chevy 350 good mpg?? 1991 suburban

hi i am 15 years old and i have a 1991 gmc suburban with a 1978 350 motor. it costs $125 to fill up the tank (40 gal tank) witch lasts about 1.5 to 2 weeks. with me working part time i basically work to drive. i figure i get about 9 mpg.i know that carb is messed up and and i am goin to get an edelbrock 600cfm. it has 33's with stock gears.i figure ill put a carb, intake, and cam and lifters.  so what can i do without breaking the bank to get better mpg.  

To put is simply, you are not going to do much better with current setup. The tires are costing you MPG from the effective gear ratio change and extra drag and the 78 low compression carbed motor is not helping you either. I have a 89 4x4 burb that I bought new and is averages around 17 MPG with the urban driving I do with it. My average never drops below 16 and sometimes it is as high as 18 or so after 180 K miles. What is my secret? First it is STOCK ( except for a custom 3 inch single exhaust with a quieter flowmaster)and with stock sized smooth treaded load range "C" tires at near 50 PSI and stock 700R4 with 3.73 gears. Next I NEVER use 87 octane as a mostly use 93 octane with a advanced baseline timing curve and it runs super and has very crisp response too. (it was a slug on 87 when it was newer and never did better than 14 MPG either on trips) It is not a daily driver and is mostly used today to run kids to college and back on weekends and tomorrow I am using it to move them home for summer and we use it for vacations too. It too has a 40 gallon tank and I can easily get 600 miles out of a tank on a trip with good reserve. Also also change oil every 3K miles and change and tranny fluid every 15K to 20k or 2 years which ever comes first. Rear axle lube gets changed every fall with 80w90. It uses no oil and is quiet and smooth too You have two basic choice here, loose the tires and the carbed low compression engine (600 CFM carb is not going to help MPG) and use a 700R4 if you are not already or bite the bullet and keep getting 9 MPG. There is no magic fix to make that big heavy burb with that old carbed motor and 33 inch tires get a lot better MPG. BTW, if you have dual exhausts you are hurting not helping MPG in town driving. Al of this is likely what you do not want to hear though.  

i have a 86 gmc sierra with a mostly stock rebuilt 350 with a 4bbl quadrajet carb, single exhaust, a 700r4, 35" swampers and 4.10 gears, i get 6mpg in town and 8mpg hwy, but every time i fill it up i remember its a truck and its fun to drive off road, i sugest you do what i did for better gas mileage, buy a $500 four cylinder front wheel drive compact car and build your burb for fun not mileage  

86GMCSierra:
i have a 86 gmc sierra with a mostly stock rebuilt 350 with a 4bbl quadrajet carb, single exhaust, a 700r4, 35" swampers and 4.10 gears, i get 6mpg in town and 8mpg hwy, but every time i fill it up i remember its a truck and its fun to drive off road, i sugest you do what i did for better gas mileage, buy a $500 four cylinder front wheel drive compact car and build your burb for fun not mileage

Quite honestly if you installed some 4.56 or 4.88's in this truck and used 89 or better octane and advanced base line timing about 6 to 8 degrees you would likely see 8 or 9 around town and maybe 10 or 11 on road.  4.10's sound deep but with 35's it is not and my guess is the OD is pretty worthless in that 4x4 and if you do use it is lugs badly in it. Deeper gears would let engine operate at a more efficient RPM and better fuel with advanced timing will help not only MPG but overall response too. Also, these days, fun and MPG tends to go together so you need to compromise and try to maintain a better MPG average so you have more money to have fun with and more time between fill ups for this fun.  Today, a truck that costs over 50 cents a mile for fuel is not "fun" if you drive it much. It is only going to get worth in next year or two.  

