When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Small Block Oldsmobile and Fuel Mileage What Was Your ?
On the search for fuel mileage. Please list what you had and what it's was what you got etc.
Mine was a 72 Cutlass factory dual exhaust, 2.73 rear gear, th400 and a 1971 olds 350 with a A4 307 intake manifold a random qjet and a HEI 11" open air filter.
I was getting 18.9 -20.2mpg hwy at 60-65mph.
Same car got 12-14mpg with a poorly running 455.
I am attempting to find parts / combinations that helped others achieve better fuel economy and exactly what they were. My goal is somewhere between 26-30mpg with a V8 in a 1970 Cutlass.
I'm considering
Headers and a msd 6a or 6al for added low speed spark.
200-4r or possibly a th350c and 2.56 gears
Last edited by JLawrence; Aug 31, 2025 at 06:06 PM.
I drove my 1970 Cutlass Supreme 350-4 bbl, headers, dual exhaust, TH350, 2.56 rearend, 28” tall tires, to and from my first real job 60 highway miles each way. At 60 mph it got 16.5 mpg average.
What you were getting is extremely good. A 2004R added to a similar combination might get close. The best I got was 26 mpg Imperial in a 3750 pound 1988 Oldsmobile Cutlass Supreme Classic Brougham. It had a stock 260 with shorty headers with 2.5" dual exhaust with an X pipe, TH2004R and 3.42 gears with 215/75R14 tires. I got 20+ mpg according to Holley's monitor with a 9.5 to 1 compression using the Mahle 1mm ring pack pistons in an Olds 350 with a 204/214 cam with SEFI Terminator X Max with a 4L80E and 2.78 gears and 275/60R15 in my 1970 Oldsmobile Cutlass S.
In my first Vista Cruiser (stock, 1970, 350 2-bbl, 2.78 axle, 4200 pounds, single exhaust, T-400, 28.1" tires) I drove 150 miles twice a week to a job. I got 18 mpg overall at 55 mph when I concentrated on keeping the accelerator steady.
I've experimented with getting better fuel economy for over 60 years. You problably know most of these helpers...
Lower cruise rpm. I found that about 1300 rpm at 60 mph works OK on a 350
Manual transmission. Saves about 5% fuel
Lower exhaust back pressure. Use headers and dual exhaust
Even fuel distribution to each cylinder. Port injection is ultimate for this
Good control of fuel:air ratio. Electronics with an exhaust gas sensor on each bank gets you beyond 20 mpg easily
Smaller engine displacement.
Mild camshaft with less overlap.
Keep your accelerator foot in the same place to avoid power enrichment of the mixture. As opposed to keeping an even speed
Low vehicle weight.
Reduced speed. Around 45 at 1200 rpm is best
Synthetic oil in engine, transmission, and rear axle.
Last edited by VC455; Sep 1, 2025 at 05:39 PM.
Reason: corrected axle ratio
Get a 83 - 88 Cutlass 307 2004R. And never let the secondary air doors on the Quadrajet open. I could always hear the secondary start opening. I got 24 -27. My 84 H/O front and rear bumper came with aluminum inner frame.
72 Cutlass factory dual exhaust, 2.73 rear gear, th400 and a 1971 olds 350 with a A4
I was getting 18.9 -20.2mpg hwy at 60-65mph.
Originally Posted by olds 307 and 403
What you were getting is extremely good.
Agreed, superb MPGs for that setup.
Originally Posted by olds 307 and 403
A 2004R added to a similar combination might get close. The best I got was 26 mpg Imperial in a 3750 pound 1988 Oldsmobile Cutlass Supreme Classic Brougham. It had a stock 260 with shorty headers with 2.5" dual exhaust with an X pipe, TH2004R and 3.42 gears with 215/75R14 tires.
22 mpg U.S. is very good and on the money for your combo.
