Miles Per Gallon versus Set-up

[ubtripn]

I want to start by saying that I know there are those who say, "It's not about the mileage or if you have to ask" that type of thing and I respect that. I am asking the following question because MPG is a factor - for me. I use my truck as a daily driver around town. I am about to purchase a Crate Engine and my goal is to set up a naturally aspirated 350 to get the best MPG I can get. I realize that it won't be high but I want to see how high I can get it. Power is not a concern because even the least tricked out 350 will have enough power to do what I need. Not having a truck payment already helps off-set the cost of fuel but I would still like to know how others are fairing so:

1. What are you currently getting with your 350, your carburetor, your transmission and your rear-end combination?

2. What do you think the optimal set up would be to maximize mileage for a daily driver without using fuel injection?

This is for a 2 wheel drive. I currently have a 350 with an Edlebrock 650 and a 700R4, rearend is still not identified. I am getting around 12-13 now. (I think it used to be better.) I keep the oil and filters fresh along with the plugs.

Thanks for any information you can provide.

[trac209]

Well I'm getting about 9 to 10 mpg to my best calculations. I have a 350, ported 2.02-1.60 heads,approx 8.5 to one compression,mild camshaft. Trans is a 350 turbo with 2800 stall. My 55 mph cruise rpm is about 2900 rpm best guess as I don't have a tach.
I've towed my 1500 pound tent trailer and had 500 pounds in the bed and the mpg doesn't really change lol. Next summer I'm looking to get more mpg out of it maybe a gear swap or od.
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[ubtripn]

Ah, thanks for the info. My truck runs 2100 RPM at 60 so it must be the tranny differnece.

[truckster]

You really should figure out your rear end ratio - it's a good place to possibly pick up a couple of MPG.

You could always pull the cover; that gives you the opportunity to change some very old gear oil. Otherwise, lift one wheel off the ground and put a mark on the tire. Also mark the driveshaft. Turn the wheel and count how many times it rotates to make one complete rotation of the driveshaft. That's your gear ratio. For example, if the wheel turns 3 3/4 turns for one turn of the driveshaft, your gear ratio is 3.73.

[ubtripn]

Yeah, your right. I do need to do that.

[69gmcc10]

what size of tire are you running? I might be able to calculate your rear gear ratio for you.

[CC69Rat]

http://700r4.com/faq/ratiocmp.shtml

Check out this site when you can. Here's what I think you might be running into. Let's say your truck is a 3:08 rear gear from the factory. I'll go into how to check in a sec, but you will need to pull the cover. If the gear in the rear end is 3:08, and you swap to a 700R Trans, where you used to have the fist gear of a TH350 .. or whatever was in it before. Just kinda thinking as I type, this would have REALLY lowered that final drive once you hit 4th gear. My bet is your engine is just barely turning over at 65mph? Maybe 1800rpm ? ..

I used to be into Jeeps a lot too, and if you think about it like this. If I lower the gear ratio WAY down it causes the Jeep / Truck to kinda 'tractor' around. For example the engine is not getting into it's power band to make true power / efficiency so it's goning to drink gas.

If you changed that rear gear to say a 3:55 (?) It will not only compensate for the 700R first gear (More seat of the pants power) But when it hits 4th gear it will not do the 'tractoring' thing as much. HAve you ever noticed a car sort of has it's sweet spot? In my wife's CRV about 63-65 mph it gets it's best mpg. If I get it in the 70 range it starts dropping quick. .. or if I drive 60 .. it doesn't seem to do as well. It's almost as if the engine sort of has it's happy spot.

In short, what I think happened is while your 700R is a good thing, and good idea to swap, .. that 4th gear final drive is not enough to keep it from lugging around.

I could be WAY off here .. but that's just kinda how I see it.


Think about a stock Jeep with big tires. If you don't re-gear it, on the freeway in 5th gear it won't fall out of a tree. It will need to be geared down just to climb a hill.

Honestly, check the rear gear. Pull the cover off and while it's on a jack, turn the rear ring gear. There will be some numbers stamped on it. For example 14:43

Divide the large number by the small number. 43/14 = 3.07xx 3:08 gear. (that's what's in my truck) Most trucks with a V8 and 3OTT were 3:08 I6 and 3OTT = 3:73

Hope this helps.

[ubtripn]

You are dead on except I hit 60 at around 2100 in 4th on the highway and that is the sweetspot (more like a cliff) because anything over that I have to push the pedal waaaaaaaay down and it slowly gains speed.

I know it's bad thaty I cannot recall the tire size but they are the stock size equivilants.

I will get my gear numbers this weekend. Thanks for such a great post!

