Gas Mileage AGAIN and the effect of speed

["Cadillac Kid" Greg Surfas 15364]

Just got back from a 335 mile round trip to Corpus Christi TX.  Driving my wife's 76 Coupe de ville with a fresh tune and essentially a stock car.  We do that trip frequently so I have some historical data.  When the speed limit was a bit slower I tried to maintain a steady throttle position around 65-70 and the best I got was just approaching 16 MPG.  Basically flat, straight highway.
Same trip this weekend. Steady throttle position, essentially just in the primaries of the carb.  Pushing that 5200 pound barn door with the speedometer indicating between 85 and 90 most of the way surprisingly the mileage dropped off to 12.212.
Not amazing but interesting(to me).
Greg Surfas

[savemy67]

Hello Greg,

Your post might have been titled, Gas Millage AGAIN and the effect of wind resistance.

To see the effect of wind resistance and the amount of energy needed to overcome wind resistance, take note that two humans on two separate occasions rode a bicycle behind a wind resistance barrier on the Bonneville Salt Flats.  The bicycles were powered solely by the energy of the human cyclists.

With no wind resistance, one cyclist achieved a speed of over 160 MPH.  The other cyclist did better than 180 MPH.  These speeds are not typo's.

Wind resistance is proportional to the square of the speed.  Going twice as fast, a car would need enough energy to overcome four times the force of wind resistance.  Going three times as fast, a car would need enough energy to overcome nine times the force of wind resistance.

Hence the significant drop in mileage at the higher average speed.

Respectfully submitted,

Christopher Winter

[EAM 17806]

Just got back from a 335 mile round trip to Corpus Christi TX.  Driving my wife's 76 Coupe de ville with a fresh tune and essentially a stock car.  We do that trip frequently so I have some historical data.  When the speed limit was a bit slower I tried to maintain a steady throttle position around 65-70 and the best I got was just approaching 16 MPG.  Basically flat, straight highway.
Same trip this weekend. Steady throttle position, essentially just in the primaries of the carb.  Pushing that 5200 pound barn door with the speedometer indicating between 85 and 90 most of the way surprisingly the mileage dropped off to 12.212.
Not amazing but interesting(to me).
Greg Surfas

HI GREG!  How in the world do you ever get that kind of good mileage on your 76 at speeds of 65-70? I have the same 76 and believe me it is tuned to specs but the best I could ever get driving at an average of 60 MPH is 13 MPG.  I would like to know all your mechanical wisdom you put into your car so all of us can follow suit and learn something we're missing.   I always us Regular Top Tier gas (Shell, Exxon, etc.)  Good going on your part.   EAM

[Scot Minesinger]

That sounds about right.

There is no flat level highway within 500 miles of my house in VA.  The Northeast corridor of the US is comprised of older highways with lots of interruptions to high speed.  I can drive North 500 miles and it takes 8.5 hours, and the same distance South with newer straighter highways is a 7 hour drive.  Anyway, driving my 1970 DVC with electronic ignition at speeds around 65-70mph and behaving myself I get about 14 mpg on the hilly, slow down speed up highways around here.  I drove 90 mph in 2007 returning from my first GN 2007 in Savannah, GA for a short part of the trip, and my mpg decreased. 

Visiting TX in GN 2018, I loved it - drive 90 mph all day long on straight highway - awesome! 

The topic of mileage and speed is interesting, and as Christopher pointed out all to do with wind resistance.  (my high wind resistance 1978 Olds 88 got about 17mpg, and my 1985 low wind resistance Chevy Caprice V-8, delivered 27 mpg - similar speeds and conditions).  For my Cadillac, it loves fuel and it is worth it.  Because I'm not using much more than 2 or 300 gallons a year in my 1970 Cadillacs, it is not a big impact on finances.

It still amazes me that my 1995 Fleetwood RWD got about 25 mpg highway at 70 mph and 24 years later 2019 V-8 sedans remain about the same mpg.

[Eric DeVirgilis CLC# 8621]

The topic of mileage and speed is interesting, and as Christopher pointed out all to do with wind resistance.  (my high wind resistance 1978 Olds 88 got about 17mpg, and my 1985 low wind resistance Chevy Caprice V-8, delivered 27 mpg - similar speeds and conditions). 

