JamesPDX

I’ve look further into the idea changing my rear gears for a combination of performance and economy. Changing the rear gears to improve the MPG will depend on several factors. Such things as manual or automatic, the change in gear ratio, the environment, and city to highway driving percentage, are where planning with a purpose plays an important role.

Another consideration is personal pleasure. The performance and personal pleasure factor derived from a change in rear gears should easily off set the minor decrease in MPG. The small change from 3.23 to 3.42 would not be worth cost because the noticed benefit would be hardly noticed. The added cost of gear oil, additive, and the gears would surely out weigh the benefits. However, changing from 2.93 or 3.08 to 3.42 would be noticed behind the wheel and cost just a few dimes more to run.

Running with an automatic and installing a high stall torque converter is a real sleeper a method. The advantage comes in to play with a 2500+ RPM stall converter when you launch from a stopped position and a higher set of gears (i.e. 2.73).

Most of my driving will be local city with occasional trips from Portland to Seattle, So CA, and Mt. Hood with my family. I will not be racing, and have designed the engine primarily for reliability and temperature control for increased longevity and added some components to boost the low to mid range torque. There are some hill climbs therein that would pose a minor challenge for the higher gears. I have a TH200-R4 OD and prefer the lower gear set of 3.42 gears (over the high stall direction) to replace my 3.08 (originally 2.73) for the best all around performance and economy in the very near future (a delay due to the current budget restrictions).

I built my engine because I was tired of the lesser quality work at higher costs associated with many of the shops out there. I decided to put my money into superior components, shopping around, and specified machine work at known quality shops. And so far, it has paid off with my complete satisfaction with the results of a job well done. This might be construed a boasting. So be it!

erke88

Wow I need to see some of your guys set ups lol. I have a freshly rebuilt 350 slightly better than stock, 4 BBL rochester, and OD transmission. I only get like 10-12 MPG and people say I drive like a grandma... Dont know my gear ratio, but everything is stock for a '71 cutlass supreme

nonhog

Back to 350 MPG! thanks erke88

erke88

No problem Also got 10-12 MPG on the original 350 engine with low compression in one cylinder

nonhog

Hey I actually have a Olds 350 now! Well see if I keep it long enough to check gas mileage. Its a 74 350 in an Omega low comp. Probably not the best platform to start with. Wait and see.

DJS70cutlass

well put sir

Just around town driving with Holley 4 barrel and 2.56 peg leg 13-14 mpg, but I don't have the carb tuning dialed in yet. Gets higher with more freeway miles.

coppercutlass

my 350 almost 400 hp with 9.75 to 1 compression 3.73 gears and a 2200 stall im getting about 12 mpg and im constantly beating on it

OLD SKL 69

I took mine on a 700 round trip and got about 18-20 mpg. 350, 4bbl, 2.56 rear.

Aceshigh

When my modern drivetrain swap is complete......

I'll have 3.73's with a 27" tire and I'll get 600 miles to a tank all highway with a car that can do 12's in the 1/4 .

Vega

But will it rattle the house when you pul up? haha, just messin with you, those LS motors are mean and i love the idea of modernizing. For my build half the love is in the ground pounding big block, i dont want people to just see me pass, i want them to FEEL me pass LOL. But in my case mileage isnt an issue, if it was id build an LS. For people on a budget as far as parts/gas goes, id recommend considering an LS swap (i mean, you can always swap back if you really hate it that much)

Keeping with Olds though, a 350 is about the best bet for V8 mileage. Have you ever considered fuel injection on the 350s? Ive seen a lot of discussion on the rear end gears and overdrive but havent heard about fuel injection much yet. Not only would it help mileage but itd make it a more streetable ride, just fire up and go.

nonhog

EFI 350 sounds cool.

Was hoping CutlassEFI would chime in re: his MPG.



"Wondering if others have good results with 350 Olds for MPG."

MG! LOL wont use a lot of fuel if it don't run.
LS engine....yeah so?

