Question for you 5.3L guys

[67 cst swb]

What kind of fuel mileage are you 5.3L guys getting when installed in these 67-72 trucks?
I am assuming being that they are a lighter truck than they came out of, that they are better MPG. Obviously if you have added performance parts that may change things abit. So, I guess this question is more for the stock 5.3L guys that simply put in the motor and trans, redid the wiring and ran it as is...

[67chevy1series]

Having a aluminum block sheds a lot of weight and may produce a better fuel economy in our trucks then having a iron smallblock. (Our small blocks vs an aluminum block LS engine is 200 pounds difference roughly)

[57larry]

who cares about fuel mileage, when you put a built 350 in these babys, you can really smoke the tires! and I'm 64

[67 cst swb]

I agree with you both. But, neither reply answers the question.
I am curious if these 5.3L motors are getting much better mileage when installed in our trucks than when they were in the original vehicle.
I would think they would, but what is the typical MPG for a stock 5.3L in a 67-72 Truck?

Reason is... I am considering a LS motor in my project I am about to start. But I drive between Rapid City, SD and Las Vegas, NV about 4-6 times a year.
I can routinely get 18-20 mpg out of the 283/TH350 set up that I already have. I also have a 2003 5.3L GM Tahoe (6400lbs) that gets about 13-16 mpg.
So, I am considering the LS idea due to the tire melting capabilities of the LS motors, but if the mileage stays about the same... I may keep what I have without spending more money.
But if these 5.3L do get 18-20 mpg because they are in a lighter vehicle. Then I would be up for the swap to get comparable mileage with much increased performance.

I know, I know... if you add it up it is worth the $ in entertainment alone. But its just a matter of spending money to spend money or don't spend money and keep what I got now.

[67ChevyRedneck]

You're going to get realistic numbers and some that are BS.

My 5,600 lb 2000 5.3L Z71 ext cab 4x4 gets 13-14mpg city and 18mpg highway. That's 10+ years of checking it randomly... it's always pretty much the same.

Expect a realistic 13-14mpg "city" and 18-20mpg "highway" depending on gearing, etc @ around 70mph.

Yes, these trucks are much lighter than the new trucks, but they're also as aerodynamic as a brick. Believe it or not the 99+ trucks are actually pretty "slippery" through the wind.

Of course, start adding power and those numbers will drop.

[Ronzo]

Originally Posted by 57larry

who cares about fuel mileage, when you put a built 350 in these babys, you can really smoke the tires! and I'm 64

love the attitude Sir.

[tsetsaf]

4l60 3.73 gears 265 70 17 averaging 16. Best on the highway has been 18. Working with Blackbear Performance to get the tune good and just installed a update. Goal is 20 highway and I think we are close.

[tsetsaf]

I would do the swap again in a heartbeat. My wife does most of the driving and the reliability can not be beat. Just turned 19k on it and do you know how many spark plugs I have checked? Timing adjustments? Carb checks? Etc

[gottattooz]

4.8L/4L85E/3.73 gear ratio and I get 16-18 mpg in my 68 Shortbed Fleetside.

-Josh

[Lees68GMC]

Turning up on 3000 miles since my 5.3 4L65E was installed and I can't honestly say because I drive my truck like the Camaro it thinks it is, fast, fun and hard so Gas Mileage ??? I'm 63, my 68 GMC had a good 4 bolt 350, Edelbrock manifold , 4 barrel and HEI, Turbo 400 with 3:42 gears and it couldn't hold a candle to this 5.3. My truck is 3680 Lbs across the scales and runs GREAT!

[jimcaf]

I completed my install a couple months ago,I haven't calculated the MPG yet but seems substantially better then my old 350/granny 4 speed and 308s ,now have 5.3,4l60e,2500 stall and 410s ,much smoother and fun to drive..definitly worth the effort

[schwoch1]

When I converted my '71 GMC to a 2004 5.3, I got in the area of 13 in the city and about 17-18 on the open road. 5.3/4L60E/4.11 rear gears with 255/70R15 tires in the rear.
I would do the swap in a heartbeat again, was one of the best mods I did to the truck!

