LS vs SBC mpg calculations

[Billett]

Couldn't sleep so I decided to do a fun little exercise. My truck is getting some pretty terrible mileage with the stock 350 and np435 and I've been thinking about doing a swap to an ls. I wondered, is the initial cost worth it? How long will it take to break even on fuel savings? Well I did some calculations based on some mpg recordings I've gotten from swaps on this site. Here's how it went.

Starting with my current 350 I get about 11 mpg. That's about .09 gallons per mile. Given a gas price of $3.50 it costs me $0.318 per mile. I pay 30 cents a mile!!! That's insane! Anyway, cost to swap is $0 since it's already in there, but I have the feeling that this thing is going to crap out on me within a few years. Also, just for comparisons sake, it'll cost me $15,909.09 to go 50k miles.

If I were to swap in an LQ4, which I'd really like to do, I'm hearing an average of around 17 mpg. This gives me 0.05 gallons per mile, $0.206 per mile and a savings of $0.112 every mile I travel. That's a third of my current cost so it sounds pretty good so far. Cost to swap, (being conservative and not doing anything other than a rebuild), I'll say is $5000. That means I could travel 44,523.8 miles in my current setup for the cost to swap. Thing is, I doubt my 350 has that many miles left in it anyway before a tear down is in order. Oh, and this would cost me $10,294.12 for 50k miles.

Keeping up? Good, it gets better. Let's say I went for a 5.3 instead. Personally I like the cool factor of the 6.0, but as far as mileage goes the 5.3 should get closer to 20 mpg from what I've seen on here. Also, the 4.8 has been damn near the same so consider these numbers the same for both engines. 20 mpg gets me 0.05 gallons per mile, $0.175 per mile, and a savings of $0.143 per mile. That's almost half of my original cost, for you folks following along at home! From what I've seen this cost to swap is cheaper due to the price of the engine itself so I put it at $3000 to swap. That will get me 20952.4 miles from my 350. It's dead even at $8750 for 50k miles.

In regards to the 50k mile price, that's a $1544 difference. I thought it would be higher, but still, that's a good chunk of change. Personally I think $1500 is worth the price of having the extra power and cool factor, but either way you're saving a minimum of almost 6 grand over the sbc. These numbers pretty much made the decision to swap for me. It's a no brainer.

Also, for those of you who haven't fallen asleep yet, I did another set of calculations for my motorcycle just for the fun of it. I have a Yamaha xv535 cruiser that get's 45 mpg or so, but you could pretend it's a civic hybrid or something as well. I bought my motorcycle specifically with fuel economy in mind because I knew the truck was so bad, so in the summer I tend to use it much more than my truck when going back and forth to work. The 78 mile round trip would cost me just shy of $25 PER DAY in my truck vs $6 on the bike. Now then, back to numbers 45 mpg gets 0.022 gallons per mile, $0.078 per mile saving $0.24 every mile. Initial cost was $2500 for my bike. That would have gotten me 10,399.2 miles in the truck. Money well spent believe me. This thing will cost me $3,888.89 at the end of 50k miles.

Here's a quick and dirty, figure that a BBC or 305 will get roughly the same as my 350, and the 4.8 is the same as the 5.3:

Engine - MPG - Cost per 100 miles
350 sbc - 11 - $31.82
6.0L - 17 - $20.59
5.3L - 20 - $17.50
Motorcycle - 45 - $7.78

Moral of the story: Need a cheap way to get better mpg's? Buy a motorcycle!

In all seriousness though, I really hoped this has helped some of you guys as well. I have the actual spreadsheet I did this on if anyone wants it, and I can do more calculations if people request it. If you feel some of the mpg estimates are off I can tell you what the changed calculations would be, but I really just wanted to give an idea for just how much these things save over the Gen I engines.

[67swb72klb]

wow you have been crunching some numbers but as far as the motorcycle goes you didnt add the cost of plates and insurance lol just sayn

[Billett]

Haha or a helmet and test for that matter Not to mention the fact that I feel much safer in my truck and you can't buy that!

[jorgensensc]

Ok, throw these numbers in your formula for me:
Very tired oil burning big block with th400 tranny. I get about 7 mpg as a daily driver and have to put about 2 quarts of oil for every fill up at about 2.00 quart. So figure in the cost of a rebuild also. I am putting in an 08 6.2 with 6 speed auto. Doesn't need a rebuild at 48k miles. However time is not on my side so I buying things I would normally do myself (harness, brackets, mounts, etc). So I'm going to be in this for roughly 8500 if I stay on track. And cheap gas here in the land of fruits and nuts (california) is on average 4.29 for reg 87 octane. How do my numbers come out, if of course you don't mind running them.
Shawn
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[mattcrp1]

I like the break down, here in California our fuel is the at least $0.25-$.0.50 higher. And we have ethonal in our fuel as well that does and that does not help mileage.