Originally Posted by Fun71
I drove my 1970 Cutlass Supreme 350-4 bbl, headers, dual exhaust, TH350, 2.56 rearend, 28” tall tires, to and from my first real job 60 highway miles each way. At 60 mph it got 16.5 mpg average.
Ideal.
My signature has all my cars pertinent information although looks like I have to revise my rear gear as its apparently 2.56 like yours. I've seen 15.0 but it was wide open highway and I intentionally drove 60 mph/2000 rpm maximum and steady with 26.6 inch tire to see peak mpg. Kept myself in a position where I wouldn't need to slow down or speed up. Fueled up highway station to highway station so essentially no stop and go in-between whatsoever. To same location in fill tube, leveled.
By the book even though our cars are extremely similar yours weighs about 100 lbs more and you get better mpg from birth.
Highway is up to 87 mph. Extra urban is up to 62mph.
69 310hp Supreme = 13.8 peak highway and 15.7 peak extra urban. (but with 2.78 gear)
70 310hp Supreme = 14.0 peak highway and 15.9 peak extra urban. (but with 2.78 gear)
The headers and tall tires worked like a charm for you !
Anybody got a W-31 and wanna weigh in? Just curious.
Till an owner chimes in. I researched ratings for key models and moments.
350 CID / TH350 EPA ratings peak highway
1969 Cut S coupe 2bbL 250hp 2.56rear 3474curb = 15.0 mpg (first year of combo)
1970 Cut S coupe 2bbL 250hp 2.56rear 3545curb = 15.2 mpg
1970 Cut S coupe 4bbL 310hp 2.78rear 3545curb = 12.4 mpg (worst mileage for this particular combo)
1970 W31 ______4bbL 325hp 3.91rear 3620curb = 10.9 mpg (most powerful combo)
1971 Cut S coupe 2bbL 160hp 2.56rear 3585curb = 15.3 mpg (least powerful combo)
1974 Salon coupe 4bbL 200hp _______4034curb = 14.8 mpg (heaviest)
1977 442 ______ 4bbL 170hp _______3873curb = 17.4 mpg (most fuel efficient)
1980 D 88 coupe_4bbL 160hp _______3638curb = 16.5 mpg
1980 442 ______ 4bbL 170hp _______3309curb = 15.1 mpg (last year of combo)
Of note.
The 1977 350 442 was rated 19.9 mpg U.S. extra urban, peak.
JLawrence you mention Olds V8 for you new proposed combo but not exactly size. The 260 will get you the most mileage for sure. The 1977 260cid/TH350 442 was rated 22.4 mpg extra urban. So maybe a tall tire 2.41 rear geared headered 260 with a 200-4R at 3500 lbs curb could see 25 mpg extra urban. Going by the fact that your are hoping for 26-30 mpg just outside of my best guestimate, indicates to me that you already thought this through thoroughly, and are hoping you missed a magic combination ?
Originally Posted by HighwayStar 442
72 Cutlass has a brick wall for a front end. It's hard to fight Physics.
HighwayStar 442 I think you may appreciate this.
1977 350 442s seem to be the most fuel efficient 350/350 combo in Olds production history. EPA rated + 0.6 extra urban (wide open highway up to 62 mph) that's plus 11 miles per tank full. Over 1977 Cutlass S coupe with same combo. The only Cutlass S that came with a slant nose for 1977 was the 442. The regular Cutlass S has the brick wall nose you speak of.
P.S. I only researched key models and moments that made sense to me to focus on, not every single model with that combo ever offered.
My signature has all my cars pertinent information although looks like I have to revise my rear gear as its apparently 2.56 like yours. I've seen 15.0 but it was wide open highway and I intentionally drove 60 mph/2000 rpm maximum and steady with 26.6 inch tire to see peak mpg.
