The value of a lower final-drive ratio and limited slip in a 1967 Eldorado?

[67_Eldo]

I was looking at the Applied GMC site (http://www.appliedgmc.com/index.itml) and noticed that they offer a range of final-drive options as well as a limited-slip modification.

For a GMC Motorhome, the emphasis is on moving toward a higher ratio so you can haul more stuff. I am, however, looking for a lower ratio in search of better gas mileage.

It looks like replacing the original (1967) Eldorado final-drive ratio (3:21) with the original GMC ratio (3:07) would cut the revs at 75 MPH from around 3000 RPM (where the 429 makes maximum torque) to around 2850 RPM. Is a 150-RPM difference worth $1100?

Since the Eldorado (with the switch-pitch THM425) already has gobs of torque available, I don't think that this ration change would cause a noticeable degradation of acceleration in a vehicle that is much lighter than a GMC Motorhome.

Adding limited slip costs another $670.

I doubt that the modifications would be worth $1800 (plus labor), but as one who really wants to bump up his gas mileage, I thought I'd throw this idea out there.

[cadillacmike68]

You might get one half of a mile per gallon improvement at 50-55 MPH or so.

A working cruise control can beat that.

[Scot Minesinger]

Changing the differential ratio for the sake of improved mileage is beyond not worth it.  Like Mike wrote and he was being generous, 1/2 mpg improvement at most.  Doing the math at say 12 mpg, a 1/2 mpg improvement is about a 4% reduction in fuel cost.  For every 1,000 miles driven using about 83 gallons of gas at $3/gal = $249.  If you save 4%, that is $10.  If you drive 3,000 miles per year, that is $30 saved per year.

Differential rations work this way: acceleration/fun is inversely proportional to good mpg.  If you raise mpg via differential change the car will be slower at the stop light.

Presumably you have a working Cadillac, when you mess with stuff like this often things the factory spent millions engineering to avoid problems is not discovered until after the work and you have to put it back the way it was.

[cadillacmike68]

That's what I used 4-5%. That's what the change in gear ratio is. 

Not much and the cruise control can beat that any day.

Now if it was an RWD, maybe the Cadillac locking differential might be appealing. It would be to me, but I don't know where to get one.

[Carl Fielding]

Locking diffs have little use in front axles. They "fight" turns. Overcoming low traction for very short distance is one use. I can see where the GMC motor home might occasionally benefit from one. But this would be able to switch back to an open diff after gaining traction. ARB Air Lockers are such a device. What information do you have on the device used for the GMCs ?       -  Carl

[67_Eldo]

All I know about the limited-slip option is this:

http://www.appliedgmc.com/prod.itml/icOid/876

I've been reading about Oldsmobile's "Turnpike Cruiser" option that came out for the 1967 Cutlass Supreme. It sported 400 cubic inches of displacement and a two-barrel carb with a switch-pitch THM 400. The engine made max torque at 2600 RPM. In its 1970 (and final) incarnation, max torque arrived at 2400 RPM! Of course, it also had a super-low diff ratio. Numbers that folks have thrown out there differ, but tend to run between 2:41 and 2:53.

With Oldsmobile-esque torque, the Turnpike Cruiser didn't suffer badly at the stoplight and it could wind all day on the highway at 17-18 MPG. Also, since the engine revs were low, Olds touted the lower amount of pollution generated by the TC.

That old-school Olds cleverness started me to thinkin' 'bout building an updated, Eldorado-based Interstate Cruiser.

[Eric DeVirgilis CLC# 8621]

Grease is supposed to be changed at regular intervals on limited slip differentials, unlike regular units that rarely need to be touched.

Firm believer of "if it ain't broke.." principle - especially where the final drive of a 67 Eldo is concerned. If it's healthy now, don't mess with it!

[WTL]

Wouldnt stuffing taller tires in there give the same reduction in RPMs, cheaper, and easily reversed? 

IDK about the aesthetics or availability or rubbing issues. 

[Eric DeVirgilis CLC# 8621]

Not a good idea either which may even have a negative effect on mileage.

[67_Eldo]

Yeah, I suppose you guys are right, particularly since my final drive was just rebuilt last year.

I'll bite the bullet when the next round of gas-price increases rolls around this year. :-)

Thanks!