SkylinesSuck

I average 12mpg mixed highway and city. Original low comp 350 with just under 100,000 miles It was under 10mpg before I tuned it up, replaced the vacuum advance, and reset the base timing (somebody had set it to 12* instead of 20* for my stock HEI.)

*Edit* stock TH350 and 2.56? peg leg out back.

Lady72nRob71

350, 4bbl, 2.56 rear
15 city, 20 highway.

Over 1700 miles when driving from South Carolina to TX in the hot summer, I averaged 19.3mpg.
Includes driving through the Smokey Mountains and the Blue Ridge Parkway (6000ft), slow sight-seeing, searching for lost attractions, and A/C on 1/3 of the trip.
Cool!!

oldcutlass

Well, as luck would have it I just filled up the OLDsRat!!!! A whopping 103 miles of in town driving 7.23 MPG!! I drive it everyday to and from work, 7 miles per day, thank god!! I did not build this car for MPG, it was built for MPH!!

Yes, it does rattle the house when it goes into the garage!!!!!

citcapp

My 57 with the 455 gets 7-8 around town and 12 on the highway, built for speed not for mileage. Less if I get on it a lot which i usually do. Just love those long on ramps

84oldsDelta88

84 olds delta 88 with 1970 olds 350/th350 w/ 307 emissions controls, dist, and carb, differential has stock 2.41 open gears. My average mpg is about 16-20. City driving i usually get 15-17, highway is about 18-20 sometimes more if i dont pass people constantly.

So all in all i get almost the same mpg my old 307 used to get but with more horsepower and torque

Lady72nRob71

So the computer is nice and happy with the setup? Do you ever have get smog tsting done? I would be interested to see what the emission numbers are compared to the 307...

Rickman48

My 69 442, 400, w/3:42's averaged 16 mpg combined - 1/2 city - 1/2 over 40mph! No x-way, but babied.
About the same on tollway, if I stayed under 75!

84oldsDelta88

Yup the computer loves the new motor no CELs or anything. And yeah in NJ we have smog tests every 2 years i almost failed, but in the end I passed with flying colors. I'll type up the results below from the test.

Although i had to take the car in 3 times before i passed the first 2 times my HC's were way too high. And every time i went to inspection they'd always pop my hood i guess to see if anythings been tampered with and to monitor my rpms with a wire hooked to my #1 plug wire. Yet they never did this to my truck or other cars and i had a tachometer on my dash board they could of used but they refuse to do so.

Maybe its just me but i don't like those idiots at inspection under my hood or even in my car for that matter. They always rev my engine like a race car and then i wanna punch the guy in the face for it afterwards.

2 years ago the lady at inspection revved my engine to hard and blew a rad hose ever since then i hate state Inspection and the people there.
Haha but those idiots know nothing about oldsmobile engines so i didnt have a problem with them saying its not the 307

IDLE TEST - PASS
my reading/ NJ standard
HC PPM: 21/ 220
CO%: 0.00/ 1.20
CO2%: 15.1
O2%: 0.2

HIGH IDLE TEST - PASS
my reading/ NJ standard
HC PPM: 14/ 220
CO%: 0.80/ 1.20
CO2%: 14.6
O2%: 0.0

I almost failed for CO% on the high idle test, i think the timing is still off a tiny bit its probably at 22 degrees right now.

brown7373

Any of the old cars I have owned over the past have had big engines and lots of power. Only 350 I ever owned, besides the 72 Cutlass 350, was the 11.0:1 compression 350/350 69 Corvette, and that car was fast! Every ting else was 400 or 455.

