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Help with bad gas milage!!

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by 76_Cheyenne, Apr 11, 2002.

  1. 76_Cheyenne

    76_Cheyenne 1/2 ton status

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    I came from 3.07 gears with a stock K10 with 30" tires, I think, to now having 35"s. I get about 8 MPG freeway I think. Its hard to tell when you odometer gets stuck everytime the .9 miles switches to 0 again. Would getting new gears like 4.11 or 4.56 help improve or give me worse gas milage?
     
  2. dyeager535

    dyeager535 1 ton status Premium Member

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    You did realize that your odometer is going to be way off right? So mileage readings are not going to be anywhere near right unless you re-gear or change your speedo gears.

    Your in town will suck, depending on freeway RPM's now, changing gears might or might not help. In town, anything numerically higher will help you.

    IMO, when you put 35's on there, you forfeited any notion of getting any kind of decent MPG, and might as well give up on it. You aren't doing the drivetrain components any favors with 35's and 3.08 gears, and to change those might (or more likely will) decrease freeway MPG. In these trucks, which are heavy, freeway and city MPG will typically not be close, unless in both cases, you are turning the same rpm's, such as with no OD.
     
  3. 76_Cheyenne

    76_Cheyenne 1/2 ton status

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    I have no overdrive, its a auto 3speed TH350 tranny. ON the gear calulator on the DriveTrain direct website, The ratio of 4.11 or 4.56 was in the area for best gas milage and power. But I was assuming I would get worse gas milage with new gears because the revs would be higher, and I'd lose freeway speed. Is this true?
     
  4. Hossbaby50

    Hossbaby50 3/4 ton status

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    You could actually get better gas mileage. With 3.07's you have to give it more gas to get it going off a stop then you would with 4.11 or 4.56's because you have more torque multiplication with the 4.__ gears then you do with 3.07. You have more mechanical advantage over the tire with the lower gears. There for it takes less torque from the motor and less RPM's to get you truck moving.

    On the highway you could actually get better mileage too because our trucks are about as aerodynamic as a brick, and to keep the motor in a power band to keep up with the MPH you have to give your truck more gas then you would to have to with 4.__ gears because those gears are keeping you more in your power band. You get better mileage in your motor is in a good band for torque then if you don't.

    At least this is the way I have been told and understand this issue.
     
  5. 76_Cheyenne

    76_Cheyenne 1/2 ton status

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    I see, thats good reasoning. All I need is 500$ for some gears! Oh and I loved your description of how areodynamic our trucks are. That was hilarious. haha
     
  6. tacoma972

    tacoma972 1/2 ton status

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    think of it like this, when you were running 31's with 308 you were running about factory set -up for the best gas mileage all the way around. when you upgrade to 35's you need need to compensate for the heavier and larger tires. that is why you need the gears to get good all around gas mileage. if you stick with 3.08's then like the other guy said, you will probably see a mile or 2 n improvement on the highway, but how much highway driving do you do. you live on the interstate and drive highway all the time? then what are you losing when you offroad. different needs are for different types of terrain. do you want to break stuff rock crawling because of the rpms required to climb, not to mention tire damage from spinning out. hitting the mud is good with higher gears but if you bog down, is just your engine enough to power you out? Things you need to ask yourself. I run 4:10's with 36" swamper tsl's and haven't had a problem with either because it is the right gearing for the right tires.
     
  7. 76_Cheyenne

    76_Cheyenne 1/2 ton status

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    well this is my daily driver, thats why I care so much about the MPG. But a few times a week we go to the mud holes and trails. So I really want an all around gear ratio. When I wheel, im usually in 4LO, because I need to be able to move the tires without having to slam the peddle to the floor. I do need new gears but how high should I go. 4.10 or 4.56, based on my driving habits. Im on the hiway/freeway almost everyday, and will be driving from Washington to Montana back a forth next year for school. BUT I LOVE OFFROADING, thats why I have the truck. I just want the best all around gear ratio, I like going a bit over the speed limit on the freeway, but i want to be able to crawl better offroad, and I want better MPG, is that to much to ask?
     
