mph. i hope i have expressed my thoughts properly.