Fuel economy slowly improving after rebuild

[1SG.Rob]

Good morning! I recently had a stock rebuild done on my 396. When I first got it back from the shop, I was appalled at how bad the gas mileage was. I know these trucks aren't known for great mileage and that's not the reason we drive them anyway, but I was barely getting 100 miles out of a tank. However, the mileage has been slowly improving without me doing anything to the engine. I have about 600 miles on it now and am getting about 10 mpg. Is this normal? Does a rebuild improve on mileage as it progresses through the break in period? I haven't really changed my driving style that I know of, I drive pretty conservatively in the truck.

[1SG.Rob]

Though now that I think about it, I did put brand new tires on it about 2 weeks ago. That could make an improvement also.

[cleszkie]

Rings need time to properly seat after a rebuild. So do other components in the valve train. It is not uncommon for rebuilt engines to "sweeten" over time. They will also run cooler once everything is seated and self-clearanced.

[Coley]

I don't think you are going to get too much better than that....you might make 12mpg at some point.
If you want to bump that up a bit have you thought of switched over transmissions to a newer 4 speed automatic? (HD 700 series or maybe the 4L60 or 4L80)
...just thinking out loud here.

Coley

[1SG.Rob]

Originally Posted by Coley

I don't think you are going to get too much better than that....you might make 12mpg at some point.
If you want to bump that up a bit have you thought of switched over transmissions to a newer 4 speed automatic? (HD 700 series or maybe the 4L60 or 4L80)
...just thinking out loud here.

Coley

I don't think I will make a transmission switch just for the improved mileage, it would take forever to recoup that cost. Like I said above, we don't love these trucks for their mileage haha. But when I was getting 5 mpg I was a bit worried that it would be too expensive to be a daily driver. Now that I'm up in the 10-12 range I'm not too worried about it.

[trac209]

10-12 mpg with a BBC is decent. With an od you'd probably see 14-15 ish. Lots of things come into play like heads and cam. I had a suburban with a 454 that got around 10-11 mpg and when I stuck a Sbc the mileage was exactly the same.

[Marine-58]

Earlier this year I put a fresh built 383 in my 72 C10. Like your's my mileage was horrible. Mileage improved over the next 800 - 1000 miles. Now if I can keep the foot off the floor, it get's about 12 mpg. But that is with a good tune and taking it easy.

But like you said, it isn't about good mileage... And even my 2016 truck only get's about 16-17, so not bad for one that is 45 years old without all the computer crap on it... Not to mention how much more fun it is to blow the doors off the rice burners and set off car alarms in the parking garage at work...