4.10s are not deep at all, but they work just fine for me until I get bigger tires and axles, I see no point in spending more money on my axles, and no the OD is not useless, I regularly pull my camper and the bed of the truck is full of supplies as well as a large aluminum canoe on the roof and I have no problems running it in OD, it does not lug or downshift to third, I do not notice any change in MPG whether the truck is loaded down or not  

4.10's are not "fine with 35's in a big heavy 4x4. It is abut the equal a 3.42 with stock tires which is not a fine gear for a heavy 4x4 by any stretch of logic. And this is before you consider that this is BEFORE you factor is extra drag from tires and lift in this "equivalent" comparison or stock tires with 3.42 va 4.10 with 35's and if you think this is a a acceptable two combo and it does fine in OD towing, you have very low standards. You "forget" that I have a 89 4x4 that I bought new and I have driven it more than 180K miles and across the rockies more than a dozen times and in temps below minus 40 at times too. (18 years experience with it)  It still runs well and is cherry and likely the cleanest stock 89 you will ever see in midwest. It has 3.73 gears and a stronger motor than yours does especially at lower RPM's. I ran 31x10.50's BFG's on it for a lot of years but it always felt lacking in OD with them on hills and at higher elevations that i used to frequent. 4.10's would have been a better match with the 31 x 10.50's than 3.73's. I know well how you burb runs with 4.10's and 35's without even riding in it. I went back to stock tires several years ago and I am never going bigger again unless I gear it deeper. In my current configuration I have 4 fully usable speeds and it very rarely ever down shifts out of OD on highway on hills even loaded and with A/C on (A/C is never off in warmer weather as I do not ever drive it with windows down.) OD in a 700R4 is not very strong design wise  and that together with .7 gear ratio is straining your tranny trying to tow anything in OD in your setup and strain in OD when not towing  (one reason your MPG is really bad) I get more than double your MPG city and highway consistently so I guess you can say that I get a lot more driven per gallon than you do. (my last MPG check for it urban driving usage was 16.6 MPG)  

well for starters its a pickup not a burb, and mine came stock with 3.53 gears and 31" tires so I guess im not to far from stock right now am I? I dont have a lift so I dont have too much extra drag to worry about and maybe you "forget" that my motor is not fuel injected like yours so you are going to get better mileage than mine did brand new. this is my forth gm truck of this body style and and only one had fuel injection (87 gmc half ton, 305cid,4speed manual,3.42 gears) which got 16 to 18 mpg all others were around 10 mpg. I do agree that deeper gears would be better both on and off road but im only going to gain maybe 2 more mpgs, and that would be a total waste of money considering i will be getting different axles within the next year. Oh and i highly doubt that your motor is stronger than mine at any rpm considering yours has 180k on it and mine has about 5k, also mine is a '78 block with '70 heads, very mild cam, and rams horn exhaust manifolds, it dyno'd at 287 horse and 331 torque,mine is not stock by 1986 standards but is stock by 1970 standards  

First, there never was any 3.53 gears ever in a GM. It is a 3.42 or 3.73. Next if you have 35'5 you have lifted it higher even if you did not lift suspension and increased cross section for aerodynamic drag and increase drag of tires too so it takes power power just to roll down the road forcing engine to work very hard at lower RPM's to move it and spend more time on converter stall and this all equals poor MPG. Also I have owned about 7 350's in my life  and currently 2 of them (also a few BB's too) and I know what a 350 can do. My 180K motor is solid and it uses no oil between changes and runs like a top and has a quiet smooth idle. The TBI motor in it is not a high RPM motor and it does it best below 3500 RPM but it does have better torque at lower RPM's than any smoggy carbed 350. I also have a VorTec 350 in my 2000 K3500 and while it has great mid to upper RPM power, it is no match for my 350 TBI below 3000 RPM. The strongest low RPM motor of that size I have ever owned  (and still do) is a AMC 360 in a old Jeep truck. It is a stump puller below 2500 RPM or so but it has no top end at all. Whether you think you might swap axles in a year or so you are loosing MPG and performance now big time and you may buy a new tranny before you re-axle because of strain on it. It is your nickel for gas and tranny but today I would not drive a truck that gets 6 or 7 MPG (most motor homes do better than this)unless it was maybe a supercharged BB with proper gearing and that was down right invincible when your foot was in it because in that setup up at least you are paying for a dog thats bite is as bad as its bark.  

