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  1. #1
    Sr. Apprentice
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    Default Would Moving from 18" Wheels to 17" Wheels improve MPG?

    I just recently bought a used 2010 GMC Sierra 1500 ext. cab 4X4 with the 6 speed/5.3 combo. In the city, I'm getting 17-18.5L/100 km. That's a far cry from the sticker which stated it gets 14.4 L/100 km. The previous owner has the original Bridgestone Dueler tires on the truck; 18" in size. Would moving to 17" wheels improve mpg? I would have to change the rims too to lower the truck as well. Also, do Michelin tires do better with mpg than the Bridgestone ones would?

  2. #2
    Master Mechanic
    AMac's Avatar
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    Default

    I believe if you change the wheel & tire combined diameter, you are changing the final drive ratio. Increase in diameter lowers the final drive ratio which may make it harder to accelerate the mass of the truck. I made a note of this from another thread but unfortunately do not recall who said it so forgive me for quoting and not giving credit directly.

    Since you are doing the opposite I think it would increase the final drive ratio which would make it easier to accelerate the truck (better MPG), but I am not sure. Hopefully the person who gave the information above will see this and take credit, along with making corrections where necessary.




  3. #3

    Default

    how it works, the bigger the tire and rim effectively makes the final gear taller. so if your rear end is a 3.42, and you put on taller tires, you go to say a 3.32 the flipside is smaller tires, a 3.42 could become a 3.52. the taller the gears are, the better you will do on the highway, if you do more stop and go in town, a little lower gear ratio is better.

    your truck will work less to move itself, but will be high revving at highway speeds. thsi is really only noticalbe with large changes. I.E. 3.42 down to 4.10s. a size larger or smaller tires will make a mimimal difference.

    make sense?

    Alex


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  4. #4

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    Stock 18" tires and stock 17" tires are the same diameter, I say stock because the tuck has been programmed for a particular diameter tire. If you change diameters performance and mileage will suffer, the only saving I can see would maybe be weight, but the savings would be negligible.
    Adam


  5. #5
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    Default

    Yes...overall diameter change is the key here, not the rim size. If your looking to reduce the diameter, then all you would have to do is change tire size versus changing the rims and tires. And as others have mentioned, it would also change your gear/tire ratios. See the attached chart via link below and line up your current tire size with your gears and you should be able to see if moving to a shorter tire will make a difference. However, if all your after is an increase in mileage, in my opinion, there are much better ways of going about it. Check out this thread as it contains several ideas regarding increasing mileage http://www.gmtruckclub.com/forum/sho...crease+mileage

    http://www.4lo.com/calc/geartable.php


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  6. #6

    Default

    Changing tire size (to a larger diameter, not smaller) will not make a big enough difference in fuel economy to offset the miscalibration it will cause in your speedometer, in my opinion.
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  7. #7
    Sr. Engineer Dana W's Avatar
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    Default

    For the same, or far less money than new tires and wheels, you can get an ECM tuning module, of which there are several versions. These actually do offer mpg AND power improvements through changing system variables in your onboard engine computer. More economical options such as merely using a K&N drop in air filter might be beneficial.

    Once you have the ecm tuning module, then you can mess around with changing your wheel and tire diameters because you can then tell the ecm what you've done.

  8. #8

    Default

    Something else that folks tend to forget about is that if you change your tire diameter, it makes your speedometer inaccurate unless you fix that as well. When figuring out your mileage, you have to do the math to find out how many miles you have actually driven since your odometer will be off by the same percentage as your speedometer. (And YES... it is a PERCENTAGE. Your speedo is not off by 6 mph across the board!)

    So... for the sake of round numbers, and I am not going to "real world numbers" here, Rather, here is an extreme example: Suppose you slap on a matching set of 13" donut spares originally designed to fit an '81 Ford Escort (OBVIOUSLY NOT POSSIBLE, but follow me here) Now, all of a sudden you are getting prius-type mileage. You first have to figure out what the actual TRUE speed of the vehicle is. So, if you are driving 50 and your GPS shows that you are actually only going 35, then your speedometer/odometer is of by a factor of 30%, which means that once your odometer shows that you have traveled 100 miles, you have actually only gone 70 miles.

    So then... If you go to the service station and fill up the tank with 20 gallons after driving 500 miles, you will think that you achieved 25 miles per gallon. Once you do the math, however, you will find that you actually only traveled 350 miles and that your actual mileage was only 17.5.

    You will not really gain/lose much in the way of MPG by using different size tires unless you seriously alter the tire diameter and width... Maybe a small amount, but nothing groundbreaking. There are variables that will change your mileage such as gearing, weight of the tire and additional/less rolling resistance, amount of power the engine needs to produce to move the vehicle, etc., but to move only a size or two up or down will not give you hybrid-like results. If 30 MPG were even remotely possible with a full size truck, nobody would buy "Smart Cars".
    Last edited by moogvo; 03-06-2012 at 11:06 AM.
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  9. #9
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    Default

    Yep ^^^^^

    You'll need a programmer/tuner to make the adjustment, which will run you approximately another $300 or so. Or maybe the dealer will change it for you, but if you're going to spend that kind of money, you might as well spend an extra few $$ and get a tuner.

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