That 15 mpg is what my car routinely got. For around 20 years it had either 255-60-15 or 245-60-15 tires on back. The 28” tires were a short term thing while driving 62 miles each way to a job site; I was trying to see if I could get better mileage with taller rear tires and tuning for economy instead of power. The 1 to 1.5 mpg increase wasn’t worth the performance loss.
Most online sites say taller tires typically decrease gas mileage by increasing vehicle weight, rolling resistance, and air resistance.
But I like the look and feel larger tires that has more sidewall improve ride comfort.
My favorite tire and wheel calculator. Helped me get the tallest and widest wheel and tire on HighwayStar. 1/2'' wheel spacers and a heat gun and baseball bat. To roll out the fender 2'' at the top of each fender.
When you're looking at the art of the possible, its best to look around for other examples that achieve your goals. Modern Corvettes can sometimes get into that range of MPG. They're running significantly lower aerodynamic drag, significantly better combustion chamber designs, coil on plug ignition, multi port fuel injection (and for the very modern ones, direct injection), VERY tall overdrive gearing, and a tune that is fully optimized for fuel economy.
So just do that with your Cutlass and you'll get the same results.
In all seriousness, you're not going to significantly improve on what you've already got. You may be able to get more power for the same fuel economy, but short of swapping in a diesel or modern 4 or 6 banger, you're only talking about another couple mpg at best.
FWIW, my bone stock '68 350-2bbl / Jetaway / 2.78 open Cutlass S holiday coupe would routinely get just over 20 MPG highway for my dad doing the speed limit (55 then). I could replicate that and on one stretch on I-70 from Kansas City to Indy doing closer to 70 MPH its best ever tank was 23.x MPG (both fills near the exit / entrance ramps). It seemed like it was geared optimally for 70 MPH (imagine that). That, or I inadvertently topped off a little higher at the start of that run.
Here is another one. 1981 Delta 88 4 door, stock Canadian non CCC Olds 307, TH250C and 2.41 gears with 215/75R15 tires. A best of 27 US(32 Imperial MPG) the usual was 22 US(26 Imperial MPG mostly highway. By the end at 450,000 Km it was probably in the teens for MPG if I remember correctly. Engine wear and carb cleanliness along with the state of tune definitely affect MPG's.
That 15 mpg is what my car routinely got. For around 20 years it had either 255-60-15 or 245-60-15 tires on back.
Reiterating that for a LONG time my 70 Supreme was my daily driver through high school, college, and first job, and 15 mpg highway / 12 mpg city was typical.
It's important to look at the total cost picture, not just mileage. Obviously gas prices and mileage jump into the forefront but it's only one factor in the equation. The cost of the car, insurance premiums, maintenence etc should all be considered. Its not bad to get 15-16 mpg in a car that is paid for and cheap to maintain vs getting 30 mpg while paying 600 bucks a month in payments and insurance, then try to factor in depreciation.
Good point on payment and insurance. My Challenger was $750 a month. Now it is paid for $133 a month insurance. I can license my Cutlass for that same $133 for the year. But I can get $800 coverage max for collision repairs, which basically does nothing. The insurance for the Challenger buys the gas I burn in my Cutlass per month, so there you go. But the Challenger can get as good as 40 mpg vs maybe 20 mpg, probably in the teens as is, probably low teens in town.
It's important to look at the total cost picture, not just mileage. Obviously gas prices and mileage jump into the forefront but it's only one factor in the equation. The cost of the car, insurance premiums, maintenence etc should all be considered. Its not bad to get 15-16 mpg in a car that is paid for and cheap to maintain vs getting 30 mpg while paying 600 bucks a month in payments and insurance, then try to factor in depreciation.
Exactly the rationale I used since my daily round-trip work commute was ~5 miles up to '08, when it increased to ~10 miles. Crappy mileage never hurt my wallet enough to justify a gas miser.
