I never tested a 500 but my 78 Eldorado with a 425 gets 14.8 mpg (US 4 quarts per gallon).

TED

 

I never tested a 500 but my 78 Eldorado with a 425 gets 14.8 mpg

That's pretty good! Is that city or highway or a mix? I get about 25 on the highway, 14 in town, and about 19 mixed in my '92 deVille!

 

My 1970 Sedan Deville with 472 gets 12 MPG average.

 

I've done as high as 15 city in my 75 Deville/500ci, driving like a grandma. I'm usually around the 12mpg mark. Highway isn't much better.

 

While that isn't terribly encouraging, at least I won't wonder if there is something wrong with the engine. lol

 

Right after I did a carb rebuild on my 71 Eldo 8.2 she would pull 19 on the highway...:thumbsup:

But it dropped into the cesspool on the way to a tossed exhaust valve... :bouncy:

 

Heavy cars with poor aerodynamics don't make for good mileage. Neither do large cylinders (thermodynamics and flame front issue)

My '70 SDV with a 472 gets 10-15 depending on the mix of city/highway.
That was with no AC though, now that the AC is back working I wonder if that will fall since the climate controll runs the AC anytime it is above 32F outside.

Give the car a good tuneup, new plugs, wires, cap, rotor points and condensor, clean filters, synthetic lubes and keep the tire pressure up (I run 35psi with no ill effects on the ride). Also check the compression - that'll give you a good idea how healthy the engine is. It's not uncommon for these cars to still put up the factory specs on compression, but if you're losing compression in one or more cylinders, expect the mileage to suffer as well.

Also remember that since yours is a '70, it has a 10:1 compression motor - this means nothing but premium gas or you'll risk engine-killing detonation.

 

BlueEyes,

Thanks for the info! I actually just picked up the car today. It was empty and I did fill it with premium, but mostly because it had sat for a long time and I knew I'd be driving it over two hours. I wanted to blow as much crud out as possible during the 160 mile drive. Putting premium in it all the time is going to be painful, but not as much as a dead car. Such is the price of excentricity.

Most of the tune-up items had been done. However, the car then sat for two years. So I'll have to inspect it thoroughly. Hopefully, I'll get to that this weekend. I'm sure I'll have a decent list of to-do's when I'm finished.

Thanks again,

Brandon

 

But it dropped into the cesspool on the way to a tossed exhaust valve

Leeb,

When your car threw an exhaust vavle, did you fix it? And, if so, what was involved? I know that I'll have to deal with that inevitability at some point. But I don't know how bad that really is apart from just being expensive and unpleasant.

 

Leeb,

When your car threw an exhaust vavle, did you fix it? And, if so, what was involved? I know that I'll have to deal with that inevitability at some point. But I don't know how bad that really is apart from just being expensive and unpleasant.

Now see... THAT's the part that bothers me...
I didn't attempt to repair it altho I should have... It would have involved replacing the Left head (due to valve seat damage) and the #1 piston due to complete loss of compression surface... where the valve head WENT THRU. Probably also honing the cylinder tho I really didn't bother to check its condition....

Was going to school at the time and it took 3 months to get the $$$ for the replacement engine... I should have simply fixed the old one.

The replacement came out of a 73 and wasn't quite the same... HP had already dropped by then.

Oh well....

 

Expensive? Nah, bolt on a junkyard head and go..

 

Expensive? Nah, bolt on a junkyard head and go..

Nope... it's the incidental damage that is the 'killer'!
:bouncy:

 

This was overall on a 1,100 mile trip. About 750 was freeway but at 70mph plus, I did not hold back. The rest was mostly country roads 30-50 mph. There's also was a couple of passes in there. It would go over the mountain pass at 70mph and never kick down to 2nd.

This car has 51,000 miles and had not been run since 94. It started right up and I had no problems. But when I came back I did fine a small gas leak in one of the rubber lines back by the gas tank so maybe it should have been better. This was a very nice car to drive.

I part these car out and sell them on eBay so now it sits without the seats so I can't drive it anymore. Tons more great parts to offer. A shame to part it out but I put it on ebay whole and my high bid was only $100.

If anybody is western Washington needs a great 425 let me know.

TED

 

I think I've found at least one reason for my lower-than-average gas mileage. One of the gas lines, next to the inlet for the gas tank, is rotted and cracked. It only leaks while the car is running and squirts gas into the inside of the back bumper. So I'll be replacing that line and checking the others.

 

A tune up helps to ensure the engine is running properly. Remember emissions equipment, checks and replacement. I have had three variable models with 500 all got about 12-15mpg in town and 18-20mpg on the highway. No AC on though. That gets ugly and fast.

 

Yeah, I couldn't really accurately calculate my mpg returning from buying the car, but it was in the neighborhood of 8mpg. I've done a lot of work since then and need to check it again.

