So I have an 85' delta with 2.73 gears, the overdrive and 28" tires and im only pulling 14-15 mpg on average and 16 highway (limited hwy sample though)

It may just be my expectations but I feel like it should be better than that for the reasons that I dont abuse it off a stop and more importantly in the same driving conditions, my 83' delta with the 3-speed/28" tire and 2.41 gears gets 16 avg around town and 19-20 highway.

The difference seems odd as the supposed fuel economy that the 200-4R boasts isnt present.

Theyre both CCC carbs, but is there any common issues with the later model that might cause the lower MPG like simple sensors or factory tuning. For starters, I see no visible leaks.

I will also acknowledge that there are many factors beyond what ive listed and also the fact that one just could be a lemon but im just going to be optimistic in saying it is not. ¯\_(ツ)_/¯

 

It may be the fact you are lugging that swirl port turd of a motor way too much with such tall tires and OD. Nearly every complaint of a gutless car was the swirl port version of the motor but mileage was usually Ok. I bet your 83 is faster as well. Even a swap to a 3.08 might help, a 3.23 or 3.42 won't be far off the 3 spd combo rpm wise. Our 81 Delta with the factory Canadian non CCC Qjet Olds 307, TH250C and 2.41 and a 27" tire got as high as 32 mpg Imperial MPG, usually mid 20's and got lower and lower as the car wore out, probably the pancake cat plugging. How many miles on the car? I got 24 mpg Imperial weighs 4000 pound with me in it, 88 Cutlass with a factory 73 Olds 350 with shorty headers and 2.5" X pipe exhaust, 2004R and 3.42 gears with a 26.3" tall tire, around 1800 rpm at 60 mph. Another guy on ROP with basically the same mid 80's Cutlass but not a Brougham probably a few pounds lighter with full length headers, same stock mid 70's Olds 350, 2004R but 2.73 gears with a fairly short tires, high 20's imperial. The factory exhaust is super restrictive with the lousy crossover pipe and the tubular manifolds. Just shows better exhaust can help mileage, not hurt it.

 

http://tech.oldsgmail.com/axle_RPM.php
I use this calculator, very accurate takes the lock up converter into account which your 83 will also have. Just put a 1 in place of the .67 for trans. You will actually run slightly lower rpm with a 3.42 gear with OD than your current 2.41 3spd in your 83. Your 85 runs like 1350 at 60 mph. My 70S with a dying Olds 350 and nearly the exact same combo only got 17 mpg Imperial with a 2004R, 2.78 and 28" tall tire, too much of a good thing. I am planning 3.90 gears and a 424ci 403 stroker, so mileage won't be it's strong suit.

 

Yea I will totally agree with the swirl port 7A heads choking the motor from the start on the later model. That was the first difference I felt when I first got the 85' as my 83' definitely feels faster.

Another notable difference regarding exhaust setup is that my 83' doesnt have any CAT or AIR system on, so it feels much more free flowing. However; my 85' is clocked at 59k with a CAT but I did immediately use a CAT cleaner and few tanks of 93 octane when I got the car back in July so im hoping that removed any potential blockage. My 83' is at 96k but still runs strong.

I do actually have everything I need to swap 3.42 gears into one of them, all except the experience and a few tools. I was actually intending on swapping those gears into my 83' because of my novice skills, it would feel like less of a burden to screw up the rear in that car. My 85' just feels too pretty to tinker with!

 

If you do swap the gears, do a 2004R at the same time, a nearly direct swap and will totally kill your 85 in mileage and acceleration.

 

I'd actually check the timing in regards to the freeway mpg. Most often they have been turned down from 20* to like 16 or worse, perhaps for smog testing or something. They like 20-22* Next might be the O2 sensor if it is original. Only Delco unit, no Bosch. And then check for Vac leaks..

 

Confirm that the odometer is accurate by comparing it to mile markers on an interstate highway...nothing like a bad test device...

Can you feel the torque converter lock-up?

 

Do a speedometer search on your smart phone, find one you like and get spot on speed calculations. I use one on a spare IPHONE. The phone is not in a service plan but, many apps still work. Plus, my Cloud based tunes play thru it into my car's stereos.

 

Im glad you mentioned timing as I was previously suspicious of it being off as I had some run-on/dieseling issues come up after my tune-up. I broke out my timing gun and set it to 22 and now the engine does seem to run better after a test drive as well as run-on/dieseling not occuring in the small sample, however it could just be placebo...

Now that you mention the lock-up, I dont recall feeling it engaging during my previous driving or test drive today. Im due for a tranny fluid change at 60k so I suppose ill just change the solenoid then, when I drop the pan.

Thanks for all the help fellas!

 

I haven't located an EPA rating or published road test for 83. But your 85s performance may not be as bad as you think.... EPA vs reality is often a big difference. Which involves the everyday driver never touching the EPAs number. You on the other hand are remarkably close.

For 1985 the EPA said the overdrive Delta 88 does 15 city and 22 highway and the 3 speed does 15 city 20 highway.

There it is, overdrive is good for a 10% increase in fuel economy. Highway only, which makes sense.

You are matching city on the 85 and city and highway MPG on the 83 after 35 years... ! Outstanding ! ( of course assuming 83 and 85 EPA mpg matched...)

So the potential issue on the 85 is likely highway related / overdrive related etc. I would consider the advice from fellow members with that in mind to help pin point the issue...

https://www.fueleconomy.gov/feg/byma...bile1985.shtml


P.S. For reference, Car and Driver tested a 1983 Hurst Olds which has overdrive. EPA said 17 MPG city. Car and Driver saw 12

 

What was true in '85 is true today. Diagnosing a computer-controlled engine without the aid of a REAL scan tool is frustrating, inefficient, and often inaccurate.

Put a scan tool on that thing. Look at the data stream. "Codes" are handy and useful. Data stream is ESSENTIAL. What is the O2 voltage and cross-counts, for example. What is the engine coolant temperature? What is the knock sensor activity?

When I was a service advisor in '84--'85, the guys installed a small mountain of mixture control solenoids.
Canister purge valves were a big deal--the canister would get saturated, then the charcoal broke down and entered the carb float bowl.
Plugged catalysts were a known problem, as were pellet-spitters. Either the cat wouldn't flow, or the cat had no beads left in it, they'd all migrated to the muffler. Cars would fast idle with beads getting thrown out the tailpipe.

Of course, there's all the usual "tune-up" issues--fouled plugs, low compression pressure, flat cam lobes, incorrect timing, faulty plug wires, plugged air filter...and that list goes on and on.

Dragging brakes, under-inflated tires are additional potential causes.

Has that "CCC" carb ever been rebuilt? Heavy float? Leaking needle 'n' seat? Sloppy air valve? Faulty choke or choke pulloff? What does a vacuum gauge, connected to manifold vacuum show? Does it agree with the MAP sensor data?

How fast are you driving on the "Highway"? These cars came from the era of the National Mandated Speed Limit; Tricky Dick's twenty-one-year disaster more commonly called Double Nickel or "55". They were geared for low revs, with cam timing and ports to match.