  8. DesertDueler

    DesertDueler 1/2 ton status

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    IF you go mudding I would say 3.73s or 4.10s. For mud you need alot of wheel spin to get going. Its the other way around for us rock crawling guys. We need to go really slow to avoid breaking things /forums/images/icons/smile.gif We do anyways.

    Dan
     
  9. 90K5

    90K5 1/2 ton status

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    I'd go for 4.11's if I were you. I had 3.42's with 33's, which would be about like your 35's if you had 3.73's and I didn't like it, but it wasn't bad...you may even want to consider it because of your future highway trips with a TH350...I have 4.11's with 33's, and I like them, but would have liked something a little higher for the highway next year like 3.73's...I'm going to college too...3.73's with 33's would be about like 4.11's with 35's. Thats what I would do. Just my $.02
     
  10. MudbogAD

    MudbogAD 1/2 ton status

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    Your MPG may get better. It all depends how you drive it. When I had 3.08's and 35's I got about 9 MPG. Now that I got 4.56's I get about 10 or 11 if I drive it easy. Since I got the gears it is much quicker and peppier and it is very tempting to hot-rod the thing around which gives me about 6-7 MPG. So if you go lower drive it easy. If your on the highway all the time and your only in the mud alo,t I would go with 4.10's because you may do better on the highway.
     
  11. ChevBlazin

    ChevBlazin 1/2 ton status

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    Right now I'm running 3.42's with 35's and a th350 tranny with a healthy GM 350 crate motor. On some trips that are strictly freeway I have gotten as high (or low depending on how you look at it) as 12 1/2 MPG, that was pulling a small utility trailer with my 4 wheeler in it. I compensated for the tire size and odometer being off with a little bit of math. It's not too hard to figure out an equation to figure out how many miles you have actually traveled compared to what your odometer says. I am changing to 3.73's since I want a little more low end, but dont want the hi revs on the freeway. IMO you should use 3.73's or 4.10's. either would be a good improvement. Good luck at school too.
     
  12. dyeager535

    dyeager535 1 ton status Premium Member

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    FWIW, in a 1:1 final drive ratio problem, I personally think that the gear ratio the differential needs to "see" (IE, the actual gear ratio changes when tire size changes) is about optimum at 3.00 (subject to engine of course)

    Now, this is kinda complicated, but from the factory our trucks had typically 29" tall tires. (31's optional) If you take a 29" tire, with a 3.07 ratio, then change tire size to 35", the actual ratio is now 2.54 Thats not really good obviously : )

    In my truck, I'm getting 18MPG freeway. (maybe better now that the advance works!) With 3.42's, 33" tires, and 1:1 tranny final ratio, from 29" tires to 33" tires, that works out to 3.00

    Interestingly enough, if you were to go from 29" to 35" tires, and change to 3.73 gears, that puts you at almost exactly 3.00, which is the same that I am actually running. ( http://yeagerd.home.mindspring.com/gearcalc.html is the calcs that I used)

    Of course, there are some major variables, like engine size and engine specifications (small or big V8, carb, cam, exhaust, etc) that will change MPG, that won't give you the same results I have. But at 2200RPM, I seem to have plenty of power, and thats where I come up with 18MPG. City/Freeway combined is consistently 13.5

    On top of that, I've got a better "2nd" gear ratio than you do in first, so with 3.73's, my driving enjoyment in town would be greater than yours, (SM465 3.58 2nd vs TH350 2.52 1st) even if I was running 35's instead of 33's.

    I think 3.73's would probably be pretty close to optimum RPM on the freeway (keep it at 60MPH) and still give you DECENT acceleration. It's going to be night and day between 3.07's, thats for sure. And if you only mess around off-road once in awhile, and your main use is on the road, don't kid yourself with a numerically higher axle ratio. It works better for acceleration and off-road, (in most cases) no doubt, but It doesn't help the motor to spin high RPM's constantly, and it doesn't help economy.
     
  13. TopOff

    TopOff 1/2 ton status

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    I have 35" with 3.73s, I get ~13-14 MPG.

    I used to get 15 - 16 MPG with the Edelbrock carb. I know my ignition is not quite right, as I had to change to a HEI for the TBI.