I suspect the OD transmission in the 85 Chevy was the major contributing factor to its MPG advantage over the '78 Olds.

Same goes for the '95 Fleetwood which has other advancements as well.

[TJ Hopland]

Wasn't that one of the main reasons 55 was chosen to be the national speed back in the 70's?

Where I'm at there is a wide variety of terrain and speed limits I can hit within a days drive.   Nothing extreme but quite a range.   I think with modern cars its worse than it used to be.   With forced induction and direct injection fairly common and way more than 3 speed transmissions cars and trucks can get incredible mileage if you can keep them in the ideal conditions.   Once you get out of those conditions instead of a slight hit like you would with your 3speed 4bbl you take a major hit.   I have talked to people that say what they thought a minor increase in load, speed, or wind can cut the mileage down by half or more.   

[walt chomosh #23510]

Thanks to Greg for his fuel millage specs. My wonder is in regards to gearing and cam for best peak performance for highway touring. I have a 1994 Roadtrek RV and can't get the small block Chevy over 16. With the 4:10 rear gearing and OD trans I simply can't get any better. (speed doesn't seem to make as much difference as I would have thought)My 1955 CDV has NEVER gotten 20MPG as a number of other owners have claimed. It has factory A/C so the rear axle ratio is 3:36. Possibly others who have gotten 20MPG did not have the A/C package. Then there's the fuel issue. Possibly fuel in the day is the difference. (not to mention wind and speed of their vehicle when they got their 20)   

["Cadillac Kid" Greg Surfas 15364]

Chris,
Drag for my .41 CD Barn door increases from 265# to 380# going from 70 to 85. Adding rolling resistance increases in the HP required goes from 35.66 to 56.58.  That puts it about 1.3 times the speed increase % wise.

EAM, besides using the correct vacuum advance and timing curve, I've got my Quadrajet pretty well tuned for mileage on the primaries.

Scott, The short version is that most of the newer engines operate in their higher output range.  Most anything after 1980 has a lower CD than the 59-76 Cads.

TJ, 85 MPH puts my engine at just about 3300 RPM.  That puts it just about maximum Torque and coincident maximum volumetric efficiency.

Walt, Of course there is about a 4% "hit" in mileage for the Ethanol in todays gasoline. Keeping the engine speed steady at or near the top of the Torque/VE curve is the "trick" to mileage, along of course with the correct timing and carburetor tune.

Greg Surfas

[pmhowe]

My 1935 Cadillac will probably never see an honest 70mph. The speedometer has clocked 60 a few times but I suspect it is a bit of a liar. At forty to fifty miles an hour (according to its advertised speed) it does a thirsty 8 to 10 miles per gallon. No transcontinental trips planned, but I love it. In fairness to it, it only sees windy mountain roads, so it is not at its most efficient.

P

[The Tassie Devil(le)]

........with the speedometer indicating between 85 and 90 most of the way surprisingly the mileage dropped off to 12.212. .....Greg Surfas

It is surprising just how inaccurate the factory Speedos are, and were designed with an over-indication of actual speed.

I hear a lot of times how people say that they could travel so fast back in the old days, because they completely trusted their speedos as though they were correct.

Drive your car and compare the speedo reading with that of a GPS, and the difference is very different.

The modern cars with electronic speedos, trip computers and Cruise Controls are probably the most accurate devices ever put into a car, as the customers are sort of demanding accuracy these days.

I have adjusted the Speedo in my '72 Eldorado to read as accurate as possible, and when I go cruising, I am getting around 16 to 17 Miles Per Gallon when travelling at the posted speed limits, that being up to 65 MPH, but cruising at 5 MPH under that, I can get better figures, and turning off the AC, even better.   But, I don't mind the worse figures with the AC on, as I appreciate the comfort.

Bruce.

["Cadillac Kid" Greg Surfas 15364]

Bruce,
Speedo and GPS coincide in that car (of mine).
Greg Surfas

[Cape Cod Fleetwood]

"barn door"... LMAO!

With a 1" x 3" piece of NOS script costing $100 + shipping do really care about MPG? JMHO.

