1970 Deville engine RPMs @ highway speeds

[chrisntam]

Took the car out on the highway this morning and was surprised to see that at 60 mph, the tach said the engine was turning about 2200 rpm.  I thought it would be less rpm.  I understand the factory gear is a 2.93.  I have a 28 inch tire.   I got on line and found a calculator that said it was about 2166 rpm @ 60 mph.  Calculator says a bit under 2500 @ 70 mph.

For those that have these cars with this ratio and engine (472/500), what speed to you run them at?  At 70, no wonder they get crappy mileage.  They're spinning too fast..........

Or is that rpm this engine's sweet spot?

Just wondering.

[35-709]

Why they went to overdrive transmissions with locking converters.  Why 700R4s and 200-4Rs are popular conversions for older non-overdrive and non-locking converter cars.   
Folks used to more modern transmissions and engine RPMs at interstate speeds think there is a gear missing when they get in a '70s Cadillac.
 

[35-709]

My 472 powered, square box '35 Cadillac sedan with a 700R4 and 3:73 rear gears gets about 2 miles per gallon better mileage on the interstate at 70 MPH than my 472 powered '73 Caribou with a bone stock drive train at the same speeds. 

[Coupe Deville]

no wonder they get crappy mileage.  They're spinning too fast..........

Why they went to overdrive transmissions with locking converters.  Why 700R4s and 200-4Rs are popular conversions for older non-overdrive and non-locking converter cars.   
Folks used to more modern transmissions and engine RPMs at interstate speeds think there is a gear missing when they get in a '70s Cadillac.

He is correct. This was right around the era that the edge was just tipping over for the OD transmissions with lockup converters. Relating to this, the first time me and my dad drove my 72 CDV on the highway he said, I think were stuck in third gear....the revs wont go down..... We were still able to get 14 mpg at around 75-80 miles per hour cruising down the highway but who cares.   

-Gavin

[olds34dude]

and my '37 60 series with a 1965 429, and a 3:73 gear, and a 700r transmission, turns just over 2400 at 80mph, at 60, it's around 2100, I feel like with the highway (interstate) driving I get in the 16 to 18 mpg range- normally cruise mid 70 mph, I wouldn't feel comfortable without the O/D, with my gear, but as Geoff says, it would take a long time to pay for the trans with the fuel savings, then again , many of these cars by now need a trans rebuild, and that being the case, there wouldn't be a large difference in cost, Dave

[Bob Hoffmann CLC#96]

My 472 powered, square box '35 Cadillac sedan with a 700R4 and 3:73 rear gears gets about 2 miles per gallon better mileage on the interstate at 70 MPH than my 472 powered '73 Caribou with a bone stock drive train at the same speeds.

Geoff,
Are you pulling 3.73 or 2.73 gears?
Bob

[35-709]

Sorry for the late reply, I have 3.73 gears as I am running a 1999 Ford 8.8 rear end with disc brakes.  A good ratio for a 700-R4 with the locking converter.  Lots of low speed stomp with a low cruising RPM.

Edit:  This is in my '35 Cadillac.

[dadscad]

My 63 turns about 2443 RPM @ 70 mph with the tires I currently have. I have a 2.94 differential, the transmission is 1-1 in high gear. A formula for drive shaft RPM works for me because of the 1-1 transmission high gear. MPH x 336 x differential ratio divided by tire height = drive shaft RPM. There is probably a little fluid slip in the torus which would increase the RPM a little at times.

["Cadillac Kid" Greg Surfas 15364]

Chris,
At 60 MPH the (ca) 5000 pound car requires the amount of power the engine produces at that RPM (ca 2200RPM) Fuel consumption is just about 1/2 pound per horsepower hour.  If you were to slow the engine down it would not produce enough power to keep the car going at 60 MPH.
Besides, in 1970 gasoline was something around $0.29 per gallon and I think I would come up empty trying to find someone who bought a Cadillac in 1970 for economy of transportation.
Greg Surfas

[chrisntam]

Besides, in 1970 gasoline was something around $0.29 per gallon and I think I would come up empty trying to find someone who bought a Cadillac in 1970 for economy of transportation.
Greg Surfas

Not to challenge your logic of why someone would buy a Cadillac in 1970 for economy purposes (which I agree), but I did a quick internet search on your gas prices.  I found that regular gas in 1970 sold for $0.36 and the cars then required ethyl, which is prolly 10% more so I'll say $0.40 per gallon.  I then found an inflation calculator that calculated that $0.40 in 1970 is $2.45 in today's dollars.  That's not real far off what premium is selling for right now (due to the dip in gas prices).  Last premium I found was about $2.90 a gallon.

I think what one of the differences is in our mindset these days.  We are now keenly aware of mileage.  Attitudes and ideas change over time.  Who wore seatbelts in 1970?  We also think that spending between $0.29 and $0.40 is nothing and we would like to be paying that amount for gas today.  I also keyed in $7,000.00 in the calculator (about what a Cadillac cost then); today (according to the calculator) that amount is $42,943.74, which (to me) is expensive for a new car (I like to live in the past), though I understand Cadillac was top of the line in luxury and today, a top of the line luxury car would sell for, what, $80k to $100K?

But you know what?  The '70 (and for that matter all old cars (mostly)) look good going down the road and that you can't really put a price on.