76CutlassSalon

How difficult would it be to install a later GM overdrive automatic transmission into our '76 Cutlass Salon coupe? Even with a 2:41 rear, the fuel economy is so abysmal that we tend not to drive it on long trips.

edzolz

Define abysmal? 10, 12, less, more?

joe_padavano

A 200-4R will be a relatively easy swap. You'll want to have the 200-4R built to handle your engine and the heavy car. You're unlikely to recover the cost of the swap in fuel savings unless you plan to drive that car a LOT.

And now we'll hear from the people who think this will cause "lugging".

Inline

Not positive that lowering your rpm's even further than you now have, will positively effect your mileage. It may actually even drop when and if you need to floor it more to retain the speedlimit.

Whats your mileage? When i had my worn smog 76' 350 + th350 running on Cutlass S, on PURE travel i got as amazing number as 11 liters per 100km / 21.4mpg. Tho, that was ultimate low, after fill-up next stop was when tank was empty and required re-fill. Usually it was 14 to 15mpg street driven.

Whats yours?

Olds64

x2

You could also consider a Gear Vendors unit, but the 2004R build and swap would be cheaper.

https://www.gearvendors.com/index.html

joe_padavano

There is some truth to this.

76CutlassSalon

Inline,

About 12-14 mpg whether on interstate or back roads.

Jeff

svnt442

This was 31 years ago, so take it with a grain of salt. I drove a 1976 Cutlass Salon from Schaumburg. Ill to Greeley, CO and used about 66 gallons of fuel. I was traveling at between 70 and 80 the whole way. That's about 15-16 mpg. I'm sure that If I have kept the speed down to even 65 it would have been much better, but I was 17 and on my way to my Aunts wedding with my mother and sister.

So 14 mpg at today's highway speeds with the ethanol fuel? It sounds about right to me.

brownbomber77

My brother's 77, stock everything except a dominator intake and headers, rebuilt quadrajet, 2.73 gear, got 22mpg on the highway doing 70. You might have a tuning problem. How new is the carb? I'm assuming it's a 350? Might even get better mileage with a different gear like 3.08 or 3.23.

Consider the 77-96 B-body's (Caprice, Perisienne, Impala, Fleetwood, Roadmaster) suspension and rear axles share the same dimensions as the 73-77 A body cars. You can find 3.08/3.23 gears easily from those cars, especially in the early 90s.

Try tuning first. Maybe the catalytic converter is clogged....another place to look.

76CutlassSalon

brownbomber,

Yes, a 350.

I restore carburetors for a living (built and owned this business for 30 years); Q-Jet was restored (with stock jetting) only a year or two ago.

I'm not sure how a numerically-higher gear ration would improve fuel economy. I live in the country and do most of my driving on the "open road" so there is not a lot of stop/start going on. It would seem that "higher" ratios would only save fuel during take-off due to less torque required to get the two-ton son-of-a-gun rolling.

I guess I'll resign myself to living with the lousy fuel economy; it's just that we have to run the engine on ethanol-free 93 octane (currently $3.19/gal 'round here) unless we want the engine to ping and the carb to heat-soak (i.e. boil the gasoline out of the bowl).

Thanks to all who responded.

Jeff

Olds64

Jeff, if saving fuel is a major concern you might also consider swapping in EFI. Unfortunately; like an overdrive swap, this is quite pricey.

76CutlassSalon

Jesse,

Thanks for the suggestion but I really don't want to alter the appearance of the running gear topside since I do enjoy showing this car in AACA's DPC class. Besides, what would people say about a carburetor guy performing such a swap? ;-)

Jeff

matt69olds

I think the car should get better mileage than 12 as is. How much power does a engine need to make to push a car 65-70mph? How much fuel does a engine need to make that power if it’s tuned properly?


Before swapping transmissions, I’d play around with the tune. If your carb has an adjustable part throttle (APT) lean the mixture until you get a lean miss at highway speeds on a day with typical weather. Once you find the lean misfire point, richen the mixture just a little. Add a few degrees of vacuum advance, timing plays a huge part in fuel efficiency. And obviously, make sure the tires are inflated to the maximum listed on the sidewall, make sure the alignment is spot on, etc.

As far as the overdrive trans, unless the 350 is worn out, I seriously doubt you will gain anything from a overdrive trans swap with the gearing you have now. If the trans shifts into overdrive at normal highway speeds, the engine would be at a RPM far below its torque curve. Move up to a 3.08 gear, and I could see the 2004r making a big improvement in off the line performance, and with overdrive and a lockup converter, probably cruise at a lower RPM than now.

76CutlassSalon

Matt,

Sounds like good advice. It seems as though the APT actually enhances acceleration and so I have it tuned for that; I will instead try your suggestion. Tire inflation will be checked as well.

It was aligned earlier this year so i think we are good there.

Jeff

bccan

If engine is stock & you have to run 93 to avoid pinging that indicates that there is a tuning problem. Carb? Distributor (frozen advance weights)? Vac advance inop or stuck? EGR? All or combo of the above?

Maybe a carb rebuild, advance kit & some tuning will net you improvements without emptying your wallet.

As mentioned above you are not necessarily that far out of range for modern mix fuel.

olds 307 and 403

With high 7 to 1 factory Olds engines on 87 with 10% you should still be able to run factory timing, which is a lot. As said check your timing and advance weights. Basically the real gain from OD is with lock up torque converter deeper gears like a 3.42 will run the same highway rpm, actually slightly less compared to your 2.41 with much better off the line acceleration. With a 2004R your 2.41 gears it would run around 1200 to 1300 rpm at 60 mph! With 225/70R14 tires, 3.42 gear with a 2004R and the torque converter locked at 68 mph my 88 runs at 2000 rpm. It actually makes a 260 acceptable to drive. The lowest the factory put with 2004R was 2.56 gears I believe, pretty sluggish.