454 to 350 (Excuse the year model in question)

[jaspencer]

This question could go for most any year truck. I have a 1991 Silverado 3500 (454 rated at 255Hp). It'll tow anything I put behind it but only gets about 10-12 mpg. Would a 2000 or so smaller displacement engine of the same or better HP net enough gain in gas mileage to make it worth the swap. The 2000 Silverado came with a 5.3L 285 hp V8 and got 16-20 mpg with overdrive. In my way of thinking. I would still have the sturdier frame of my trusty 3500 and the gas mileage of a newer 1500. Give me some thoughts please. I know just enough about this stuff just to be dangerous.
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[zieg85]

Torque is what is getting the job done with the 454 not HP.  The mileage from the 2000 is probably a 1/2 ton, a lot less rolling resistance and weight.  I have ridden in the 5.3L and it was impressive but will it do the towing you need???  What gear do you have in the 91?  How much driving do you do?  All valid questions when considering a ROI if the effort and expense are worth it.

[1980c10]

there is a lot more to it than displacement. I am in the 16-21 with my carbed 350. However I also have an overdrive manual trans, electric fans, very carefully tuned from the carb to the tailpipes. this combination dynos at 393 hp@5400 and 432 tq. An improvement in every way over the originial inline 6. You really should be looking at your entire set up i don't see why you cant see 15-17 if set up right.

[jaspencer]

Not sure about the gearing, and that year did not have an overdrive trans. I didn't think about the torque. Though not technically legal, I have pulled a 10-ton rated Hudson trailer with a full sized backhoe and had more issues with stopping than taking off. A 350 probably couldn't have pulled it off. I'm gonna try taking the lund sunvisor off and a couple of other things to see if I can get a little better cruising mpg. I don't expect much gas mileage hauling horses or a dump trailer. With only 84,000 miles, this truck is still in excellent condition inside and the clearcoat/paint issues can be solved easily. Thanks, and any ideas for mpg would be appreciated. (Transmission suggestions??)

[zieg85]

A 700R4 or equivalent OD transmission would do wonders for mpg I would think.

[Edahall]

The only way I know to significantly improve your fuel economy and keep your towing ability would be to install a Cummins 6BT 5.9 and an appropriate transmission.  My 2500 Suburban with this combination gets 26 mpg on the highway and it'll pull anything you put behind it.  When it had the 350, the best fuel economy it would get was 15 mpg.

[jaspencer]

Thanks guys for the suggestions.