But my 72 Olds 350 4bbl surprises me with how well in performs. It gets 16-17 on a trip, maybe scratches 18. I live in south Florida, so it's mainly flat. Now I'll lose any bets with you to ****** the $20 bill off the dash on an acceleration run, but I am amazed at the good power and torque it has for normal driving. Mid range or interstate acceleration or passing is smooth and effortless, and much better than I expected from a low compression 350. That is not to say it is anywhere close to a 455, but for a nice car to use, cruise around with the top down or cruise down the highway or up the mountains, it is much better than I expected. Fast cars and drag racing can sure be a hoot, but there certainly is a place for the lesser cars when you just want to cruise in style.

The thing I sometimes get a laugh about, is how some young people, raised on rice burners, go "wow! It has a 350!" I guess if you grew up with 4 cylinders in everything mom and dad carted you around in, a 350 must seem huge.

OldsManNJ

350 with performer cam/lifters/intake/600cfm, stock th350, 2.78 daily driver - I get 13mpg every tank full - I drive 50% highway and 50% around town. Never seems to stray from that 13mpg whether I am babying it our beating on it.

nitrojunkie1029

How are you guys getting such good mpg? i have a 72 cutlass with a 350 2 barrel and dual exhaust, 2:78 rear and i get 8 mpg, i dont do much highway driving to calculate the mpg of that. is there something wrong with my car? im pretty hard on the gas pedal every now and then but usually im good about it everyday

MDchanic

Yes.

I'd expect 18 from a setup like that, maybe 20 with a tailwind.

You need to go through your motor and do a full tune-up. You've probably got a few things way out of adjustment.

- Eric

nitrojunkie1029

Wow i had the carb rebuilt and the timing adjusted this summer and i just recently had my carb adjusted(it was running bad because of the cold weather) and it runs like a top. Its got less than 50,000 original miles and it has been very well taken care of all its life

MDchanic

I dunno.

I don't remember the mileage I used to get in my '70 Cutlass with 350 / 4bbl, but I know it wasn't remarkable - I never thought "Man, this thing is using a lot of gas!"

In my '70 Chevelle with 350 / 4bbl, I definitely got about 18.

In my '71 Skylark with 350 / 2bbl, I also got about 18.

With a 350 / 2bbl and tall gears, you should be getting something in that neighborhood as well.
I'd wait until a nice day, start from scratch, and check everything - something's not right.

(My '68 88 and 98 with 455 / 4bbl both got about 8 in the city, and I'm pretty sure I could have tuned them up better.)

- Eric

nitrojunkie1029

Do you think it could be my distributer? I was having trouble with it where my points would start closing up and it would run like a dog. Do you think if i put and HEI in the car it would fix it

MDchanic

If your points rubbing block is wearing too quickly, and your dwell angle is therefore closing, and your timing is therefore retarding, your mileage will decline.

A lot of new points sets have very small rubbing blocks, compared to the original ones "back in the day." Good ones, like Standard "Blue Streak," are as good as the originals, but are way too expensive, if you ask me (though they're really the best choice).

If your rubbing blocks are wearing too quickly, it could also be caused by inadequate lubrication or roughness on the distributor cam.

HEI will eliminate points problems, but should not be necessary - millions of people drove trillions of miles for a whole lot of years on these points in Delco distributors, and very few had problems like this, so you should be able to get the points working without replacing the distributor. In the end, it depends on "how you roll."

- Eric

70olds455

I am getting around 15 mpg mostly city driving with my 1971 cutlass. Other than the 2.5 inch exhaust and new air filter its completely stock.

nitrojunkie1029

I have a set of NOS points and a new condensor i was thinking about installing, would they help?

MDchanic

Only if you have a points distributor to install them in - HEI is a different unit.

- Eric

nitrojunkie1029

I do have a points distributor. I just wanted to know if i'd be safe replacing the points before i spent te money on a good HEI unit. Thanks

MDchanic

Whoops - wrong guy .

I confused you with someone who has HEI.

The answer is, Maybe.

I would recommend installing the new set, and making sure that the felt and cam are properly greased, then set the dwell and be sure the timing is set properly, and see how it goes. Feel the cam when you've got the points out to be sure it's perfectly smooth.

- Eric