jonstringer

Just a not-so-quick side note. I flew to Tallahassee, FLA to pick up my 1972 Cutlass S. I performed a fluid flush, inspection, set the timing, etc because it had been sitting a while... The engine, and transmission were both rebuilt less than 1k miles ago (documented) before I got my hands on it. It's a 455ci mated to a TH350 and a 2.73 open rear.

Here's what I have:
1. 455 w/ E Heads, forged crank, pistons, and rods. (not sure why the prior owner invested so much into the engine. Maybe he was building a racer.)
2. original Q-Jet also rebuilt
3. Cam unknown. No receipt, but a nice mild idle sound. Nothing radical.
4. Long tube headers by Hooker, mated to 2 1/4" Dual exhaust, 40 series Flow masters, w/ and H pipe.
5. Original snorkel Air Filter

I left the snorkel on, and forgot change the air filter, which was so caked with dirt it wasn't breathing at all. I drove my first tank of gas with this set up, (Avg speed 65 mph) and achieved about 14.5mpg. I've never owned a GM Big Block so I figured this is how things work. I didn't know what to expect efficiency wise.

I stopped in a small town and checked the air filter, and saw how unsavory it was. I changed it out with a $23 open element 2" air filter from Autozone and tested it with my second tank of gas (avg. 65mph) and it was a much different story. I was averaging just over 17mpg, no joke. I monitored my fuel usage the entire way home, and it was consistently between 16.5 and 17.2mpg depending on my foot.

I was a little more than impressed by this fact. Also, the engine doesn't drop or burn any fluids whatsoever. Money well spent.

captainehh

It's nice to hear something positive amidst all the bad stuff in the news lately. Man, i wish all mods/fixes were that easy with such positive results!

jonstringer

No kidding. This puts a 455 Olds at the same highway efficiency as my Small Block Chevy engines. Unheard of.

Redog

That's very impressive for a 455. My 3.4 liter Alero gets around 20 MPG with a combo of city/highway driving.

Dying to know what the Delta's doing with the 350 in it. I'm guessing between 12 and 15 with city/highway driving, but with no odometer, I can't really do anything but guess. Speedo is now hooked up

68conv455

Thats it! I'm scrapping my 250 cu.in. 6 cylinder for a 455. Rats! Ive already done that.

Oldsguy

2.73:1 gears and a quadrajet on the highway with the driver exercising some foot control.

87cutlas

i drive a 87 cutlass with a 307, no internal mods, rollercam engine, just put on a 600cfm edelbrock carb and scrapped the computer, minus the fact that the tranny won't kick down unless you manually down shift or slow down, i went from 7mpg to 17 to 20, inner city. i just have the 200m 3 speed tranny and some 2:73 rear gears

oh ya its still stock and it beats the mustang gt's with ease

88hurstolds

When I drove to the Olds nationals last year I calculated roughly 16-17 mpg on the highway with my C headed 455 as well, not too shabby for a bbo with a 3 speed trans and 3.08 gears. Of course it doesn't have a wild cam and it has stock intake and exhaust manifolds. I started to use the argument that my carb fed Olds was better than most SUV's when criticized about fuel efficiency. Of course street driving is a different story

It actually gets better gas milage than my stock 72 350 daily driver (11mpg street, 15 hwy) with 2.14 gears and a three speed trans.

Omicron

Exactly right.

Gaz Hog

Mine is a '69 Delta 88 with a 455. I had a 2 bbl on it, but since changed the intake manifold, put HEI on her and a Quadra-Jet. So long as I stay out of the double barrel, I get around 14 or 15 (better than my 86 bronco). When I stomp, I probably get more like .14 or .15!!

Deltarocket

I'd be willing to bet, that it would get even better with a 4 speed overdrive.

rrielley

...but I'd never be able to keep my foot out of it and I'd be back down to 10-12 mpg

oldscsc

Those Quadrajets are great for mileage. Good economy until you romp on it

dr.rocketman

Grandpa had a 72 98 with the 455 . Best riding car we ever had, it was handed down to my Dad and then me and my brother. When tuned up with fresh fluids and filters the car would hit 21 mpg on the hwy between Houston and Austin rolling 60 mph. Car had 2.41 ratio. Can't say enough about the car, engineering perfection. What a car.

bob p

I'm glad to hear that someone with a big block gets decent mileage. It must be the 2.73 rear end.
My Suburban has a 454 and a 4.10 rear end and I'd be lucky to get double-digit mileage coasting downhill with the cruise set on 55.

Octania

I am pretty sure my '68 when last running got about 10 mpg on a good day.

Oh, but it is worth it.

