View Full Version : 1979 sedan gas mileage?

03-08-05, 01:23 PM
well i almost feel silly asking this question but what kind of gas mileage can i expect from a 1979 sedan deville 425/ turbo 400 i have the opportunity to by a 51,000 mile all orig beauty and i was thinking to drive this as my summer ride. i have a real nice 85 fleetwood garage queen and gas mileage was never a concern there.

03-08-05, 02:13 PM
1976 Olds Delta 88 350 Olds 4BBL (2.73 gears) was rated 13/17
1976 Olds Delta 88 and 98 455 Olds 4BBL (likely 2.41's) was rated 12/18

So I would expect you SHOULD be close to these. If not, there is likely something out of tune.

I had the 350 Delta 88, I got 13 driving for Dominos, and averaged 18-19 highway. And a few all time bests of 23.5 on long trips.

The Delta 88 weighed in at 4860 lbs with no one in it, and 1/2 tank fuel (26 gal tank)


03-08-05, 03:08 PM
My '79 Sedan Deville used to get 17 MPG on the highway. Not bad for such a big car with a huge engine. I get a lot worse now that it has a 472, and twice the power!
But if it has the standard 2.28 rear, you should get reasonable mileage. My '94 Deville only gets 22 on the highway, with a much smaller engine and more aerodynamic body!

Night Wolf
03-10-05, 11:17 PM
my '79 gets 11mpg around town... becuase I can't keep my foot of the gas.. when I drive more conservativly, 12-13 is easy

on the highway I get between 18-20... that is when I can keep the enigne in it's peak efficenty range (70-75mph with 2.28 gears) but that never happens either as I am always running it wide open and burrying the speedo (I need to get cruise control fixed.. maybe then I will slow down)

For the time being, I have been using my '79 as a daily driver... worn tires that are off-balance are the only issues... and it's thirst for gas... but that is why I have a fuel-sipping V6 to drive around... normally :)

03-11-05, 01:13 AM
Be happy if you get 12. 10 is probably realisitic for city driving.

03-11-05, 02:41 AM
my '79 gets 11mpg around town... becuase I can't keep my foot of the gas.. when I drive more conservativly, 12-13 is easy

on the highway I get between 18-20... that is when I can keep the enigne in it's peak efficenty range (70-75mph with 2.28 gears) but that never happens either as I am always running it wide open and burrying the speedo (I need to get cruise control fixed.. maybe then I will slow down)

For the time being, I have been using my '79 as a daily driver... worn tires that are off-balance are the only issues... and it's thirst for gas... but that is why I have a fuel-sipping V6 to drive around... normally :)

Drop in a THM200 4R transmisison with a set of 3.23 gears and you will have an awesome punch out of the hole as well as slightly lower highway cruising RPM. The THM200 4R can be built to take anything a 425 can dish out. The THM400 is an awesome trans, but a power hog, not the best for fuel economy.

There was an article on the the "4L70" trans that a guy named Yank (last name) built for a 4th Gen Z28. It has better gearing than a 4L60 that is more suited for high torque BB's that aren't so rev happy. (2.74/1.57/1.00/0.67). Level 10 Trans has some good info on beefing the THM200 4R also.

Still all things considered, my LT1 V4P FWB is only getting between 18 and 23 back and forth to work, and that is trying to keep my foot out of it. And that is with modern Sequential Port EFI as well as OD, lockup, and decent aerodynamics.

Is that 2.28 gears the 7.5in 2.29's that are in the A/G Bodies? I have never heard of a 2.28 from GM, just the 2.29's. But never in the 8.5in axle. Just wondering. I have a good set of 3.23's (7 5/8, which will bolt in a 7.5) just dying for a new home, carrier, shims and all, just not a posi set.


Night Wolf
03-11-05, 12:00 PM
it is a 2.28 GM 10-bolt rear end.

from my reading, the TH400 is perhaps the best trans to put behind a BB Cad... they like to tug and not rev... and with that, once you get your engine to where you want it, you find the peak RPM range that the engine is efficent (according to which cam you have) then what speed you normally drive on the highway, and you set up the rear gears on the car so that when you are going that speed, the engine is turning over most efficent RPM.... = best gas mileage. There have been cases where people put OD on the car (or truck) and actually got worse gas mileage with OD becuase the enigne was just out of it's efficent range, so even though it was turning over at a lower RPM, it was working harder.