I finally got around to replacing the breather octopus of gas lines in the rear. Photos are up here for your amusement.

The PCV valve had been ham-fisted into an aftermarket valve cover. The gromet was too big, so the last owner cut a piece of radiator tubing, stuffed that in the hole and stuffed the PCV valve in the tube. You could hear the sucking sound inside the cab. I've replaced all of that with tight-fitting, propper parts.

I still need to adjust the carb. It stalls when cold and will stall out if you apply gas without "tapping".

After all of that, I'll check my gas mileage again.

 

The first one I had was a 74' Eldorado purchased for $800 flat. It was smoking like a smoke screen out the tail pipe but it just smelled like gas. I took a chance with buying it. The inside was nice and the outside was okay. I figured if the engine was crap 2 out of 3 was not bad. I filled the tank and drove it home. This was a 35-40 mile drive. It went through half the tank. Two and a half weeks later with some tweeking and tuning this was a completely different story. I guess by most standards still crappy gas milage but imcomparison, wow.

 

In my 1975 Eldo with a 500, 8-10, but a lot of it is stop and go traffic. And I have a VERY heavy lead foot...

 

Well, replacing the hoses in the breather assembly improved the fuel economy by over 2mpg. I'm now in the process of rebuilding the carb, which is sort of a soaker hose wherever there is a gasket. Every surface of it had fresh fuel on it when I took it off. I'm guessing simply cleaning it and replacing gaskets and rubber parts will help quite a bit.

The last time I filled up, I put about 11 gallons in and had gone about 150 miles. Assuming that the odometer is accurate, this puts me over 12mpg. That's largely highway without putting my foot in it.

 

I get 10 mpg real world driving in my bone stock unrebuilt 1970 beater of a Coupe.
From Vegas to LA I once got 15 (highway)

 

That's not too bad. Is that a DeVille with the 472 engine?

 

Yeah. 472 Coupe Deville. It smokes on startup now,...so I think it needs some engine work...still gets 10 real world driving

 

Just a brief puff of blue on a cold start is 99.9% of the time the valve seals. Nothing too much to worry about except that it can be embarassing. It doesn't tend to cause much noticable oil loss but you should still keep an eye on the dipstick as always. Good news is that valve seals can be done without even removing the heads if you have the right tools.

 

I finally got the time to pull my carb off. It was slowly leaking gas from every gasket and plug. Once I got it apart, it actually was really clean compared to others I've seen. However, someone had cobbled the power piston and bent one of the primary metering needles. So I'm rebuilding that and expect that my gas mileage will improve simply due to not dribbling it over the intake manifold.

The car doesn't smoke at all and doesn't seem to burn or leak oil (which amazed the bejeezus out of me). At 115,000 miles, I expected a lot more leaking. So far, the power steering pump is the only real leaker.

 

wow im seeing numbers like 12 and 10 an 15 ive got a '71 500 and i only ever get 8 i dont stomp it too much and i dont smell any gas anywhere.

 

I don't really want to know what MPG my '73 Eldo Convertible gets, but if I had to guess I'd say 8. All city, no Hwy, 2 miles to work, 2 miles home.

It's CRAP BABY! But if I cared about MPG I wouldn't have bought a car with an 8.2 in it, I would've gotten a Honda..

 

I don't really want to know what MPG my '73 Eldo Convertible gets, but if I had to guess I'd say 8. All city, no Hwy, 2 miles to work, 2 miles home.

Your engine probably never gets a chance to get fully warm in such a short drive with a 500ci.

Unlike my 1970 Eldorado, the 76 had a batch of emissions items (like the EGR) that kind of put their foot in performance in the name of cleaner air.

Actually, stuff like EGR can help economy by letting the distributor be calibrated for more part throttle spark advance. If the EGR is hooked up correctly it only does anything during cruising and cuts off at idle and WOT so power is not affected.
What hurts economy on the early emissions cars was the POS catalysts used in the '70's, low compression and the miserable spark advance settings that were specified. Fix these things and the cars will do quite well.

 

My 472 gets about 7-8 mpg and I know it has alot of carb issues and maybe more than that.

 

Yeah, now that I have the rebuilt carb on it, I need to fill it up and go through another tank to see where the mileage is now. I don't expect it to be fuel-efficient, but I've stopped enough fuel seepage that I should see more fuel being turned into power and less becoming a stain on the car.

It's amazing to me how badly tuned and poorly maintained a car can be and still run. I haven't checked the timing yet and have no idea when the points were last changed. That may have an impact as well. This car just wasn't loved for a looooong time.

 

I have a 76 sedan deville with the 500ci motor. the motor is rebuilt along with the carb, once I sorted out a stuck egr my mileage went up to 12mpg city and freeway mixed. The exhaust is still pretty black so I will start tweaking the carb now. but 14mpg mixed driving does not look impossible
cheers
Barri