Mike

[windmill]

for the mileage if you want it,,, you have to keep rpms down below 2000.. look at the vette's with the 6 speed they get the best 6th gear is 0.50

[Andy4639]

Not a 5.3 but a 6.0 LQ4 iron block aluminum heads. I have the 4L80 E transmission all is stock except for the headers. I have 4:10 rear end gears.
I get about 16-18 on the highway. I haven't checked it since getting back from the 2014 HRPT back in June, those where the numbers for the highway nothing else matters to me. No matter which LS motor you go with you are way a head of the game over a old small block.
Mine has 289RWHP at 305 ft. of torque. My motor came straight out of a wrecked 2500 HD with nothing done to it. Had 89,000 miles on it when I bought it.

[67 cst swb]

Wow... for the same mileage, why wouldn't a guy go with a 6.0L/4L80E with... say, Ford 9" with 3.90 gears...?

[67ChevyRedneck]

There's "typically" about a 1-2 mpg loss with the 6.0L (assuming using same rear gearing as 5.3L), but yes, a lot of guys go with those for the power.

But really, you can do about the same with gearing like you said... and a 6.0L is *usually* a little more $ to get and a little harder to find, but a 5.3 with a 4.10 will have about the same "grunt" as a 6.0L with a 3.73.

The 4.8L is also an "underrated" motor. The early ones are a little low on power, but by the mid/late 2000's they were making the same power as the early 5.3L's and they tend to be a little cheaper to get.

[jimcaf]

Originally Posted by windmill

for the mileage if you want it,,, you have to keep rpms down below 2000.. look at the vette's with the 6 speed they get the best 6th gear is 0.50

Seems like most believe in this theory that gas mileage is related to rpm,I believe its more related to load,I take advantage of the gearing the 4l60e offers with 4.10 gears and my 5.3 dosnt even know its running up a mountain hill to get me home... just my theory wouldn't be the first time im wrong though

[67 cst swb]

Originally Posted by jimcaf

Seems like most believe in this theory that gas mileage is related to rpm,I believe its more related to load,I take advantage of the gearing the 4l60e offers with 4.10 gears and my 5.3 dosnt even know its running up a mountain hill to get me home... just my theory wouldn't be the first time im wrong though

Well, maybe that decides it for me then.
5.3L or 6.0L
4L60E
4.10 gear
30" tall rear tire

[scottybaccus]

My 2wd R1500 with 6.0, 4L80e, with 3.08 gears and 29" tires is getting 20 mpg. I can drive it around town, on the highway, or cruise at 85 mph. It gets 20 mpg.

[Aruba1]

It would be nice if we can also get some before MPG figures and the type of engine the 5.3 replaced. But I am subscribed.

[boraxman]

Dont know about my mileage but will say I have excellent low and mid range power w/5.3/

[cleanfreak]

I can't answer your first question , but having a bunch of older ls trucks , the 6.0 is definitly my choice between power & mileage. As said earlier , the 4.8's are good , get great mileage numbers , 6.0's are not as thirsty as some may lead you to believe,if you can drive them calmly. I would skip the 5.3 , stock they just don't have the 6.0 grunt , but still drink alot of fuel trying. Just another opinion - good luck

[Possmguts]

At 3000 RPM at 55 MPH I get about 2 MPG lol Im afraid to even figure what it really is But Ill do it once its back on the road with an OD tranny.

Its for sure not a freeway truck the way its set up.

[jimcaf]

I like the 6.0 too,, around here used they cost used about 30% more then a 5.3 in my case $1200 vs $1800 since I cheaped out when I did mine and settled for a 5.3 my next mod is going to be a set of long tubes from what I read u can great a great bump in torque should then be real close to a 6.0 and at cruise a little edge in MPGs..

[Andy4639]

Originally Posted by Aruba1

It would be nice if we can also get some before MPG figures and the type of engine the 5.3 replaced. But I am subscribed.

I had a 305 TPI with 700r4 in mine before the swap. MPG was about the same. The HP was nothing compared to what I have now. It will put a smile on your face now!