I usually cruise between 70-75mph, and with that, the enigne is turning over at 2,000RPM... which is peak torque. So for now the 2.28 are actually perfect (for fuel mileage atleast) but once I do the cam swap that'll raise the most efficent range to around 2,600RPM, that is when I need to see about gears... I was thinking 2.73-3.08 would be best for me.... Posi really is a must as it spins the right tire all the time now, anytime I pull out it spins while the car hardly moves... although maybe it has to do with the near-bald rear tires :)

the TH400 will get a shift kit and a nice tranny cooler... and I am going to run Mobil 1 synthetic ATF in it.... besides that it is staying how it is... really isn't much more to do with a TH400 as they are already bullet proof to begin with...

03-11-05, 07:32 PM
The ideal tranny for these big blocks has to be a 4L80-E, 4 speed that's solid as a TH400 with an OD so you can still get acceptable mileage with 3.43s or even 3.73s.

03-11-05, 08:57 PM
The ideal tranny for these big blocks has to be a 4L80-E, 4 speed that's solid as a TH400 with an OD so you can still get acceptable mileage with 3.43s or even 3.73s.
Do they make 4L80E with a BOP bellhousing or you'd have to get an adapter plate? I was just wondering...

Night Wolf
03-11-05, 10:39 PM
The ideal tranny for these big blocks has to be a 4L80-E, 4 speed that's solid as a TH400 with an OD so you can still get acceptable mileage with 3.43s or even 3.73s.

Thats the thing, you DON'T need 3.43 or 3.73 gears with these enignes...

they do not like to rev, they like to tug....

the smaller Chevys need the low gears so they can accelerate quick, but then the OD so they don't burn them self up... or use more gas thent he Caddy would...

the Caddy likes the high gears, it has such a high amount of low-end torqe, that even though the gearing is high, it still accelerates very quick.... but once it is moving (highway) the overall gearing is still pretty high because of the high final drive gear.

Again, you want to keep the engine in it's most efficent RPM range when cruising.... lower RPM dosn't exactly mean better mileage.

03-12-05, 04:15 AM
No, this is the thing, I don't care how much low end torque you make, 3.73s WILL get your car moving faster than 2.28s. And with a .75 OD ratio, 70 mph even with 3.73s is just under 2400rpm which will net relatively good mileage.

Edit: that calculation is with 28" tires. 3.43s and a 4L80-E is probably the best combo for cars that see the highway often.

03-12-05, 11:37 AM
The 4L80E has some of the better gears. 2.48/1.48/1/0.75, so the ratio is closer, better drivability with a high torque engine.

My 94 FWB and my 96 Suburban both have a 4L60E and I don't care of the wide ratio of the 3.06 to 1.6x ratio. Too much RPM drop, especially with a Vortec headed engine, they actually make nice low end torque so revving like that actually hurts lighter acceleration. But it does help you get the barge moving.

But the closer ratio of the THM400/4L80/THM350 is much better.


Night Wolf
03-12-05, 03:04 PM

read all 17 replies.

03-12-05, 06:45 PM
I agree with some, but not all.

1. An engine's peak torque at WOT is NOT peak torque at part throttle cruise. Maybe some GM Powertrain Engineers can shed light here.
2. If cruising at higher rpm netted better gas mileage, my V4P FWB would easily get better mileage than the 2.56 and 2.93 geared cars. it does NOT. I am geting around 18.5 city/highway and haven't seen better than 22.5 no matter what. But I haven't had a long trips yet. But all things showing is it won't getting better than 23 or so. My cruise RPM is right about 2000 rpm at 65. The 2.56 and 2.93 geared cars are lower cruise rpm and do better in mileage. It still takes fuel to turn the engine at those higher rpms.

I drive 500-600 miles a week to get to work and back, I watch mileage closely and what make a difference.

These things are (not in any special order):
1. Stops and starts kill mileage more than anything.

2. Weight. the more weight you have to get up to cruise speed and slow down affects mileage alot. My Suburban with 3.73's takes a lot longer at 1/3rd throttle to reach 75 mph than my V4P FWB does with 3.42's, the overall gearing is nearly identical. This longer time burns a lot more fuel. The Sub is nearly 1000#s heaver. Yes, the Suburban is worse than my FWB in Aerodynamics, that has some effect also.