you are dead wrong about the gears and i will try to get a picture of the build sheet that shows it came with 3.53's so unless you are going to tell me that you worked for Gm in  1986 and that my build sheet is a typo you can quit telling me that my truck didnt have 3.53's stock, and there is absolutly no way your engine puts out anywhere near 331 ft. lbs. of torque at 2700 rpm so i would have to say that my fresh 350 has way more torque at any rpm than yours with 180k on it. you have less power, stock tires, stock gears and fuel injection which adds up to way better gas milage than my truck, and like i said with deeper gears i am only going to gain about 2mpgs so is it worth it to me when i only use the truck for off roading and pulling my camper 4 or 5 times a year, no its not, especially when i plan to change axles this winter, and there is no way i am hurting my tranny, its a built 700r4 and if your math was correct my ratio comes out to about 3.42 which is pretty close to my original 3.53 and 31" tires so if the truck went 170k miles in stock form im pretty sure it will last 3k miles with my current setup  

Again there was no 3.53 EVER in a GM P/U or SUV. You can take it to the bank. If it says it, its is a typo. For that fact nobody ever offered a 3.53 (Spicer, Ford and Dodge offered a 3.54 ins a few axles and when GM still used Spicer axles in 70's they had a 3.54 option but there is no 3.53 for any 10 bolt or 14 bolt axle period. Also before you trash my engine you should ride in it because it is VERY strong on torque at low RPM's and will pull very steep hills at speeds as low as 35 MPH in OD with converter locked with so much ease it makes you wonder how it does it. Motor is stock except for the timing being set to 8 BTDC instead of zero or TDC and I run 93 octane in it year round and have for years. It also has a 3 inch stock routed single exhaust with a quiet flowmaster on it. Last tank fill I put on 401 miles over a 3 week period (it is not a daily driver) It was mostly urban driving and a trip to bring kids home from college which is about 160 mile round trip. It took 21.8 gallons filled to brim from same pump and station I normally use (they have lowest 93 price and it is local) With 21.8 gallons of fuel it works out to 18.39 MPG or 18.4. This last tank I never exceed 60 except a few times on trip to college and A/C was always on. Truck coasts really well on Michelin "C" range LT tires with smooth street tread running 44 PSI front and 49 in rear. You just touch the gas and it jumps to action crisply and the very slightest pressure will get you moving and up to 55 or 60 fairly quickly. It is typically shifts to OD and converter lock by 35 to 38 MPH and never down shifts once in it unless you crowd it really hard but there is no need to because it pulls OD strongly with very light throttle pressure. (I wish my 2000 350 VorTec pulled so good at lower RPM's which by the way has a custom spark curve too and uses 89 or better octane fuel all the time) BTW, the speedo and mileage on odometer is correct because I used a GPS to calibrate it with proper speedo gear long ago so MPG is correct as hard as it seems to to believe (I have trouble believing it too) BTW, Built or not, a 700R4 is not a tower in OD especially with 3.42 gears (there is no 3.53) and 31 inch tires. Due to design limitation and ratio it is not recommended to two with even if tranny is built. There is no surprise that your MPG is bad. BTW at link below is a online tires size and axle ratio calculator if you want to do the math.

Click Here  

SnoMan,
Great info.  I have a 1970 k5 stock tired mill and 1-ton axles.  I was convinced that I needed a 700r4 but after your input I'm not so shure.  The gearing on current axles is low 4.56 and engine screams at 50mph.  I thought the tranny was the way to go vs. gearing changes.  I plan to use the truck as more towing/highway use.  What is your opinion?  

