Bo185

I have a 55 2dr post and a 5.3L lying around and I am thinking about swapping it in, but want to know what kinda of gas mileage everyone with an Gen III/GEN IV V8's are getting with an auto or stick, in a 55/56/57 Chevy car is getting.

I hoping to make this my daily driver and want good mileage so a 5.3L will make plenty of power and hopefully good mileage. I hoping 25-28 MPG.

I know, I know just swap it in! But I am on the fence about swapping it into this car because I really need a 4dr. And I can build a nice car vs going and buying a $20,000 new one!

Thanks
-Scott

OldSchoolFormula

I think you might see 21-22MPG freeway at 65 or so, but I doubt much better. It is an aerodynamic brick.

Bo185

Yes but it's a light brick! Lol...


Still looking for real woruld numbers. Anyone?

jeepinpete

I pulling 18 mpg @ 65 mph in my old full size Jeep Cherokee. Similarly poor aero, 4500 lbs. It does have oversized tires and tall gears, and a 5 speed. I think 21-22 mpg is a realistic number.

Ribbedroof

If that's as good as anyone is getting, I'm not sure the swap is worth it. I get over 19 on the highway with a 100K mile carburated 305 and no overdrive in the brick to the left.
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Buddy had an 85 Camino with EFI 4.3 and a 2004r that gave him 27-28 highway MPG before it got totalled. No powerhouse, but I would have thought that the LS engines should come close despite the larger displacement by virtue of the benefit of an additional 20 years of technology.

Even with the poor aerodynamics of the shoebox, I would have thought the numbers would be better than 21-22, unless running an old non OD trans.

Bo185

The aerodynamics are not to bad, I think. But, I don't want to mess with it if it isn't at least 25mpg. It's just not worth it for a DD right now. I have a TDI VW now and get 45mpg with the diesel. I was hoping to let my wife drive the VW and I'll drive the 55 with the 5.3L swapped in. I rather have it than buying another car.



Still looking for anyone with actual mileage numbers?

JZHeyde

iTrader: (30)

Wouldnt some of this have to do with the tune put on the car? rear gearing? cam selection? etc?

I am no expert but I dont think you are going to get any closer numbers than what is already posted here. To me the difference between 4-5 mpg is not enough to stop the swap. I think you would have an absolutely bitchin DD with that swap. Dont let a few MPG spoil the plans. If you are building it for MPG then get some 3.23 rear end gears or lower and build the car for the MPG.

Just my .02 and its worth what you paid for it.

Jamin

Quick Double Nickel

iTrader: (5)

My '55 weighs in at 3,250 without driver and that's with an all-iron small block. I can manage at least 20mpg with a Carter carb on top of a Victor Jr. and backed by a TH350 and 3.42s. So I think 25mpg on the highway could be feasibly.

NX74LS6

I get 18-19 city / 25-26 on the hwy in my 74 Camaro. Ls6 stock cam '02 headers( they picked up 1.5 mpg in my 01 SS) 700 R4 3:42. But know I have paint on it it should glide thru the air and net me atleast another 4 mpg. But seriously the aerodynamic issue with the '55 is the large frontal mass - one of the biggest problems as far as cutting thru the air..... even still 23-24 mpg I think is possible but you must have the car wideband tuned ...even stockers respond ( power /mpg) and don't forget the headers if at all possible... really helps on the hwy. I'm in the middle of my C6 Z06 brake install I'll post picks later. God bless Tobin at Kore 3. great kit !!

67rsss

iTrader: (4)

We have knocked down a consistent 27mpg going to shows all over this summer (over 3K miles now), that is the average of city/highway/idling at shows. This is in a 3200lb 67 Camaro, which is a little bit better on aerodynamics. We are currently running a 3.08 rear. It is hard to say what you will get, but we get 21mpg highway on our 02 Suburban with a 5.3L in it, and it is a true brickwall! I think you should go for the swap, the difference in drivability is amazing, and the fuel injection is really nice when the weather changes.

Jhankinson

iTrader: (1)

I think I got something seriously wrong then in my tune. I only get 14 city and 18 highway with my LS1 S10. And thats with cruise on... So I guess I should go get a wideband tune done and hope it fixes my MPG slump. OOO yeah...this is with 3.42 gears and 255/35/20 tires. At 75 its at like 2200rpm....so 18 isnt good IMO

kirtondog23

so what you are all saying is I should be unhappy with my 8.5 mpg 6.0 very large brick

OldSchoolFormula

I can eek out 24-25 MPG if its strictly freeway cruise at 70MPH, but generally don't see that. I seem to average 18MPG or so...but I tend to drive kind of fast for highway cruise. To be fair, it hasn't had a tune, but it does have headers. But, I'm also running on lousy California fuel oxygenated and with ethanol..

NX74LS6

You have to tune after headers... my SS( w headers) without a tune felt weak , it wanted more fuel and as soon as it was tuned that's when the mileage jumped. And picked up 1.5 mph in the 1/4. Tune that puppy you'll be very happy.

OldSchoolFormula

Shoot, I thought the 12.9x's with 3.08's was pretty good as is! LoL. The motor/trans is getting yanked though for a 224 cammed '00 LS1 with a T56. I'll get that one retuned.

Bo185

I really wanted a 6l80 and change my rear gears.

I think with a 5.3L 6l80 and 3.08's that should get the mileage up! Guess I'll start looking for a 6l80.

NX74LS6

I would expect to run mid to low 12's in my Camaro. You should be right there with some good tires with the 224/T56..... IF you can launch it ( sub 2.0 '60). I've seen cammed LS1s run low 13's .... great launch (1.8 '60) and power shifting and you might be damned close to an 11 sec pass .

sparkwirez06

iTrader: (18)

I have a 02 silverado 5000+ 4x4 brick and I can get 20+mpg 5.3 with 373 gears. I would think your car could do much better.

jasons56

I used to have my 56' chevy set up for mileage to make a 250 mile trek to and from college. Muncie 4 speed, Ford 9" with 2.75 gears, 350 .040 over with single plane intake and Holley 3310 750 vac secondary, Cam with 244degrees @ .050 and .507 lift. Electric cooling fan. Mildly ported 993 smog heads. Ran LOTS of advance, vacuum dissy, stiff spring in secondaries.
Driving it hard and the interstate, would get 17 - 18 at 80mph. If I drove for mileage, 21-23 was do-able at 65mph. My best was 24mpg with an absolutely still 75 degree day. PS usually ran tires quite over inflated. Car was also sitting pretty low. Helps frontal area some.

Same tune with 3.89 gears would get 13mpg or so.
I would sure think low 20's should be a no brainer with the right gears/tranny.

Ackattack1

iTrader: (7)

Like other said, my 05 4wd ccsb with a 5.3L will get 20+ on the highway (5800 lbs). It has 3.42 gears and 32" tall tires, so it keeps the RPMs down.

Other things will also affect your mileage.

In my 72 pickup, I have a TPI 350 and 700R4. When I first did the swap, I had the stock 3.08 gears, and at 60, I was running around 1500 rpms which dogged the engine bad. I switched to 3.73 gears which brought the RPMs up to ~1900. Fuel mileage went from 14 to 18.5. (Just some things to think about)

The gen3 motors seem to work well at lower rpms. The vette is under 1500 at 60 and is happy there (I've gotten over 31mpg).

I think if you could shoot for about a 1700ish cruise rpm, you'd be looking at 24+ mpg (maybe more) in an auto. 6 speed manual probably 26+.