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MPG with 33s accurate on dash display?

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GMC207 Enthusiast

GMC207

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I added 33s to my 2015 sierra and am wondering how accurate the dash display of MPG is. If its reading 17 and i have 33s is it less or more than 17?

GMC207 Enthusiast

GMC207

Posted
  • Author
Posted

I thought that but wasnt sure. Any idea how much its off? Not that it matters just curious what im getting compared to my ecoboost I had before.

TxTruckMan Senior Enthusiast

TxTruckMan

Posted
Posted

3.4% higher is what it was for me. I previous went from stock 275/55R20 (32") to 275/60R20 (33"). So you're going 3.4% faster than speedometer says, and you're getting 3.4% more mileage than the DIC says. Add about 0.6 mpg if you're around 17 mpg.

Rawyzf Senior Enthusiast

Rawyzf

Posted
Posted

My $.02.

 

With factory size tires, I always found the DIC to read about .5 MPG too high. So, if it said 17 and I hand calculated the MPG it was closer to 16.5.

 

Since I've switched to 33" tires, I have found the DIC to be almost directly on as far as MPG. However, the way it is calculating that is not really accurate.

 

Best way to calculate your MPG to see how close the DIC is: Reset Trip, fill tank. Drive until you feel the need to gas up again, and fill the tank. Check Trip miles and multiply them by 1.045. (33" tires are 4.5% larger than OEM 265/65/18 tires). Then take that number and divide by how many gallons it took to fill back up.

 

Ex.

300 miles on trip. X 1.045 = 313.5

313.5 / Gallons to fill up (ex 17.55 gallons) = 17.86mpg.

smrr1 Senior Enthusiast

smrr1

Posted
Posted

My $.02.

 

With factory size tires, I always found the DIC to read about .5 MPG too high. So, if it said 17 and I hand calculated the MPG it was closer to 16.5.

 

Since I've switched to 33" tires, I have found the DIC to be almost directly on as far as MPG. However, the way it is calculating that is not really accurate.

 

Best way to calculate your MPG to see how close the DIC is: Reset Trip, fill tank. Drive until you feel the need to gas up again, and fill the tank. Check Trip miles and multiply them by 1.045. (33" tires are 4.5% larger than OEM 265/65/18 tires). Then take that number and divide by how many gallons it took to fill back up.

 

Ex.

300 miles on trip. X 1.045 = 313.5

313.5 / Gallons to fill up (ex 17.55 gallons) = 17.86mpg.

X2 exactly what i would have typed. Exactly what i do.
GMC207 Enthusiast

GMC207

Posted
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Mint. I will try that. I didn't think of the x1.045 idea, but it makes perfect sense.

Rawyzf Senior Enthusiast

Rawyzf

Posted
Posted

Mint. I will try that. I didn't think of the x1.045 idea, but it makes perfect sense.

 

I haven't heard mint for some time. Is it making a comeback? (I'm old and out of the lingo loop) Thank you for that, gonna use it tonight while having a beer with some buddies for the game. Want to see how it plays out.

GMC207 Enthusiast

GMC207

Posted
  • Author
Posted

 

I haven't heard mint for some time. Is it making a comeback? (I'm old and out of the lingo loop) Thank you for that, gonna use it tonight while having a beer with some buddies for the game. Want to see how it plays out.

Haha. I'm bringing it back!

Rawyzf Senior Enthusiast

Rawyzf

Posted
Posted

Oh, and make sure your calculation is correct.

 

1.034 if your truck came with 275/55/20s

1.045 if your truck came with 265/65/18s

SgtShiv Enthusiast

SgtShiv

Posted
Posted

Have to agree, mine read lower mpg than actual after getting new tires and before getting odometer/speedometer corrected. Now back to reading higher than actual.

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