3. I can take 2 different routes to and from work. One has around 26 "opportunites to stop", one has around 17, but is 10 miles longer. The longer route has a cruise speed generally around 65 mph. The other around 45-55. But more stops, less cruise control time. The amount of fuel used is very close, nearly a wash. The longer route can occaisonally save $, but not very often.

4. Cruise Control. use it, the small variations in speed affect mileage. Keep consistent. Especially with a carb. The accelerator pump squirts in fuel with the slightest movement in the gas pedal with a Q-Jet. Stay steady.

5. Heavy throttle starts. They will kill mileage. But they are fun. Lower gears help get the car moving easier, but they rev higher for engines that don't need it and burn more fuel. remember, ever time the eninge turns over more than you NEED it to wastes more fuel. So that 4L60E with a 3.06 first gear and a THM400 with a 2.48, the 4L60 needs less power (torque) get going, but if abused can burn more fuel doing it.

6. Coast when you can, If I am at 65mph, and I know the speed limit goes to 45, I judge my speed to cost to 45 by the time I hi the zone (I drive in rural towns, do the speed limit, or the cops WILL get you...). I only do this when there is low traffic, as I don't drive to be a meanace to traffic.

7. Typically Ethenol "enriched" (that is joke!) fuels cost you around 10% in fuel economy. Avoid. But the higher the compression car the less it hurts, as the higher compression makes it less of an issue. Low compression like your 7L will hurt more. Remember that gasoline has a stoichometric ratio of 14.7:1, Ethenol is around 9:1. So add 10% Ethenol to your gas, and your ideal fuel mixture is now 14.1:1. The ECM (for those that have them) doens't know what fuel it is running so it is off making it run worse for efficiency because it is trying to make it run at 14.7. Carb guys can tweak for this. Noice the only people who tell your Ethenol is GOOD for your car and the environment are politicians? Ever find one you can trust? Thought so.... Gasoline without crap is best. There is some adeabatic cooling and some other things you can take advantage of with Ethenol, but by default it screws up more than it helps. Remember that even fuel additives like solvents take away from the BTU's that are in the fuel. More BTU's per gallon the more power.

8. 87 octane is easier to ignite than 93. If the car doesn't NEED 93 octane, don't waste the $$ running it. I have tried running higher timing and more octane and I have never got the cost savings back in fuel economy. Never saved anything, virtually no effect in mileage at all.

9. Wind. My 91 SDV mileage dropped like a rock in a headwind.

10. Specific Gravity of the fuel. I know it makes a difference, but don't have a way to control it. Aviation fuel is lower SG than car gas. Higher is better.

11. Winter blend fuel vs Summer. Makes a difference. Summer is usually better.


03-12-05, 07:05 PM
A TH400 with a switch pitch and GV overdrive might be a better choice if you've got $2000 for the GV OD and want to do gear splitting. Otherwise 4L80-E > TH400 as long as you've got a rear gear that compliments it.

Night Wolf
03-12-05, 10:37 PM
you really can't compare an LT1 to a 425/472/500.... the LT1 is a revver and the Cads are tuggers

I think what I am saying is... cost no object... 4L80E, stand alone computer to shift it, and all that good stuff, or the TH400.... the 4L80E would be a good choice...

but to buy the trans, do a shift kit, maybe refresh it... whatever... not to mention the stand alone computer to operate it, and extra tools, adapter plates or whatever needed to mount it, I think you are tlaking well over $3k

So lets just say that all said and done, you get 4mpg extra out of the gearing/OD (which unless it is matched with the right final gears, it won't do anything)

so 4mpg... so say with the TH400 you would normally get 20mpg on a steady cruise, on the highway, so with the 4L80E you will now get 24mpg (don't really see it.. but ok) so, 4 miles difference, So lets just play with some numbers here...

Lets say you drive 20k miles... all highway, 2 exact cars except trans... the TH400 car getting 20mpg and the 4L80E car getting 24mpg...

