MPG Highway

I have a 2000 Eldo that consistantly gets 29 - 31 MPG on the highway. Anybody know how it does it? Everyone is amazed and actually doubt me, but I’ve checked it manually myself and always come very close to the “info” # on the dash.

Newton’s first law and high gearing. That big V8 barely has to work to keep the car moving and since it has enough torque to turn a very high gear, it can run at relatively low RPM’s at highway speeds. Lower engine speed means less pumping losses and less fuel burnt just to keep the car moving.

This used to be how you made a fuel efficient car. During the 50’s and 60’s, the small V8 version of a car was often more efficient than the 6-cylinder version because the V8 had enough torque to run really steep high gears. Nowadays, most cars use a small engine-low gearing approach, which yields better city mileage, but the big engine-high gearing still gives pretty decent highway mileage.

I bet cruise control has something to do with it. Could this engine also be one that disables some of the cylinders when cruising at highway speeds? I bet variable timing is involved too.

I dunno… cylinder deletion was such a disaster for Cadillac with the 8-6-4 motor back in the early 80’s, I don’t know if they’ve tried it again even now.

No, this car doesn’t have variable displacement. The OP’s car should have the 4.6L Northstar V8. I agree with GreasyJack, the V8 doesn’t have to work that hard to keep the car moving. Another good example of this is the Z06 Corvette, even with a 500+ HP a 7.0L V8 it’s been known to get 30 MPG on the highway. It has a 6th gear with a very tall ratio (around .50 IIRC) But the engine is so torquey that it can maintain highway speeds at little more than 1400 RPM or so.

Just curious - what speed do you cruise at?

Someone else shares your experience. Check at under 2000 Cadillac Eldorado. Another driver from Florida gets 29.1 MPG with 90% highway driving. Unless that’s you, of course.

Cadillac didn’t advertise this, but in the early 1950’s the Cadillac, especially the models 61 and 62, got higher gasoline mileage than many other cars. The modern for its time 331 cubic inch V-8 engine coupled to a 4 speed automatic (Hydramatic) transmission did much better under most conditions than the Buick with less displacement and the Dynaflo automatic transmission which depended on the torque converter alone. These Cadillacs even got better mileage under most conditions than the 6 cylinder Chevrolet with the PowerGlide transmission that was much like that of the Buick Dynaflo.

When Studebaker brought out the Lark in 1959,a Lark withthe 259 cubic inch V-8 got virtually the same mileage as a Lark equipped with the 169 cubic inch 6 when both cars were equipped with the same transmission.

Don’t brag about the mileage of your Eldo or people will think you are driving an economy car. Cadillac owners don’t want that kind of image.

Thanks to all you guys!
I believe GreasyJack nailed it. At 70 MPH, on level ground, I tach at just under 2K.
And I wondered if the engine had cylinder deletion. Now I know.
Thanks again to all!

And I won’t tell anybody. But jeez, you’d think Caddy would want people to know. No wonder GM is bloody.

Does it have variable timing?

Would 13~15mpg be reasonable then for a 283 2bbl V8 with the powerglide in mostly city driving?

I think that would be very good for the Chevy 283 with Powerglide in mostly city driving. By the time the V-8 engine was introduced in 1955, the PowerGlide had been revised to start in low and shift into direct instead of depending completely on the torque converter unless one started in low. The original Chevrolet V-8 displaced 265 cubic inches–the 283 came along in 1957. The 1957 Chevrolet with the 283 and Powerglide could probably attain this mileage with careful driving in warm weather. I’m not certain if the later Chevrolets which were heavier, but had this engine/transmission combination could do as well.

I agree with good design, high gearing, light foot, flat roads etc. I’m sure your driving habits has the most to do with it. The difference in mileage on some cars between 55 and 70 mph can be substantial. My 4 Runner advertises 21 highway. I can do 26.5 and higher easily…at 55 mph with little traffic and light load. That’s not realistic for most…at 65-70, it’s the advertised 21 mpg. So I still bet at 55 you could do even better.

Neither the OP’s Caddy nor the Z06 have variable valve timing.

“cylinder deletion was such a disaster for Cadillac with the 8-6-4 motor back in the early 80’s”

Back in '83, I used to drive a Fleetwood limo, and it was nicknamed the V-8-6-4-2-zero by the drivers. Unless the engine was warmed up for at least 20 minutes–even in the summer, it was very prone to stalling. And, even when you were driving on the highway for an hour or so, flooring the accelerator to pass could yield a nice rush of torque or it could produce rapid deceleration. You just really never knew what to expect when you tried to accelerate with that engine.

That cylinder shut-down system was a real dog. When you couple that disaster with the Olds diesels of the same era, I think that these were some of the first indications of GM’s vast slip in engineering prowess.

The 1967 Olds Cutlass came with a “Turnpike Cruiser” option. Tall gearing, low lift cam, small carburetor = big fat cruiser with 400 cubic inch engine that gets 18MPG on the highway.

The Buick Lesabre had a special motor setup in the late 90’s which turned out over 30 mpg at 65 or 70 mph. I read it and found it unbelievable, but in late 98 some guy plowed into the car behind me and totaled our Pontiac, so we rented one to drive to the Midwest, around 1000 miles. It got well over 30 mpg, and we ran the limit which in most states was 70 out in the country. I don’t know what motor it had vs. the “eldo” but if one can do it, why not the other?

There wasn’t anything special about the engine, it was the 3.8L. But it did have very tall gearing (speeds of 85 MPH could be achieved in 2nd gear). All the GM H-Body cars were like that.

That’s what I was averaging with the 65 Chevelle I had. Not sure what it weighed, but it had been on a diet when I first got it. Got rid of the cancer in most places and it went back up. Only t hing I didn’t get replaced was the floor pan, which I’m sure has been replaced by now by the person who bought it offa me 2 years ago.