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1998 Gmc Sierra Increasing Tire Size

Discussion in 'Chevy Silverado Forum (GMC Sierra)' started by Wyatt S, Jun 18, 2013.

  1. Wyatt S

    Wyatt S New Member

    I'm Wyatt and I'm new to the site. I have a few questions involving tire sizes and what kind of gas mileage or side effects others are getting.

    Mechanically Bone stock 1998 gmc sierra 4x4 with the 5.7L and a 3.73 rear end. The truck came stock with 245/75/16 size tires and I have already moved up to a set of 265/75/16 and still managed to get about 14-14.5 mpg at best. I'd like to go up another size to a 285/75/16 for looks but don't want gas mileage to kill me. If anyone has any information they could provide on this setup or similar it's be greatly appreciated.

    Im looking at the Yokohama Geolandar AT/S for my next set of tires. Any reviews or thoughts on those tires would be helpful also.

    Thanks everyone in advance
  2. 99'HEARTBEAT

    99'HEARTBEAT Epic Member Staff Member 5+ Years ROTM Winner 1000 Posts Platinum Contributor


    :sign0016: To the Site....:glasses:



    Here are the Specs for the 245/75/16's and the 285/75/16
    [TABLE="class: withlines, align: center"]
    [TR]
    [TD="align: center"]Tires:[/TD]
    [TH]Tire 1[/TH]
    [TH]Tire 2[/TH]
    [/TR]
    [TR]
    [TH]Tire size[/TH]
    [TD="align: center"]245/75-16[/TD]
    [TD="align: center"]285/75-16[/TD]
    [/TR]
    [TR]
    [TH]Section width[/TH]
    [TD="align: center"]245 mm (9.6'')[/TD]
    [TD="align: center"]285 mm (11.2'')[/TD]
    [/TR]
    [TR]
    [TH]Sidewall[/TH]
    [TD="align: center"]184 mm (7.2'')[/TD]
    [TD="align: center"]214 mm (8.4'')[/TD]
    [/TR]
    [TR]
    [TH]Tire Diameter[/TH]
    [TD="align: center"]774 mm (30.5'')
    [/TD]
    [TD="align: center"]834 mm (32.8'')
    60 mm (7.8%) taller
    [/TD]
    [/TR]
    [TR]
    [TH]Rim Sizes[/TH]
    [TD="align: center"]16x6.5 to 16x9[/TD]
    [TD="align: center"]16x8 to 16x10.5[/TD]
    [/TR]
    [TR]
    [TH]Circumference[/TH]
    [TD="align: center"]2430 mm (95.7'')[/TD]
    [TD="align: center"]2619 mm (103.1'')[/TD]
    [/TR]
    [TR]
    [TH]Revs per mile[/TH]
    [TD="align: center"]662.2[/TD]
    [TD="align: center"]614.5[/TD]
    [/TR]
    [TR]
    [TH]Speedometer[/TH]
    [TD]Assuming that this is OEM tire and speedometer readings are correct >>>[/TD]
    [TD]When your speedometer reads 60mph actual speed will be 64.7mph or 7.8% faster.[/TD]
    [/TR]
    [/TABLE]


    Here are the Specs for the 245/75's and the 265/75's
    [TABLE="class: withlines, align: center"]
    [TR]
    [TH]Tire size[/TH]
    [TD="align: center"]245/75-16
    [/TD]
    [TD="align: center"]265/75-16[/TD]
    [/TR]
    [TR]
    [TH]Section width[/TH]
    [TD="align: center"]245 mm (9.6'')[/TD]
    [TD="align: center"]265 mm (10.4'')[/TD]
    [/TR]
    [TR]
    [TH]Sidewall[/TH]
    [TD="align: center"]184 mm (7.2'')[/TD]
    [TD="align: center"]199 mm (7.8'')[/TD]
    [/TR]
    [TR]
    [TH]Tire diameter[/TH]
    [TD="align: center"]774 mm (30.5'')[/TD]
    [TD="align: center"]804 mm (31.7'')
    30 mm (3.9%) taller.
    [/TD]
    [/TR]
    [TR]
    [TH]Rim sizes[/TH]
    [TD="align: center"]16x6.5 to 16x9[/TD]
    [TD="align: center"]16x7 to 16x10[/TD]
    [/TR]
    [TR]
    [TH]Circumference[/TH]
    [TD="align: center"]2430 mm (95.7'')[/TD]
    [TD="align: center"]2525 mm (99.4'')[/TD]
    [/TR]
    [TR]
    [TH]Revs per mile[/TH]
    [TD="align: center"]662.2[/TD]
    [TD="align: center"]637.5[/TD]
    [/TR]
    [TR]
    [TH]Speedometer[/TH]
    [TD]Assuming that this is OEM tire and speedometer readings are correct >>>[/TD]
    [TD]Your speedometer reads 60mph actual speed will be 62.3mph or 3.8% faster.
    [/TD]
    [/TR]
    [/TABLE]

    ....If you go to the 285/75 size Tires, you should and need to have the Trucks Computer Reprogrammed to correct Speedometer and Odometer readings, as the Truck's Computer calculates your speed based on the Diameter of the Stock Tires. By installing Taller Tire, the Trucks computer doesn’t know how fast your Truck is going, also, as a result of this change in Tire Size, the Automatic Transmission may Not Shift Correctly,

    Regarding Fuel Mileage, the Below Gear Ratio-n-Tire Size Chart, shows with
    3.73 Gears your Truck has, and going with 285/75 Size Tires, which have an Overall Diameter of....32.8in"....your going to receive the Best Fuel Mileage with this Setup, but your Performance will drop off.

    [​IMG]

    Last edited: Oct 31, 2013
  3. Wyatt S

    Wyatt S New Member

    Wow thanks so much for all that information. Definitely helps a lot! I don't tow a lot so sounds like the 285/75/16 are the best for gas mileage but will definitely need to get my computer reset to the new tires. Any idea on where and how it'd cost to get that done?

    Last edited by a moderator: Jun 18, 2013
  4. 99'HEARTBEAT

    99'HEARTBEAT Epic Member Staff Member 5+ Years ROTM Winner 1000 Posts Platinum Contributor

    Your Welcome, regarding re-programming your Trucks Computer.....check with some of your local Auto Shop, and if you know someone who has a Scanner, and last some of the Hand-held Programmers, have the ability to reprogram the Speedometer, and as far as Cost, I don't know??.....hopefully some of the other Members, will be able to help answer your question, regarding this.
    Last edited: Oct 31, 2013
  5. Wyatt S

    Wyatt S New Member

    Is there any clarification you can provide when it says poor performance in the chart?
  6. stchman

    stchman Active Member 1 Year 1000 Posts

    Remember:

    Bigger, taller, wider tires zap fuel economy. The combination of decreased mechanical advantage, higher rolling resistance, all work against fuel economy.

    Also, with changing tire size, you will need to reprogram the ECU and tell it how tall the tires are. If you do not, the ECU will not be shifting gears properly.
  7. McClintoc

    McClintoc Active Member Staff Member 2 Years 1000 Posts Platinum Contributor

    Are reprograming the ECU and adjusting the speedo the same thing or two separate tasks?
  8. mfleetwood

    mfleetwood Rockstar 4 Years ROTM Winner 5000 Posts

    They are not the same thing, as you can program many aspects of the ECU, including tire size, with the right programmer. However, you can complete the tire size change by inputting into a programer the new tire size and uploading into ECU via the connector under the steering wheel in most vehicles, thus making this a single task.

    I use the Diablo Predator, which uploads my custom tunes/performance aspects, tire size, and gear ratio all at the same time.

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