[ubtripn]

My tires are 235/75 R15's.

[CC69Rat]

28.87" tall tire, Im guessing 3:08 for 2100 @ 60 with 700R.

Bump it up to 3:55 and youll actually see an improvement in both power and fuel economy. Especially on the freeway. 3:73 it will start going the other direction. 3:42 is a good gear too.
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[dukcaln]

Originally Posted by trac209

Well I'm getting about 9 to 10 mpg to my best calculations. I have a 350, ported 2.02-1.60 heads,approx 8.5 to one compression,mild camshaft. Trans is a 350 turbo with 2800 stall. My 55 mph cruise rpm is about 2900 rpm best guess as I don't have a tach.
I've towed my 1500 pound tent trailer and had 500 pounds in the bed and the mpg doesn't really change lol. Next summer I'm looking to get more mpg out of it maybe a gear swap or od.
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same motor set up, other than 10.5 compression and TH400. 373 rear and 3k at 60... this set up I don't know yet but my old motor sb350, mild build with TH350 same rpm I was getting 9-10mpg in town only...

[ubtripn]

Thanks for the data everyone. I will do that, 3:55 it is. Thanks a ton!

[ubtripn]

With the 3:55 gears what RPM's would I be hitting at 60 do you estimate?

[dukcaln]

what size rear tires?

I think your in the ball park at 10-14mpg with either set up

[GMCJOE]

IMO, if your not make vehicle payments you are saving money. I got 3 vehicles I pay on and then have to pay the gas to drive them around. The GMC is my project, even if it sits without being driven I am only paying $119 a year for insurance.

If you want to get fuel economy out of it with out spending a ton of money, I would first make sure you have a gear ratio that is suitable to run with the overdrive transmission. It will kill your fuel economy putting more stress on the engine running something like 3.08's with an overdrive, that is if it even kicks into overdrive.

Another way to improve is to get rid of the Edelbrock. Edelbrock is not known for fuel economy, its a mid performance bolt on carb. The best way to go to stay carb'ed is to purchase a Quadra-Jet, they are the closest you can get to the fuel injection from a carburetor. You will get better fuel economy and notice better performance than an Edelbrock if tuned right. If you decide to go that route, I recommend purchasing one of these two depending on your application:

https://www.smicarburetor.com/produc...2/productID/18

https://www.smicarburetor.com/produc...2/productID/47

These are cheaper alternative than going with a Cliffs performance Carburetors and seems to have pretty much the same work involved.

Another option that is more expensive, but effective. Put a fuel injection system on it, like EZE EFI. But if you are going to spend that kind of money, I suggest just picking up a 5.3 liter with 4l60e/80e combo and doing the swap. The swap would give you the best result for fuel economy and be a cheaper bang for your buck compared to the EZE EFI. I have heard of myths of 17 MPG city and 23/24 MPG highway in the 67-72's with the 5.3 swaps with a good tune. I call it a myth cause I don't have this setup and can not confirm it.

But anyway you go (carb swap, fuel injection, or 5.3 swap) to get the best fuel economy, you are going to have to have the right gear ratio to match up with your overdrive transmission.

Seems like you are getting pretty decent MPG's with your setup compared to some as of right now.

[69gmcc10]

transmission gear calculator tools. play with theses and you can find allot of answers.

http://www.markwilliams.com/calculators.aspx
http://www.angelfire.com/fl/procrastination/rear.html

[69GWC]

I know this, I have a 06 Z71 with 32" tires( by a tape) and 3:42s and 17/18 mpg on the HWY is max lucky to see 14/15 in town. I average 15mpg out of it. Pulling a 5x10 enclosed trailer i get 11 mpg.
I have had 3 5.3L equipped trucks with the others being 2wd and have never seen better mileage out of them.
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[GMCJOE]

Originally Posted by 69GWC

I know this, I have a 06 Z71 with 32" tires( by a tape) and 3:42s and 17/18 mpg on the HWY is max lucky to see 14/15 in town. I average 15mpg out of it. Pulling a 5x10 enclosed trailer i get 11 mpg.
I have had 3 5.3L equipped trucks with the others being 2wd and have never seen better mileage out of them.
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Not to highjack the thread, but you must have the L33 engine in your truck. Love that engine. My friend averages 17 with his 05 Extend Cab Z71 L33 equipped truck at stock height and 3.73's. If your truck isn't been tuned up, I seriously recommend doing a http://nelsonperformance.com/ ECU tune. The work some magic with the GM products..... Now, back to the regular scheduled programming...lol