I would not be concerned one bit about using a 700R4 in your case. Just get a 30 spline one as they have most of the upgrades, rebuilt or not. Nice thing about 700R4 is that it is not a computer controlled tranny so it is great for swaps in older trucks. You will need a cheap after market kit with a vacuum switch to controlling torque converter lockup. You also gain a much deeper first gear in the swap to. Do run a aux tranny cooler with on but do not get carried away with the size of it. the 700R4 is arguable one of the best trannies of its type GM has ever built. People that have trouble with them do not understand them or look for reasons for failure (like big tires with tall stock gears and no aux cooling) and blame tranny as in poster above. One tip, stay away from shift kits for it unless it has upgraded sprags and drum, The stock one hold up fine with out a shift kit but a kit can tear them up (GM softens the shift for a reason but some seem to think otherwise)  

I sure hope that you are not trying to bash me  ("People that have trouble with them do not understand them or look for reasons for failure (like big tires with tall stock gears and no aux cooling) and blame tranny as in poster above.") oh and i never talked any trash about your motor, i just said that there is absolutely no way yours puts out anywhere near the power mine does, simply a fact. and one more thing, a rear axle seal went out a few days ago and toasted the ring and pinion, now i have my excuse to get the one ton axles so its a good thing that i didnt buy new gears huh. and for the last time i dont care about your gas milage or mine, my daily driver gets 45mpg, i only use the truck for off roading and occasionally pulling my camper, it doesnt go on cross country trips  

It is not a bash but rather to say the the 700R4 is misunderstood by some such as you. There is nothing wrong with one. Where they fail is when they are misused like big tires and lifts with stock tall gears and towing in OD with tall gears because you have to use tranny withing its design parameters. Failure to do this results in short tranny life and its failure blame being falsely placed on tranny design not how it is being applied. Furthermore you very poor MPG shows that you have a mismatch of engine gearing and load on it and it is nothing to brag about. I have been around 350's in various configuration since the motor came out back around 1969 and some of the 327's before it and I know that motor well, (better than most) and how to tailor it to your needs. Your ego is getting the better of you hear on this power thing. TBI motors had small throttle unit and small port intake manifolds and heads which equals higher mixture velocity at lower RPM which equals very high torque potential with proper tune. The trade off is loss of higher RPM performance. With a carbed 350 and classic intake the velocities are lower  which results is less fuel mix and reduced lower RPM potenail. I have owned a LOT off 350's in various configurations and I can honestly say that the TBI 350 in my burb has the best lower RPM torque of any of them and the worst top end of any of them too but how I use and drive it, I never really miss top end because it very rarely ever exceeds 3000 RPM. Even in town it responds well and up shifts quickly and you are in OD by 35 to 38 MPH at around 1100 RPM and at which RPM it pulls very strongly. Regardless of factory ratings I can tell by living with it it does some of its best work below 2000 RPM. Also feeding it with 93 octane allows for a good advance curve at lower RPM for high torque and efficiency that you are not going to get with 87 octane and 9.1 to one CR that TBI  motors have.  

I will say this one last time, your motor does not put out anywhere near the power of mine, can you honestly say that your stock tbi 350 with 180k miles on it puts out more than 287hp and 331torque? My father is a professional engine builder and specializes in small block chevys, he has built hundreds of them for muscle cars, drag cars, circle track, boats, and 4x4's. I have grown up working on engines and have built numerous small blocks, the one I built for my street/strip malibu dynoed at 523hp and 516 torque and it runs on 92 octane pump gas, the one I ran before that ran on 100% ethanol and put out even more power. I don't doubt that your 350 is still strong after 180k miles or that it puts out decent power, but lets be realistic here its nowhere near the power mine puts out. And just to clarify, I never said that deeper gears wouldnt be better for my setup, I simply said that it would be a waste of time and money considering what I currently use my truck for and what my future plans are for the truck. I just put on the 35's this spring to repace the worn out 33's that I had before, a couple weeks later I purchased an 8" lift from a friends show truck and decided to run bigger axles and 39.5" boggers. It would be a waste of hundreds of dollars to switch gears only to run them a couple times.