The 2006 Charger hemi does barely any better. Is it faster? possibly. I hope to find out someday.

As my departed uncle used to say, "that's some good engineering there..."

jensenracing77

My dad claims he got 20 MPG out of a 455 in the 70s in a big car. Likely had highway gears in it.

That is one thing I am looking forward to checking on my W-30. I am betting 12 to 14 MPG on the highway but that is just a guess. The carb was built with no primary rods so I may be dreaming.

ELY442

Wow! I cant even get a decent gas mileage while Im idling in my garage

fast eddie

Back in 1993 I drove my 71' SX from Salt Lake City to Houston and averaged 19.2 mpg. I did adjust the idle mixture screws halfway through the trip and noticed more pep in the throttle after dropping 6,000 ft.; 2.56:1 peg leg in the rear.

76 Regency

NICE! Last time I clocked it, my burgundy '76 98 got 18.8 MPG on a great long trip home from NC back to CT. Mind you, that was with 2 adults, a trunk full of luggage (including 2 mountain bikes), going 75 mph with the A/C on. Pretty damn impressive for such a big car if you ask me! The car was all stock with the original untouched motor. All that's been done is the new, dual exhaust with no cat. This was before I replaced the really worn out timing chain and gears and removed all of the nylon bits from the oil pump screen! The car runs 1000 times better since then so I'm really psyched to take another long trip and re check the mileage!

70cutty

How do you guys calculate this? Honestly, I find it hard to believe that a 455 in 4000 pound car can get 20mpg. I mean according to some of the comments, 455 with a 2.56 rear, q-jet and 4 speed overdrive would get around 25mpg. That's like a Honda Civic.
I couldn't get 15mpg with the 350 in my Cutlass going down Mt. Everest, I assume my BB will be in single digits.

76 Regency

I'd imagine everybody calculates it the same way. Start with a full tank, drive a long distance and then fill-up again. The total miles divided by the exact amount of gallons it took to fill gives you your MPG. When I clocked my car on that trip, I checked at each fill-up. It got progressively better along the way as I would expect.


Mind you, we're not talking about driving around town with lots of stop and go. I clocked mine on a very long trip on open highways with next to no stopping.

jensenracing77

That is a good question because most odometers are not that accurate in 300 miles. The best way today is with a GPS. In the past I would always check my odometer with a 20 to 50 mile stretch of highway mile markers. Then calculate that into how much it is off in the tank of gas.

oldcutlass

The other problem is the difference in fuel since the OP's original posting. I know that the E10 mandate equated to a 10-15% reduction in fuel mileage. Even if the speedo is off it may equate to a 1-2 MPG difference vs actual.

Fun71

I checked the MPG on my car by putting a gallon of gas in a can, putting the can in the fenderwell, and running a fuel hose from the gas can to the fuel pump. Recorded 12.3 MPG in town with EXTREMELY easy driving and ~15 MPG on the highway at a steady 60 MPH.

I obviously have something different in my setup that is counterproductive to achieving good mileage.

frankr442

You can't get a meaningful average with only one gallon of gas. Too small a test sample, something like stopping for a couple of red lights will have big effect.

bccan

It's possible. Over time that I have checked mine I got right around 20 @ 60-65 sustained on highway, trying to be good. Around town, not sure but with a majority around town combined w/ maybe one or two moderate trips on highway it will be appx 15mpg. I have to admit that I rarely get that mileage on the highway because I'm typically traveling @ 80 or a tad higher, it's just where the car is happy if traffic conditions allow. Engine turns 2300 @ 70mph. This would be on E10 warm weather blend fuel.


When we drive to drag strip we go appx 240 miles, get a good mix of in town, secondary highway, short stint on Mass Pike and 8-10 drag runs, returning w/ a hair under a quarter tank left. My speedo shows 1mph over the radar speed signs so it is pretty accurate, If I could go all highway & lock converter mileage would increase, car doesn't like to be locked up in OD under 50-55 mph.


None of this is laboratory tested but is a reasonably accurate indicator.


Granted, it is 434 ci not 455 but close. Considering 3.90 w/ appx 26.5" tires that's pretty good, largely due to OD, Qjet & tuned for drivability.


Red car w/ lightly warmed over 350, 3.08, 200-4R turns about 1800 @ 70 but I don't know what it gets for mileage. It's my kid's year round DD & I have never driven a tank of fuel through it but it seems easy on gas until you get to the infamous last quarter of a tank or stick your foot in it more than a few times.

