Ozz1967

iTrader: (4)

I'm just curious, but what sort of mileage are you all getting?

I've got an 84 Trans AM with a 305 H.O. that's carburated and I'm only getting about 22 mpg on the highway and maybe 19 in town. Now it has the 700R4 so I have overdrive...I'm wondering what's wrong? Maybe I Need to rebuild the carb? I keep seeing you guys saying your getting 22 or so in town and 28-ish on the highway...what's your secret? I need to do something...we are sitting at $2.80 for regular here now and I can't keep this up for long.

sofakingdom

iTrader: (1)

I don't know who's telling you they're getting 28 out of that combo; but I wouldn't believe it.

22/18 is not too bad. That's about right in fact.

If your car is getting that mileage, I'd leave the carb alone if I was you.

Ozz1967

iTrader: (4)

Maybe it's because they've got TPI or TBI and I'm running a Carb?

sofakingdom

iTrader: (1)

Maybe; maybe it's because they have grocery-cart 2.73 gears, and you have the highly sought-after performance 3.73 gears; or maybe, they're just exaggerating.

Ozz1967

iTrader: (4)

You are correct, I do run 3.73 gears! Love them too! I do run 2600 RPM at 70mph...if I dropped to a more pedestrian 3.42, I could probably get that back down around say...2000 RPM? That would help the gas mileage...but my performance, what there is of it, would suffer.

Mike97ZJ

Sounds like you're getting pretty good mileage to me. I had a hard time getting mileage that good with my L98 GTA. Just about never, actually.

I have a hard time believing anyone is getting 28mpg, at least with a 3rd gen. I could get that out of my LS1 Formula on the highway, but that was with a M6. In 6th gear, it barely turned 2000rpm at 80mph. During everyday driving, I got about what you're getting now.

Token

average city/hwy driving:


21 stock, with 305 TBI
13 with modded 305 (cammed, heads, intake)
16 with modded 350 (in the sig)

Air_Adam

About 10-12 mpg in town right now, and I have:

'79 350ci Z28 engine
- Edelbrock Torker
- Holley 750 VS
- GM HEI with Mallory coil
- Hedman 1-5/8" shorties
- Hooker catback, 3" cutout
- NWC T5 trans
- 3.73 gears

Upcoming in the next few weeks...

- XE274 cam
- worked over 601 castings for 9.7-10.0 CR
- Edelbrock RPM air gap intake
- MSD Street Pro Billet, 6A box
- Super T10 4-spd trans

84z28350

Im with sofa, where in the he|| did you hear 28mpg?


you should considder yourself very lucky to be getting the mileage you do. It is very acceptable for a carbed motor with fairly steep gears in the rear.


If you think your car is bad on gas ill gladly let you foot my gas bill for the next week! carbed 350, 1:1 top gear on the TH-350C with the lockup disconnected and 4:11 gears in the rear, This is when a guy should start complaining about fuel economy. On the highway i hit 3500RPM's at 100km/h (60mph...) and get roughly 2.5km to a litre of fuel(6mpg), it really sucks on the highway!

Its cool having a gas gauge that will spin around as fast as the speedo...

Ozz1967

iTrader: (4)

I was just reading some of the other posts in the forum and several members mentioned getting 27-28 MPG on the highway. Maybe they got 6 bangers instead of the V8's? Maybe I'm just feeling bad because it now costs me $36.00 to fill up on regular unleaded.

IROZINCO

iTrader: (1)

I had an 85 305 TBI that i got 30mpg out of consistently on the highway. It had the 700-r4, no clue as to the rear gears, but i'm betting the 2.73's. In a 350TPI car (i've had 3 of em) 24 seems to be the top of the line highway.

techno101

a little of topic......I have a 2006 Mustang GT. Drove 7 hours on the highway, it climbed up to 30 mpg. Not bad for 300 horses.

joe350

You could swap back to 3.43 rear gears... but the cost of that would negate your increase in fuel economy for the next few years anyway.

irocz28gal

I have a stock 305 TPI and only 26,500 original miles on it, I am getting about 16-18 mpg around town here where I live. I am guessing around 20 on the highway.....not much better than that.

3.8TransAM

Cruised my previously one owner GTA back from SALt Lake City to IN near Chicago and hit 23 to almost 26 mpg per tank depending on my particular altitude at the time and driving habits(screwing around for a tank or cruise at 90+mph)

My 383 Formula got me 21.4 highway. I avg 18+ in it around town/mixed

My GTA will do 20 mixed driving as did my Formula when it was a stock 350.

TBI get quite good mileage in proper running order when stock, especially if a 5 spd or equipped with the lame 2 series gears. Kid I went to school with avg'ed 28-30mpg with a 5 spd 90 RS w/ v8 and TBI.

Going by what I have personally verified and seen from stock 350's I would believe some of these people getitng the mileage they claim with the smaller engine and weaker gears.

later
Jeremy

MonteCarSlow

1983 TransAm

- ~2.95:1 gears w/TH200-R4 tranny
- stock LG4 with TPI (just finished converting it)

23.8mpg on the last fill up (combo highway and city)

Still tuning the car, so that above number has some heavy throttle goofing around in it, but it's mostly highway.

I have poked through 26mpg with the old carb and strictly highway driving (100% stock LG4 with original exhaust). Only mod I did to get those numbers was switch from the 3spd auto to the 4spd 200-R4.

MonteCarSlow

I agree. I put an 89 TBI engine with 700-R4 into our 4000+lbs Jaguar. It averages 19.5mpg, but it easily does 26mpg on the highway. 2.88:1 gears in this car, with a fairly tall tire too.