[bcroe]

I observed that my stock 79 Olds with 403 engine and 2.41:1 axle was
in fact pretty much copied from the Turnpike Cruiser. 70+ mph cruising
for long and frequent miles is what I used it for, and I came to love the
GAS and ENGINE saving 2000 rpm at 60 mph.  I did some experiments
swapping on complete axles (far simpler and cheaper than rebuilding)
with ratios like 2.73:1, 2.56:1, 2.41:1, and 2.19:1.  Changing tires hardly
does anything.  I finally concluded 2.41:1 was the cruiser ratio for me. 

OK, except launch about feels like starting in second gear.  The Cruiser
had a switch pitch transmission.  I put together a switch pitch TH400,
also added dual exhaust, and got 0-60 down to 7.49 seconds as
measured on my VC2000.  1/4 mile of 91.2 mph in high 15s.  Not
bad for a stock 4150 lb cruiser that routinely gets 19 mpg on crap
87 octane, at 70 mph.  Yes I corrected the speedo/odometer. 

I didn't stop there, developed an electronic switch pitch controller
to get that advantage not just at max throttle, but over the entire
driving range.  I liked it so much that in time all 5 cars got switch
pitch, I didn't like regular 3 speeds any more. 

As for that 67 Eldo, no need to spend a fortune.  Just get the
2.73:1 final drive from any later 70s Eldo, and bolt it on.  You will
also need the cross over shaft assembly, they do not interchange.
Unfortunately the crossover assembly on a Toronado is somewhat
different, can you weld?  I recommend the conversion for road
cars.  Sell your 3.21:1 final drive to a motorhome owner. 

This was most difficult on my 79 Eldo, converting from a TH325 to
a TH425 switch pitch trans.  But this is a cure for failing 79-85
final drives, I lost 2 of them from defective factory gears.  To go
all the way I used another motorhome trick, a 57:65 chain set
instead of the stock 1:1.  The 403 in my 79 Eldo is not pulling a
motorhome, so I turned the chain set around to get a 2.40:1
overall ratio with my 2.73:1 final drive.  Bruce Roe CLC 14630

[67_Eldo]

Thanks, Bruce! Since I already have a swich-pitch controller, I have a head start! :-)

Actually, it was your switch-pitch controller that brought me to realize how much torque the tiny 429 has to burn, even when pulling a big ol' Eldo. 2000 RPM at 60 MPH sounds good to me!

I haven't got around to putting on dual exhaust yet because I'm still arm wrestling with people who can't seem to stick to their schedules. (I was supposed to get the Eldorado back from the sandblaster two weeks ago. They just finished sandblasting the top today and are too busy to haul the windshield-less car back to my part of town before next Monday.) I hope the duals show a bit of MPG improvement.

I'd forgotten about the lowering of the final-drive ratio in the late (big) Eldorado/Toronado. I can do MIG welding, but not gas.

Thanks for giving me ideas on more ways to spend my time and money! :-)

[bcroe]

The 66-67 Toro/Eldo crossover under the engine is different
than 68-78, so that is part of the set.  In addition Cad (in their
infinite wisdom) put the bolt holes at the bottom right of the
engine in a slightly different position than Olds, so the Toro
crossover shaft support bracket isn't a direct bolt up.  I am
pretty sure that flat plate could be reworked a bit to fit a Toro
on an Eldo, it would have been a lot more reasonable to have
more holes so the same plate would fit either. 

There were a few 3.07:1 final drives after 67, I have never
seen one except in a motorhome.  Bruce Roe

[67_Eldo]

I dimly recall somebody once saying that the engine crossmember on the 67 Eldo is welded in while the crossmember for 68-on is bolted in. The transmission guys I dealt with last year were complaining that the welded-in crossmember made it doubly difficult to work on my car.

Am I misremembering that detail?

[bcroe]

I dimly recall somebody once saying that the engine crossmember on the 67 Eldo is welded in while the crossmember for 68-on is bolted in. The transmission guys I dealt with last year were complaining that the welded-in crossmember made it doubly difficult to work on my car.   Am I misremembering that detail?

I don't recall the 66-67 being more difficult, just different from 68-78.
Don't have that year parts book, gave all my early units to the
motorhome guys.  Don't know what is welded.  Bruce Roe

[cadmium]

You're looking for a numerically lower gear ratio in search of better gas mileage?  Frankly I say you're wasting your effort. 