[Clyde65]

The one thing your not factoring in is the tranny, which tranny you going with. To get close to the mpg numbers you are quoting you will need the 4+ auto speed tranny and a highway friendly gear/tire combo. The thing I like about my swap is I can jump in and drive it almost anywhere I want. The old SBC and 3 speed I had, I didn't have that confidence.
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[derrickmanx1]

I think your 350 needs a serious tune up and an overdrive. What is your tack reading @ your most common cruise speed? I've had 350's get good mileage with the right gearing and tuning. By good mileage I mean 15-17 mpg not in the fuel injected ls1 range of 20-22. I'm hoping to get 12-15 with my bbc and Fast injection.

[Billett]

Originally Posted by jorgensensc

Ok, throw these numbers in your formula for me:
Very tired oil burning big block with th400 tranny. I get about 7 mpg as a daily driver and have to put about 2 quarts of oil for every fill up at about 2.00 quart. So figure in the cost of a rebuild also. I am putting in an 08 6.2 with 6 speed auto. Doesn't need a rebuild at 48k miles. However time is not on my side so I buying things I would normally do myself (harness, brackets, mounts, etc). So I'm going to be in this for roughly 8500 if I stay on track. And cheap gas here in the land of fruits and nuts (california) is on average 4.29 for reg 87 octane. How do my numbers come out, if of course you don't mind running them.
Shawn
Posted via Mobile Device

Might want to sit down... I put the 6.2L at 17 mpg also, but I can adjust it. It should cost you around $89.80 to fill up now with a 20 gallon tank and $4 per fill up due to the oil. That means it'll cost $85.80 to fill up with the 6.2, but you'll be able to go about 200 miles further on a tank! $8500 could get you a little over 20k miles on your current setup. 50k miles with your current one will cost you $31,518.37 and the 6.2L would only be $12,617.65. That means that within 50k miles you'll have saved your swap cost twice over! I didn't put any rebuild costs in because that can vary greatly, and I really did this just as a rough estimate for mpg purposes alone.

Originally Posted by mattcrp1

I like the break down, here in California our fuel is the at least $0.25-$.0.50 higher. And we have ethonal in our fuel as well that does and that does not help mileage.

I know it's a lot different all over the place. I think my area is probably one of the lowest in the country, but if you think about it, that just means that you are saving even more. The fuel savings per mile goes up about 2 cents across the board if I make gas prices $4.00 instead. The actual mileage you get from different blends is just one more variable though.

Originally Posted by Clyde65

The one thing your not factoring in is the tranny, which tranny you going with. To get close to the mpg numbers you are quoting you will need the 4+ auto speed tranny and a highway friendly gear/tire combo. The thing I like about my swap is I can jump in and drive it almost anywhere I want. The old SBC and 3 speed I had, I didn't have that confidence.
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This was just about gas mileage. Different engine/tranny combos will definitely get different mileage but at the end of the day you will still end up with one mpg number for your setup. I do plan to swap to the 4l60/80 or even the 6l80. Using an older trans would save you some initial cost, but like you said you'd be losing mileage, and why bother doing the swap without getting overdrive to go with it? At least from an mpg perspective that is. I completely agree with you on the reliability aspect. I love the farm truck feel of my 350 and 3 speed, but I really wish I could feel more confident about it. It's not that I don't trust it to get me places, but it can be a bit finicky sometimes.

Originally Posted by derrickmanx1

I think your 350 needs a serious tune up and an overdrive. What is your tack reading @ your most common cruise speed? I've had 350's get good mileage with the right gearing and tuning. By good mileage I mean 15-17 mpg not in the fuel injected ls1 range of 20-22. I'm hoping to get 12-15 with my bbc and Fast injection.

Definitely needs a tune and overdrive would help, and in a way that is what the ls swap would do. I'd get the overdrive from the 4l60 and I could use a tuning program from my pc instead of fidgeting with temperamental carbs all the time. No idea about tach reading cause I don't have one, but I run an np435 with no overdrive, 3.73 gears, and 28" tall tires

Like I said, there are a lot of variables and I can't hope to cover them all, but I thought the savings in gas alone was interesting.

[jorgensensc]

Well that makes me feel better!! I figured 2500 for a good total rebuild. using some rough numbers I think I can break even on the swap factoring in the rebuild cost of the Big Block in about 1 1/2 years! I'm totally stoked. That makes me feel a whole lot better about shelling out the money now.

[derrickmanx1]

With 3.73's and an overdrive you'd be in the 2.73-3.08 range. That would help a lot. I wonder if a 5 speed manual out of a mid 90's 1/2 ton would work for swap? That would be kinda cool.