FWIW, the 23 MPG HWY tank I mentioned above was with stock size tires on the original 14 x 6 steel rims. When I later switched to 245-60-14s the HWY MPGs went down to around 17 HWY. Regardless of tires, that Jetaway gave very poor city MPG, ~13 in my experience. Its not surprising that econ-boxes have small & skinny tires. On my '99 Dakota R/T switching from the OEM 255-55-17 tires to 275-50-17s dropped my daily (mix) MPG from ~13 to ~11. I could barely push it alone on level ground it had so much rolling resistance.
Thought I’d keep this thread going for others who, like me, pursue power and economy.
I drove my ‘72 Cutlass (original 350) across the state and back this weekend, and I kept good notes on miles and gallons. I was battling winds, so it was a tailwind one way, and a headwind back. The leg of the trip I felt was most accurate for measuring fuel economy (given the conditions) was a round trip stretch where I gassed up at a stop, drove to my destination, then drove back to that same stop, and filled up at the same pump. On paper, I got 17 mpg at 70-75 mph.
The car still has the original TH350, and 3.23:1 gears out back. Stock tire size. The only relevant mods to the car are:
- The original 2 barrel intake/carb was replaced with an Edelbrock AVS-2 650cfm 4 barrel carb (metered/jetted for the altitude here) and a 2711 Performer intake.
- The previous owner converted it to dual exhaust, with tiny pipes. Original exhaust manifolds.
- Points were replaced with a Petronix Igniter III electronic ignition and matching coil.
- Recurved ignition advance with lighter springs.
I have a 200-4R w/lockup sitting in the garage that I’m ready to throw in there, so I figure that should get me over 20 mpg easy.
Very impressive mileage for what you have. The carb must tuned on the lean side, the multi spark and faster curve also probably helps mileage. What size tire are you running? The 2004R will drop about 1000 rpm. I am hoping for your current mileage with my 70 Cutlass.
Cruising 60/65 mph on Ohio and Penn turnpikes when my 67 W30 was new got 18 mpg! M21 trans, 3.55 rear, and those short F70-14 tires. Not bad for a performance 400 CID beast.
On my commute to work (2/3 freeway driving + 1/3 surface streets) I get a combined 12.5 or so MPG for my 403. I was on leave from work and just did surface street driving, from what I recall I think I got almost 9 MPG, so I estimate it's pure freeway mileage would be 14 or 15. Everything is stock.
Due to acquiring a decently rebuilt carb, my 1979 Blue Calais 260 F motor with 3:08's and 200R4 is now getting me 20 mpg pure HWY at 65 (I'm running 1800-1850 rpm at that speed). Ethanol Free gas. Tires are 205-75R-14.
Last edited by BlueCalais79; Feb 2, 2026 at 06:21 PM.
EPA rated your exact model on the low end at 20.2 mpg average for extra urban (up to 62 MPH) and as high as 20.4 mpg average for highway (up to 87 mph). But of course with a 3 speed.
I have noticed over many years of reading the forum that folks with 350-2 bbl engines reported higher mileage than folks with 350-4 bbl engines. This is with most everything else being similar - similar models with auto transmission and same rear gearing.
Is the jetting on the 2bbl carbs different than the primaries on the 4 bbl carb?
Did the 2 bbl engines have a milder camshaft than the 4 bbl engines?
... folks with 350-2 bbl engines reported higher mileage than folks with 350-4 bbl engines. This is with most everything else being similar - similar models with auto transmission and same rear gearing.
Is the jetting on the 2bbl carbs different than the primaries on the 4 bbl carb?
Did the 2 bbl engines have a milder camshaft than the 4 bbl engines?
I would think it's just about axiomatic that two barrels get better fuel mileage than four barrels. Although theoretically the four barrels should be capable of similar mileage if the driver can only keep his boot out of the secondaries.
Did the 2 bbl engines have a milder camshaft than the 4 bbl engines?
Kenneth, the 350, from 1968 to 1976 generally used the same cams regardless of carburetor. The exceptions were the W-31 and those with standard transmission from '70 through '73.