Ozz1967

iTrader: (4)

So switching to the lower gear seems the way to go...but I am so loathe to give up my 3.73's! lol They make up for quite a bit off the line. LOL Through 35-40mph I can keep up with my friend who's got a 2003 Corvette...then he just roasts me. lol

mdricken

I used to get 26mpg hwy with my 305 TBI.

Now with the ZZ4 I can get 22mpg if I take it easy on the highway. That is with 3.73s and lockup.

City driving I don't even check because I'm sure it is aweful.

Ozz1967

iTrader: (4)

I know next year it will get worse because I am going to do an engine swap to either a 350 or if I get crazy, I might throw a 383 in it. OF course then, because of the increase in Torque and HP, I might switch to a lower gear like a 3.33 or 3.42.

SaintedCorrupt

Gas prices are what's keeping me from swapping in the 3.73's I have lying around. I'm running 3.07's, a t5, and a carb'd 350, and I can barely keep up with prices as it is.

Bull

iTrader: (5)

Last time I checked about a week ago, my Formula w/350 TPI was getting 20 in mixed highway/backroads driving.

MonteCarSlow

The right gears is part of the recipe, the camshaft and driving style is the other 2/3's in my opinion.

IROCThe5.7L

Last time I checked, I was getting 18 MPG, all city driving. And that was driving like a 90 year old women.

91TG

Got a 91 Z28 thats stock except for a cat back system. It has 91K miles on the motor and 3.73 out back . It gets 25mpg at 60mph and 23 mpg at 80mph and 16-17mpg around town if I keep my foot out of it. Been around long enough to know thats not that bad compared to the older performance cars.

bahsarie

also what size tires are they running? rolling resistance makes a difference. the wider the worse the gas millage

Ozz1967

iTrader: (4)

My next road trip I'll keep it parked on 60 the whole way to see what I get. For my tires, running stock 15x7's.

Stekman

You all are lucky with your double digit MPG.

Firebreed

with my v6, im getting about 18-19 mpg mixed.. and getting only 180 miles to the tank.... though i am carbed.....

Formula350Scott

iTrader: (3)

2 words. Cruise Control. I set the cruise on my car and im getting 24mpg highway constantly and thats with the a/c blowing and my maf unplugged cause its broken. Ive found that i can let up off the gas a little and still go the same speed but with more manifold vacuum and it seems when i set the cruise control it keeps the most vacuum but still do the speed its set at. I bet it would hit 30mpg with 2.73s, 22lb injectors instead of 24lb ones, a working maf, and highway mode enabled in the ecu programming.

Firebreed

By the time i hit cruise control... I'm at work...

Ozz1967

iTrader: (4)

Lol! Not all of us are running a "built 400"

I know if I was, I would be lucky to hit 15 I'm sure.

Formula350Scott

iTrader: (3)

Well you could save all kinds of gas then by walking.

Ozz1967

iTrader: (4)

If it wasn't 15 miles to work one way, I might think about it.

Formula350Scott

iTrader: (3)

Someone needs to invent an add on cylinder on demand setup for sbc's like the older caddys and the new hemi motors.

Ozz1967

iTrader: (4)

You'd have to redesign the whole motor to do that. See my thread here...

Northstar System...

If you are interested, the best way for a chevy motor, would be to grab a 5.3 out of a Caprice of Envoy that's been wrecked. You'd have to take everything in the motor though. Not sure what kind of mods you could make to it.

Air_Adam

Not really any way to do that with a carb'd motor, unless you can shut off litteraly everything else - valvetrain and ingition, mainly

Ozz1967

iTrader: (4)

Well, it's been a few weeks, but I did that 60mph drive I told everyone about. I actually set it at 55 and watched as people made funny faces while they passed me! I eeked out 25mph at 55 which, in 1985 is what the car was designed to cruise at so I guess that's not too bad. It did make the 3.5 hour drive a 5 hour trip but hey, it was worth the test.

BLUETA

406 cubes, TPI, mild cam, 700R4, 3.23 gears, 255/50-R16 rears. All hiway driviing at 55-60 with a/c off gets 20.5. But at 80 with the a/c on, still get 19. Averaged 17 city/hiway on vacation last year for 1400 miles going to Woodward Ave. Dream Cruise.

84z28350

I measure mine gallons to the mile...

Apeiron

I get about 27/19 on the LB9, 18/13 on the old 383.

AlkyIROC

I got the worst!

Drive from my pit through the staging lanes. Make a full 1/4 mile pass at WOT. Drive down to the end of the track then back to my pit. Total trip is 2 miles. I use about 1-1/2 gallons

Apeiron

Actually, you've got the best. You make that entire trip without using a single drop of gasoline.

AlkyIROC

True, but alcohol costs more than any pump gas.

GuitarJunki17

iTrader: (1)

13mpg here with EFI, consider yourself lucky you are getting over 20.

Brent C.

I took the new engine on a road trip and got an honest 24.5 mpg in about 500 miles...not bad!! But now the engine is missing on a cylinder, not all the time. just once in a while and I can't find the root of the problem...now I get about 14 in town .......makes me mad!!!

wulff

I am running a 406 (10.8 compression) with TPI, 3:73 gears I am also running a 6-speed T56 with the double overdrive (6th gear is 50%).. on the highway my RPM's drop down to about 1650 RPM cruising and I am getting great milage.. I need to check it and I will post it here.. myself I would always opt for an OD transmission and run deeper gears.

CaptPicardsZ28

I'm getting 9mpg in the city with my 408ci motor and no complaints from me. I love to drive my car and couldn't care much about gas prices. If it goes to 5 bucks a gallon I will still drive it as much as I do now and enjoy every moment...

Trevor86TA

I got 6mpg on a short highway trip.

7plagues

iTrader: (1)

im getting around 17mpg mixed driving, all city is around 15 i imagine, and all highway was 24mpg one time. overall im ok with it.