66 gas mileage

[scotth3886]

I know, if you gotta ask, find another car, right?  Still I'm sorta curious where folks are in 60s Cads in something like 75% city 25% freeway driving fuel economy?  Mine is a 66 Fleetwood so not exactly the lightest model for the year, plus my sorta heavy foot.  What freeway driving I do is an average of 75 - 80 so with the flow of traffic on the way to Cars & Coffee every Saturday morning.  I did one run of about 100 miles driving as carefully and modestly as I can and tried to stick right at 70.  Filled up at same gas station at same pump.  That only got me 17mpg. 

Plus, once in a while, a few WOT runs with GPS timers as I'm planning on drag racing it at IFO (Import Face Off) in April.  Yeah, imported from Clark Street !!!!   

I hear from my grandson that there are a couple of guys who have taken 60s Imperials and a Lincoln or so to the event. 

I'll tell the loud mouth, snot faced brats that if you can't laugh with me, laugh at me!   I've drag raced every car I've ever owned and certainly all of the 90s and 2000s Cadillacs I had as new cars

["Cadillac Kid" Greg Surfas 15364]

Average fuel economy for a well tuned close tolerance (near new) 66 with an AFB and the 3.21 rear end would range from about 9.5-13.0 MPH for a mixed bag of city/highway driving.  Constant cruise at a steady speed, keeping your foot out of the secondaries will do better.
Greg Surfas

[scotth3886]

Average fuel economy for a well tuned close tolerance (near new) 66 with an AFB and the 3.21 rear end would range from about 9.5-13.0 MPH for a mixed bag of city/highway driving.  Constant cruise at a steady speed, keeping your foot out of the secondaries will do better.
Greg Surfas

Thanks Greg.  Did the AFB do better than the Rochester?

[bcroe]

I am happy for my 77 to get 19mpg at a steady 70mph, I have the benefit
of a 2.41:1 axle ratio.  But that is on crap 87 unleaded, not premium. 

Chances are yours is not as good as you think, because they always made
the speedometer/odometer read high.  Second thing I do when I get a car
is recalibrate the odometer (first is change the timing chain set and tires). 
good luck, Bruce Roe

[scotth3886]

I am happy for my 77 to get 19mpg at a steady 70mph, I have the benefit
of a 2.41:1 axle ratio.  But that is on crap 87 unleaded, not premium. 

Chances are yours is not as good as you think, because they always made
the speedometer/odometer read high.  Second thing I do when I get a car
is recalibrate the odometer (first is change the timing chain set and tires). 
good luck, Bruce Roe

I need to play around with speedo gears.  Odo is right on versus mile markers, but speedo read a few percent low versus GPS.  It's not real far off, but an indicated 70 is closer to 73 / 74 actual.  It's just way down the priority list compared to some other items.

Funny, older Mercedes I've owned do the same.  US market BMWs always reads 5 to 6% high

[The Tassie Devil(le)]

Years ago, it was an unwritten design feature that Automobile Speedos had to read about 10% faster than actual speed.

When specifying the requirements of the Police Vehicles for Australian Police Forces, one of these requirements was that the Speedo had to be correct to within +1 to -1 Kph at 200 Kph.

Getting back to Gas Mileage, the only way that real figures can be reached is by scientifically working the amount of fuel consumed for a given distance travelled.

Using the Odometer, and filling the tank at the bowser gives a really rough idea.

When I did my workings out following the purchase of my '72 Eldorado, the odd thing is that I got better figures whilst driving in USA than I did when I got the car back home to Tasmania with no changes to the car.

Bruce.

["Cadillac Kid" Greg Surfas 15364]

G

[scotth3886]

Years ago, it was an unwritten design feature that Automobile Speedos had to read about 10% faster than actual speed.

When specifying the requirements of the Police Vehicles for Australian Police Forces, one of these requirements was that the Speedo had to be correct to within +1 to -1 Kph at 200 Kph.

Getting back to Gas Mileage, the only way that real figures can be reached is by scientifically working the amount of fuel consumed for a given distance travelled.

Using the Odometer, and filling the tank at the bowser gives a really rough idea.

When I did my workings out following the purchase of my '72 Eldorado, the odd thing is that I got better figures whilst driving in USA than I did when I got the car back home to Tasmania with no changes to the car.

Bruce.

Exactly.  I use an electronic version of the old fashion pen and notebook as we've used for the last 120 years.  It's an Android app named Fuel Log.   

Speedometers can be all over the place for many reasons, but very easy to check now in the days of GPS. 

https://www.caranddriver.com/features/a15136801/speedometer-scandal/

[scotth3886]

Years ago, it was an unwritten design feature that Automobile Speedos had to read about 10% faster than actual speed.

When specifying the requirements of the Police Vehicles for Australian Police Forces, one of these requirements was that the Speedo had to be correct to within +1 to -1 Kph at 200 Kph.

Getting back to Gas Mileage, the only way that real figures can be reached is by scientifically working the amount of fuel consumed for a given distance travelled.

Using the Odometer, and filling the tank at the bowser gives a really rough idea.

When I did my workings out following the purchase of my '72 Eldorado, the odd thing is that I got better figures whilst driving in USA than I did when I got the car back home to Tasmania with no changes to the car.

Bruce.

Btw, Bruce, since you mentioned this, I did check speedo versus GPS the other night.   Couldn't sleep so woke up at 3:00am with nothing much to do.  So of course, go waste some gas.

It's a bad video and sideways (not the car), but turned out that the speedo was right on. 

https://youtu.be/AAdZvbCUvu4

[The Tassie Devil(le)]

Nice to know.   But, are you running the Factory size tyres?