    Of course, I have a 383 making GREAT torque at 2000 - 2500 RPM, and I have to run 2750 rpm just to keep up with traffic around here (75 mph).
     
  14. HarryH3

    HarryH3 1 ton status Author

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    Are you using the odometer readings to check your mileage? If so, then you're getting better mileage than you think. 35 inch tires roll much further for each revolution than the stock tires. But your speedo is calibrated for the stock tires. So you're going much further than the odometer indicates. I don't have the exact numbers handy, but less guess that the difference is 20%. This means that when your odometer shows that you have travelled 100 miles, you have really travelled 120 miles.

    100 miles / 11 gal = 9.09 mpg. But if we use the "real" distance, then 120 miles / 11 = 10.90 mpg. Nearly 2 mpg higher. Still not fabulous, but 20% better than you thought. /forums/images/icons/wink.gif
     
  15. Hossbaby50

    Hossbaby50 3/4 ton status

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    Yep. My trucks odometer only goes .9 miles for every mile I go. So I get 10% better mileage then odometer says.
     
  16. Blue85

    Blue85 Troll Premium Member

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    One thing you didn't mention is which engine this truck has. The 350 often gets better mileage than the 305, especially when it is overworked by big tires, small gears.

    For gas mileage, do a basic tune-up. Make sure that the plugs, wires, cap and rotor are all good. Also make sure that the choke is releasing fully. The biggest thing you can do for gas mileage is slow down. I get 15+mpg at 55mph, but maybe 9 at 80mph. If you're real serious about getting mileage, swap in a diesel and enjoy 20-25mpg on the highway.
     
  17. 76_Cheyenne

    76_Cheyenne 1/2 ton status

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    I have a 350ci, and I've put Flowtech afterburners headers on and all new exhaust, otherwise the engine is stock with about 70,000 miles on it. I know my odometer is way off not just because of the tires but because it sticks every revolution of 1 mile, and i get a great ticking noise when this happens, brightens my mornings. So I really lose 3/10's of a mile on the odometer for every mile, do to the sticking and ticking, plus the tire size. When i calculated my MPG, I drove what I knew was close to 100 miles then filled up and did the math. So will new wires, rotors, plugs help as well? I think when I get new gears I'll go with 4.11's, and I might as well throw in a locker while im at it. I just need some dang money! Thanks a ton for your HELP, it was greatly appreciated!
     
  18. dyeager535

    dyeager535 1 ton status Premium Member

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    As is my new motto, don't throw money at it. Imean if the plugs and wires have never been replaced, of course do it. But if the idle quality is still good, (this is tough to tell sometimes) then just pull all the plugs, check the gap, clean them up if necessary (if, like me, one cylinder burns a bit of oil) and replace them. Plug wires are real difficult to tell if they are "good", although if you pop the hood while its running on a pitch black night, and you see sparks everywhere, thats bad : ) Plugs WILL let you see some spark through them. Delco or NGK plugs are highly recommended. After 2 out of 8 new in box Autolites failed in 20 minutes of idle time, I won't be going back to them. Hard to tell if plugs are working ok, but pulling one or two should give you an indication if they can be re-used or need replaced. Bad idle when everything else is set right (carb, timing) more than likely is going to be an ignition problem in the plugs, cap, or wires.

    Cap/rotor, take them off and inspect them. You can SEE when they are bad. Loose plug wire terminals? Tighten them up. Corrosion on the terminals on the inside? Sand them smooth and clean. Same with the rotor. No cracks or tracking where there are weak spots, it's good. While you are in there, pop the rotor off and check the mechanical advance springs/weights/bushings. These can wreak havoc on idle and MPG, and after a lot of years/miles, these DO wear out.

    You don't have a lot of money I understand, so things that don't HAVE to be replaced, shouldn't, especially if they are fine! Just make sure the money you would have spent on those components new, if they can be salvaged/re-used, is put into a slush fund for future projects : )

    I just cleaned my plugs, re-gapped them, and cleaned the cap/rotor contacts, and the truck does run a bit better. It certainly didn't hurt, this will be the first full tank I can check if mileage changed at all. The cap terminals were very corroded though, so hopefully that was "holding back" some power that will now be translated into increased MPG.
     

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