20,000 miles divide by 20mpg = 1,000..... or 1,000 gallons of gas.... lets say gas cost $2/gallon (we'll be cheap and run regular) for a total spending of $2,000 in gas to drive 20k miles

No we have that same 20k miles, but we are getting 24mpg... so 20,000 divide by 24mpg = 833... or 833 gallons of gas needed. So at $2/gallon, we get $1,666 spent on gas to go that same 20k miles... a net savings os $333....

So $333 over 20k miles of countinous highway driving... now the trans swap cost... like I said, say $3k said and done... so you are stuff $2,666 away from even breaking even....

Obviously these numbers are not accurate... but they get a point across, that would will have to buy ALOT of gas to ever even come even on the new trans.

Cost of the TH400? FREE!... throw in a shift kit, performance tranny cooler and some synthetic ATF fluid... you now have less then $300 of work in the stock TH400, and the most solid auto trans ever built is behind your big block Cad engine.

Plus, while maybe on a Chevy, being able to use an over all lower rear gear, but have the OD to keep the RPM down will help acceleration... the Cad's don't like to rev... so performance is actually lost, they prefer to tug....

Think about the small Honda 4bangers... the cars acclerate fast, but the engine revs very fast, they need a very steep final drive gear and 5 or 6 speed transmission to keep the enigne revving, stay in it's high RPM powerband, and then keep the engine from burning itself up when crusing ont he highway...

The Cad OTOH.... is the exact oppisite, the cars speed will actually accelerate much faster then the engine revs come up... think of it like a diesel.... 500RPM on these Cad's makes a world of a difference (when accelerating) but 500RPM on a Honda 4banger that redlines at 7,500RPM is nothing.... On the same not, stick a 3speed auto behind a 4banger Honda and a very high final gear and it wont even be able to get the car moving....

So basically... unless you have very, very deep pockets and just want to say "yeah, I have this" then the stock TH400 is your best trans to get by far. If you can justify spending the money on the 4L80E and everything needed to get it work... then the correct gearing to keep the engine in its efficent power band RPM at crusing speed, then more power to ya.

Plus when you are racing.. and beat someone, and they ask what kind of transmission you have, and you say "stock 3-speed auto" and their jaw drops (much the same after an old Cadillac just beat their rice burner) it just adds to the fun...

03-13-05, 03:01 AM
I know the torque mongers. My 455 Pontiac is one of them. 500 lb ft torque at 2800 is awesome. 3.08 gears and my THM350 survived fine. And I wasn't always nice....

Just remember, the weaker trans' survive if you don't hook up.... Hook up and they are gone!

My recommendation for cost vs fuel econ is not to waste money on a 4L80E, but yet drop $1200 on a THM200 4R. They can take a LOT of power built properly. And with the 2.74 first gear it is much nicer to torquey engines than the 700/4L60's, and it has a slightly deeper OD at .67. You can keep your 2.73's with the THM200 4R and launch like you have 3.08's with a THM400 with the economy of a set of 1.83's (2.73 * .67) in the rear end! And the THM200 4R's suck up less HP than any of the other trans. If a GNX with 300 hp and 350+ lb ft torque can keep it intact for a while, I am sure the average driver can do it with a Cad V8. Unless you are making dragstip passes, this trans will survive.

If a THM200 4R can survive a built 455, then anything a Cad 472/500 can dish out as well.

I read an article about a guy with a 90 or so T/A with a built 455 Buick that pulled off 11.80's down the 1/4mile. And 23+ mpg.... THM200 4R's can take it. And that is with the 7 5/8 axle, so you don't HAVE to have the 8.5" 10 bolt. Just keep the slicks off of the car....

But you will never get a trans that shifts better then a good THM400, still the best....


03-13-05, 05:11 PM
You can find a computer controller for pretty cheap from a junkyard in a diesel vehicle that used the 4L80-E. The tranny itself isn't too much more than a 400.

03-14-05, 12:00 AM
As far as rear end ratios go, the lower the gearing, the quicker it accelerates, as well as having a lower top speed. These are a given, all else being the same. with OD, you have a whole other ratio at your disposal, and you can restore your top end. If the engine is operating out of its powerband, its not because of axle ratios, its because the shift points are off. Anyway, the solution is not to run insane OD ratios with steep gears. It becomes pointless in light of another factor: traction. If you don't have traction, you lose. period. if your vehicle is at the point where it is difficult to KEEP it from spinning under certain conditions, then you do NOT need steeper gears.