Looks like you are one of the lucky ones.

The next check is to check the distance travelled with the Odometer verses the GPS.   And do this comparison over a measured distance, like 5 to 10 miles.

That would be interesting.

With my car, when I converted my Speedo and odometer over to Metric, I was able to correct the speedo needle to read correctly at 100 KPH before reinstalling the unit.

Bruce.

[scotth3886]

Nice to know.   But, are you running the Factory size tyres?

Looks like you are one of the lucky ones.

The next check is to check the distance travelled with the Odometer verses the GPS.   And do this comparison over a measured distance, like 5 to 10 miles.

That would be interesting.

With my car, when I converted my Speedo and odometer over to Metric, I was able to correct the speedo needle to read correctly at 100 KPH before reinstalling the unit.

Bruce.

When I original reported that the speedo was running slow by a few %, that was on the 900x15 wide whites that were on the car when I got it.  I replaced very shortly with 235/75x15, which are about 8/10th of an inch shorter.  The closest in rolling diameter in a P-metric would be a 255/75x15, but good luck finding anything that size.

Yes, this is first day I own the car stuff.  If everything seems to be running ok, I do some initial acceleration runs.  I have saved my Motor Twit, C&D and Road & Tracks from the mid 50s up to the mid or late 70s so I have a reference, plus I just sorta know where these should be.  If it's not running as it should be, I start looking. 

GPS versus odo, mile makers versus odo and Google Maps versus odo over 10 miles or whatever distance.  Straight, mostly level road with no curves to throw things off a bit by what lane you're in,  (on start point >right click>measure distance>left click on end point.  You can do multiple stops and it show cumulative distance.  When done>right click>clear measurement)

[fishnjim]

If you're getting 17, you have nothing to gripe about.   I wouldn't be concerned unless it drops below 8.   Mostly you drive these from one gas station to the next, plan accordingly.
I think you're in the q mile 16 sec bracket at best, ~80s top end.   Advance the timing and take off the air cleaner.  Let 'er rip.   We used to drag mom's and pop's cars all the time in the '60-70's.   

[scotth3886]

If you're getting 17, you have nothing to gripe about.   I wouldn't be concerned unless it drops below 8.   Mostly you drive these from one gas station to the next, plan accordingly.
I think you're in the q mile 16 sec bracket at best, ~80s top end.   Advance the timing and take off the air cleaner.  Let 'er rip.   We used to drag mom's and pop's cars all the time in the '60-70's.

Not at all easy though.  It was an experiment to see what the max is that I could squeeze out of it.  It's easy to top this off to the same point in the tank is at shutoff the gas is visible in the filler neck.  Then get on St Rt 161 and head east past Newark at a steady speed of 70mph or as close as I can without cruise working (soon to be fixed) and then head back the same distance to equalize effects of wind and what little grade there is.  Then fill up at the same gas station from the same pump.  So I feel comfortable that the 17.9 is the absolute max that I can get unless I want to run slower and that ain't gonna happen.

I've run a bunch of WOT runs using GPS on this same piece of road and 1/4 mile at the track should be somewhere between 15.8 and 16.3.  I've used this at the track while running and it's within a 1/10th.  What throws it off a bit is depending whether you shallow or deep stage as there's a foot rollout.  Trap speed at the track is the average of the last 60' so that can vary just a bit too. 

I have to get this tailshaft bushing replaced before I get too rowdy again with it.  However, I have zero qualms about drag racing it though.  I'll race anything, always have and always will. 

["Cadillac Kid" Greg Surfas 15364]

Scott,
At 17.9 MPH you would have to hold the world's record for that car.
Greg Surfas

[scotth3886]

Scott,
At 17.9 MPHyou would have to hold the world's record for that car.
Greg Surfas

I went a little faster than that.  I held it right at 70 for 17.9 mpG.  I didn't stop, but hung a ramp off and over the overpass then back on the other direction.  I'm pretty certain it's the best it will ever do with the TH400. 

To do any better, I would need to convert to a 700R or a 4L80.  I did a similar conversion on my 63 GP 21 years ago, going from a rotohydramatic to a 700R for a big gain in highway fuel economy.

[The Tassie Devil(le)]

The only other way would be to install a Hone Overdrive unit.

Bruce.

[scotth3886]

The only other way would be to install a Hone Overdrive unit.

Bruce.

Or Gear Vendors.  I don't know how much the Hone unit weights, but it would be adding unsprung weight back next to the already heavy rear dif. 

I'd probably be more likely to go to a 4L80e if I didn't something like this.  It'll be a while though, as I still have other issues to take care of. 

[chrisntam]

My '70 model 500 gets 12 (give or take) on the highway at around 70 mph.  I always hope for a strong tail wind...

[scotth3886]

My '70 model 500 gets 12 (give or take) on the highway at around 70 mph.  I always hope for a strong tail wind...

My average since I've had the car is only 11.7, but that's mostly around town.  Since just about everything everywhere was cancelled in 2020 I didn't get a chance to take any trips with it. 

My expectation would be 15 driving with traffic out on the highway.  I'd like to see better than that, but I gotta eat right and make my own tailwind.

[scotth3886]

I haven't tried this since I got my vacuum leaks fixed including the big one at the vacuum modulator.

Trap speed is up and 0-60 is down, but I can't seem to do anything about that ET.  15.8 is it. 

Weather is perfect tonight in the low 40s and very low humidity so likely a below sea level D/A.  I have to try it sometime when the gas tank is down to a 1/4 or so versus full. 

This is so tempting in two weeks.  I'd likely be the only American car there

https://www.nationaltrailraceway.com/events/2021-03/spring-import-face-2