I've always noticed a direct and immediate correlation between how hard I'm pressing on the gas and the position of the needle in the fuel gauge. The Ark is happiest cruising at 80mph, much to the chagrin of my local state police. Front end gets "floaty" at 90 so I stay at 80 when conditions permit.

Using Waze to see if my speedometer is correct is a BRILLIANT idea Bruce, I'll try that in the spring.

Gas mileage? When The Ark gets to a 1/4 tank, it gets topped off. Fuel is the cheapest thing about owning this car so far.

\m/
Laurie

["Cadillac Kid" Greg Surfas 15364]

I've got Speed Box and Speedometer. Happy with both.
Greg Suefas

[Jeff Rosansky CLC #28373]

+1 for the GPS. I dont use it all the time but it gives me a good baseline for normal driving.  The 55 shows about 4mph fast at, well, 55. All things considered, not bad. However the cable broke but that is another project. The odometer wasn't too bad tho. It was about a 10th off after a 14 mile run (not using gps but it is a route I drive all the time in my daily driver).
I just re-read this and had to smirk. Using gps to verify the accuracy of a speedometer in a car that was on the road before Sputnik.
Jeff

[BJM]

3300 rpm seems high to me even at 85 mph.  I thought the gearing in a mid 70's Cadillac rear would be 2.43 or so, combined with a huge V8 would produce loafing rpm's in the low to mid 2000's tops.  You seem dialed in to your measurements Greg so you must be right but doesn't a 472/500 turning at 3300 rpm seem like 12.2 is pretty good gas mileage?  And 16 mpg would be awesome at 70. 

How can you guys go 90 mph?  I know that's speeding and all but I travel 7-8 mph over the posted speed limit and I am usually looking at 70 mph speed limits here in the Midwest, sometimes 75.  In those 75 mph zones, I will "push it" to 82 sometimes but never 90. 

[Cape Cod Fleetwood]

How can you guys go 90 mph? 

Easy, keep putting pressure on the gas pedal until the needle in the speedometer reads "90".



\m/
Laurie

["Cadillac Kid" Greg Surfas 15364]

Standard ratio for 1976 is 2.73:1. Aside from the "barn door" aerodynamics, once you get to speed, if you stay out of the secondaries and your car is tuned and set up correctly good mileage is possible.
I really was trying to show the difference in mileage rather than the absolute numbers.
Driving technique is the key.  With my '72 Eldo, I thought the 12.? MPG I was getting was chiseled in stone until my wife drove on a trip from LA to San Diego and racked up  16.7 keeping up with traffic. Go figure.
Here in Texas traffic seems to move a little faster.  The posted speed was 75 and if you were driving 75 in the far right lane you would get run into by everything from a cement mix truck to a cattle hauler.  Traffic was moving as fast as I was, so there was very little need to get into the throttle.
Greg Surfas

[walt chomosh #23510]

Greg commented about volumetric efficiency and that's why I have a vacuum gauge in my Roadtrek RV. Still, I haven't seen much difference between 65MPH and 70MPH plus. I have played with 58MPH and honestly prefer to get to my destination before this lifetime ends. My 4:11 rear axle ratio with 350 motor runs great and seems to like 70MPH or so. Hills don't hurt it's performance much either so I'm happy....being happy is good....walt...tulsa,ok

[35-709]

How can you guys go 90 mph?

Drive on I-95 here in Florida and you will see it all the time.  70 MPH is the speed limit but it seems 80 is the new 70 and then the drivers add 5 or 10 MPH to that!  As was noted, if you are doing 70 you have to be careful about getting run over.  Doing 70 in Big Red on I-95 might mean we pass a motor home, travel trailer, or tractor-trailer rig nursing a flat trailer tire along but that is about it.  The only time we see the Florida Highway Patrol anymore is if there has been an accident, they just aren't out there (too dangerous) and folks know it.

[The Tassie Devil(le)]

When I first ventured onto the US road system in 2002, I was flabbergasted at the speeds that everyone was driving at, and soon realised that if I didn't keep up, I would be a statistic pretty quickly.

Try driving at those speeds down here, and your car would soon be clamped, for 28 days and receive heavy fines, including loss of demerit points against your licence.

Bruce.