Fun71

I agree with you. I should have stated that this test for in-town mileage matched my mileage calculations based upon the traditional method of # gallons used at fill-up and # miles driven. For that test, I just had wanted to see what the mileage was in a hurry. Back in those days it took me a couple of weeks to go through a tank of gas, and there was no way I could keep my right foot off the floorboard for that long.

brown7373

Ditto on the E10. It kills fuel mileage. I also have a 2007 Dodge Charger R/T and used to have a Chrysler 300C, both with the same Hemi and transmission. The 300 used to get between 26 and 27 on the highway 60 mph and about 18 around town, both instances trying to attain the best mileage. My 2007 Charger struggles for 24 and is usually in the 16 range around town. The difference is the 300 ran on 5% ethanol and the Charger on 10% ethanol.

Destructor

My 07 Grand Marquis will consistently gets 28 on the highway going at a pretty good clip. My 70 Cutlass (350) around 15 on non highway but not really around town driving since I usually drive it on non congested Sunday drive type roads. It will go down if I'm having fun with the gas. I no longer check millage. I have a good sense of how the car is running and how much gas I use. I haven't driven enough over the past year. The slipping transmission took the fun out of it but that's all fixed now so come the good weather I will NOT be getting good gas millage. I didn't get much less fuel economy from my 71 Caddy.

cjsdad

Around 1986~87 I moved from So-Cal to Bay Area and drove my '70 Camaro with a BBC 403, TH-400, unknown rear gear in it. I followed my Dad in a U-Haul truck at 65 MPH the whole way. I was shocked to find that I was getting right at 20 MPG.

I also once drove my '71 Duster that had a 225 Slant-6 and three-on-the-tree from Las Vegas back to So-Cal and got 30 MPG. So many cops on that trip I stayed right at 55MPH. That was the speed limit back then.

Like mentioned above, I think the key is staying a consistent speed and resisting the urge to play around with the throttle.

olds 307 and 403

We once got 32 mpg imperial out of our 81 Delta 88. Olds 307, 250C and 2.41 gears. My 88 Cutlass got 20 mpg imperial in the cold of winter with a 403, 2004r and 2.56 gears. I probably could have got low-mid 20's in the summer. With 3.42 gears, it was tough to hit 20 mpg.

steel-and-fire

In June 2011, my 89 Buick Electra wagon with three adults, luggage, and no rear shocks managed 19 mpg. It was a fuel economy challenge for a road rally. We drove with a full tank from a hotel in Lincoln, AL to a gas station 116 miles away. Filled up 6.147 gallons.
Not bad for a stock 307 with the CCC, 2004r, and an open rear 2.93.

A lot of factors contribute to decent mpg. Especially for a 4500 lbs car.

oldsmobiledave

I used to drive my 1969 Delta Royale from Calgary to Saskatoon and back. Cruising @ 70 mph with 2.93 gears I got 16 miles per imperial gallon. It is 400 mi each way.

matt69olds

I drove my car on drag week pulling a small U-Hual trailer, average about 16mpg. Mid 11 second 455, TH400, 3,73 gears and a gear vendor. Oh, and a Q-Jet! Hard to beat a well tuned Q-Jet for milage and performance.

Toadmacher

Great Thread!

I have an Oldsmobile 307 in an 85 convertible Riviera. It has been well maintained, and is entirely stock. I absolutely love how smooth it is, and am not unhappy with its modest performance.

It gets around 12 mpg (US) around town, and 23 on the highway. Should I be happy with the mileage, or is there something I can do to improve it without yearning it apart?

Firewalker

My experience from 98s to 88s big block 425s or 455 was about 18-20 stock in the 60s. My current 455 with a cheap holley then and a radical Crane Fireball cam, and no headers and a B&M 3000 stall TH400 was about 20 on about a 800 mile road trip. Around town who knows, but I know my plugs after many months around town would look almost brand new, even before the MSD 6A was put on it. I think DW says he get around 20 on his 11 second 455 84.

Kennybill

If you really want to get serious about the gas mileage put a vacuum gauge in your car. My Uncle bought a new 1965 Impala SS, 327 powerglide that has a factory vacuum gauge on the console that looked like a tachometer. He said he "pretended" there was an egg under the gas pedal and watched the gauge in the economy area of the gauge. He swore by it. In my bigger cars I got 16 to 19 mpg on trips. The best gas mileage car I've ever had was 1999 Oldsmobile Royale, great riding car and always over 30 mpg on trips and once I got 34 mpg.

Olds442redberet

I've always wanted to fly in and drive and old car back home but I never had the guts.

Did you take a tool box? Did you have a plan incase the car broke down on the way home?

oldspackrat

A tool box with all the basics, a few hundred in cash & a credit card are about all you need I would think. That's what we did a few times. A while back there was a thread about what you should carry in a basic tool box & it ranged from a tackle box sized kit to a box van sized kit.