I went just the opposite way in both of my '68 Devilles in search of better performance.  They came with a 2.94 and I put in a 3.21, which was standard for several years prior to '68 anyway (in any car with air conditioning).  Then I found a 3.36 and put in one of them.  Gas mileage did not go down the slightest bit, and I checked it, but performance sure went up.  The engine is spinning faster, but it's also working easier too when it doesn't take as much effort for the engine to get the car up to speed and hold it.  At interstate speeds the engine is setting right in its power band.  A numerically lower gear ratio doesn't save anything when the throttle has to be held open further because the engine has to work harder into a stiffer gear.  Just my own personal experience.

I always wanted to get a '68 Eldorado.  They came with a 3.07 gear ratio, and I always figured that if I ever got a '68 Eldorado that I would find a '67 final drive and put in it for the better gear ratio.  But I never did get the car.  I'd buy your final drive unit if I had a need for it, but I don't.

[67_Eldo]

I just found the most technically detailed article I have encountered describing Oldmobile's thinking behind "Turnpike Cruiser" philosophy.

Engine friction. I had never considered internal engine friction and how it increases in relation to the RPMs the engine needs to turn to get down the road. This would be as true for Cadillac (or anybody) as it is for Olds.

Good stuff (if rather breathily pro-Olds).

http://www.oldsmobility.com/old/carlife_apr67.htm

[bcroe]

I just found the most technically detailed article I have encountered describing Oldmobile's thinking behind "Turnpike Cruiser" philosophy.

Engine friction. I had never considered internal engine friction and how it increases in relation to the RPMs the engine needs to turn to get down the road.

Yes and there is also air "pumping" which is reduced when the engine
turns slower with more torque and less manifold vacuum.  If it didn't
work, somebody would have to tell all those owners that their overdrive
transmission doesn't help. 

When I experimented with gears, I used this very carefully mapped
out drawing of nearby roads to recalibrate my speedo/odometer each
time, considerably better than factory.  I have all the trans speedo
gears made, and additional external correction speedo drive box
if the former couldn't come really close.  This assured measurements
were quite accurate. 

If you change the final gearing but don't go thru this procedure, of
course economy will appear the same, with the relation between
engine and odometer rpm unchanged.  Except you are not going
the same distance anymore, a higher rear number won't take you
as many miles for the same odometer readings.  Bruce Roe

[J. Skelly]

My folks bought a new 1968 Olds Vista Cruiser with the Turnpike Cruising option (400, THM400, numerically low axle ratio, 2-barrel carb., 290 gross horsepower).  Acceleration from a standing start wasn't nearly as good as our '68 Buick Wildcat, but it accelerated quickly in the mid-and-upper ranges.  I don't recall the gas mileage on trips, but it was pretty good.  When it was brand new, Dad drove from Detroit to the Canadian province of Saskatchewan and back, followed by my maternal Grandmother and her sister in Grandma's '67 DeVille.  Out in Montana and parts of Saskatchewan you could drive as fast as you wanted, and we drove slightly over a thousand miles one day.  I think the package only lasted from 1967-1969, and Pontiac also offered the package on their GTO for the same model years.  Similar packages were offered as Export models with a 2-barrel carb and lower compression ratio.  For example, Buick did this with their 401 in 1963.  Probably most American manufacturers offered something similar for export.   

[67_Eldo]

Bruce,

You think of everything. Well done!

Jim,

It's good to hear from someone who actually has experience with the original Olds.

Well before the Turnpike Cruiser days, my Dad was a hotshot musician who was based in central Kansas. That meant that for most of the big gigs he played, the distance to the gig would typically be at least 200 miles one way. Since he hated to pay for hotel rooms, he always came home after the gig, no matter what the hour.

So Dad always bought Oldsmobiles with the lowest possible overall gearing. His favorite Olds was a dark-blue 1955 88 that could run at 130 MPH all day (or night) long. He exercised that potential quite often. In the days before cameras and radar, he would happily outrun any patrolman who tried to chase him down.

But for him, the main advantage was that he could go as fast as he needed to go and not immediately destroy the car in a couple of months, thanks to the gearing.