[blackdawn]

i am in a very similar boat to you, and have been reading up a lot. here are a few questions for you, what speed do you want to run/ will be running for the majority of your driving? (i drive 50 miles per day to work and back 40 of which are freeway at 65-70mph) what kind of budget are you looking at? can you afford to replace eng/trans/rearend in one go?

i hear you on the carb thing, i am avoiding computers and anything more complex than the truck initially came with for the most part. keeping it very easy to work on. for carbs there really isnt much better for mpg than a quadra jet that is tuned in nicely (pay someone who knows what they are doing to get it just right). the quadra jets aren't good for racing etc so some people hate them, but they are great for cruising.

the setup i intend to build will be a 700r4, 3.73 rearend, and a 327 motor built for low end torque (i know this is controversial on the motor, but i have a very competent engine builder and i've done a lot of research/tinkering with the idea). i plan to run 29in tires and will be around 2000rpm at 70mph on the freeway, which should be a nice sweet spot for the motor.

anyway you go good luck with the project

[GMCJOE]

QuadraJets are very good for racing. Read up on Cliffs QuadraJets and the ET's these guys are putting down. That dude is great with the Q-Jets and builds them specifically for what you plan on using them for. But for his wait time and pricing to get one to you is another story. Better off to purchase his book and build your own...lol That's why I posted the link for the SMI Q-Jets, built on the same basis as Cliff's. I will be swapping over to a SMI Q-Jet at some point after my 700r4 swap. At that point I will have a 96 LT1, 700r4, and 4:11 ratio. Not really after MPG's, as I am looking to get the RPM's down and have a higher MPH in the end also. Though the MPG gain will also come.

[CC69Rat]

Just listen to the engine as it runs through the gears .. as compared to 'efficiency' overall. With 3:08s and the 700R it will actually be working harder to make the power it needs to turn the tires over that fast. Where if it had a little more gear in the rear end to get it up to that speed quicker .. see what I mean?

When you buy a camshaft for your engine the Cam is designed to operate in a certain RPM range to make power. If I put a giant cam in a stock 350, I have to run a stall converter to bring the RPMs up so the cam can actually make power. Same concept.

We gotta get your truck back into it's 'sweet spot' .. As I mentioned, the 700R4 is definitely a GOOD thing to have but unfortunately the truck appears to be setup for the older TH350/TH400 that was in it. TH400 would like the 3:08 because of the gear range. 700R drops the final drive because of the 4th gear, but with a 3:08 man, it drops it way down. When it hits highway speeds and 4th gear .. BLLLUUUUUUUHHH and you have to really give it the gas to make it go and/or keep the speed of traffic.

If you're at 2100 @60 mph with what we're thinking is 3:08 - IF you go to 3:55 I would guess 500 more RPM. Notice that sits you right in the range of Highway driving and Fuel Economy with a bit of around town driving. PLUS the truck will feel like it has 30-40 more hp.

Check out this site, I know it's a Jeep site but it will help.
http://www.jeep4x4center.com/jeep-ti...atio-chart.htm

Chart says 2674 rpm with a 3:55. Do you know what cam is in the engine?

[GMCJOE]

CC69Rat... you speaking the truth right there. I am running between 3500-3700 RPM's with a TH400 and 4:11's at highway speeds. I get the buttdyno feeling stomping into the gas...lol Not very friendly on the fuel and I am sure if I was to regularly take it highway it would put undo stress on the engine. Budget wise I been having to do things one at a time, so 700r4 is next step like UBTRIPN has. Essentially I be running my setup like what would have been in the 96 Corvette that the engine came out of minus have fuel injection.

So, lot of factors to consider to get your best MPG. Cam, Carb, transmission, gears, tires all go into the equation to effectively run your engine in a good RPM to accomplish this. Use them gear ratio to rpm calculators, they work wonders with researching this kind of stuff. Started a while back just to figure out what RPM's I would be sitting at with over drive and the size of tires I am running.

[PAWS 72]

With my truck,I'm running 255-70-15,350 engine,350 trans and 3:08 gears.At 60 mph I'm running around 2200 RPM. The sweet spot is 58 mph and it gets rite at 14mpg.

Now with that said I can bump it to about 60 or 61 mph and the mileage will drop to about 11 mpg. I don't understand why big drop but....

[bonnieclyde100]

13 mpg for an older truck with a carb is not bad. the best way to increase mpg is to get a newer engine with vortec heads and fuel injection.

[Frkypunk70]

I have 3.08's and 350/350 combo gets 10/12 miles to the gallon. It's always ready to pounce , but on the highway at 70 it feels like its never gonna shift. It's like I ran out of gears. I started a thread on this a while back cause I thought I needed a 700 r4. Turns out I wasn't driving fast enough or slow enough
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