Looking for advice or thoughts on conversion.

[85 silverado]

Okay, this is just me thinking right now. I have a 91 rcsb Z71 that I would like to see get makeover. I have had the truck for about 10 years now and it is just sitting not being put to use. I am thinking about selling me Jeep cherokee and putting some money back into the ol truck.

What I am wondering is if I sell Jeep which is getting about 20-23mpg then what are my options as far as an LS engine to get decent mileage. I have way too many vehicles and don't want to get rid of the 91 but I don't like the throttle body engine. I know it would be cheaper to just buy a newer truck. I do have an 03 HD2500 crewcab 4x4 that has the 6.0l so I do know a bit about how it performs in the big truck. I would like to see a 6.0l in the 91 but I am sure mileage wouldn't be that great. So would a 5.3 or a 4.8 be a better option. This truck would be a daily driver that I would use to do all my personal running in and also use for doing my estimate for my floor care business. What are your thoughts. Thanks

[gagliano7]

5.3 would be your best bet for gas mileage and performance.

[canada4g63]

4.8 or 5.3 will be better on gas. Gears, and if you 5 speed swap it would also help with mpg.

[tfi racing]

You will have to drive that 91 many thousands of miles to ever recoup or even justify your investment in a LS swap.It is tough to save fuel because these engines really wake these trucks up,most of us are too heavy on the gas pedal afterwards to offset any mpg improvement...

[85 silverado]

I don't want to be so conservative with the mileage to where I should just go but a 4cyl. or something but didn't know if it was out of line to get 20mpg or so with a newer LS engine in this truck. The truck does have the factory 3.42 gear and I am going to stay automatic. I switched the truck over to 5-speed a few years ago and its okay I just get tired of shifting for a daily driver and my wife refuses to learn to drive a stick.

As far as putting a bunch of money into this truck I don't mind. It wouldn't be much different than going out and spending all that money on another truck that is newer. Although I am sure there would be more money spent doing the conversion and everything in the long run, but I would still have the truck that I have had for the last ten years or so.

[pl4yboy]

I call bs on everyone who says 4.8s and 5.3s get better mileage. The trtturbo truck which is a 92reg cab step side got 18 hwy running 80mph... Then busted off an 11.3 pass with his 6.0. My crew cab z71gets 18-19 hwy all day and had a big cam and stall and a 6.2. My sisters stock 4.8 gets 15 hwy in a rcsb 4x4.

It has less to do with displacement and more to do with the driver and the tune.

[DBRODS]

I agree Justin, lots of bad info in this thread. I have a customer that I did a LY6 (6.0 with L92 heads) swap into a 62 Ford unibody truck. It has 4.11 gears and a T56 tranny. We were talking the other day and he mentioned that he averaged 24mph on a trip from TX to Arizona a few weeks ago with AC blasting and cruising 80mph. IIRC it has a ASA cam and all bolt ons. Nice clean truck and fun to drive.

[pl4yboy]

I agree Justin, lots of bad info in this thread. I have a customer that I did a LY6 (6.0 with L92 heads) swap into a 62 Ford unibody truck. It has 4.11 gears and a T56 tranny. We were talking the other day and he mentioned that he averaged 24mph on a trip from TX to Arizona a few weeks ago with AC blasting and cruising 80mph. IIRC it has a ASA cam and all bolt ons. Nice clean truck and fun to drive.

That's a first, but its a start haha. Now with a T56, obviously the mileage is going to be way better. Devin's 5.3 that put down 518 was getting 25mph with his 5 speed. But yes, in general, the tune and driver are going to determine mileage. Gears play a bigger part of the equation than the displacement of the motor.

[kerrige]

DIdnt frenchy get like 26 mpg to and from the shoot out in his 5.3/5speed setup. I think the key to good gas mileage has 2 parts. First a smart economic right foot and second a good setup. Very important factors are tire/wheel/ gear combos, overall weight, fine tuning at highway speeds, and of course sufficient power to allow the truck to make the least effort possible. Down shifting on every incline will not lead to good mileage. I would think with it being a rcsb/z71 it would prolly get great mileage with a modded 5.3 or 6.0 where as a 4.8 would be too underpowered to be highly efficient. I am willing to bet a higher compression(11:1) 5.3, with boltons, cammed, good flowing heads, and a spot on tune allong with 30ish" tires, light weight wheels, a t56, and 3.90s would be best for this vehicle if looking for pure mileage. Of course some trade offs would need to be made to fit budget and end user desires.

[pl4yboy]

Displacement doesn't matter as much as keeping the motor in a spot that doesn't lug and doesn't overrev. While that requires a certain amount of power, it can be done with a 408 if its set up right. My truck, that sweet spot is about 1900 rpm. That's where it gets the best mileage.