MPG’s with new tire size? - Chevy and GMC Duramax Diesel Forum
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post #1 of 10 (permalink) Old 10-26-2019, 11:14 AM Thread Starter
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MPG’s with new tire size?

I just put new toyo mt’s 305/55/r20’s with sota wheels. Speedometer is reading correct and the dic is reading 16.2 mpg. I was wondering if this is what other people are seeing with this size?

2012 Chevy CCLB 3500HD LTZ LML Delivered 12/28/2011

2004 Chevy CCLB LB7
EFI Live, 5in magna flow straight pipe, Air dog II 165, Banks CAI and scoop, Banks big head waste gate, Intake horn mod, Spek gauges, 33x12.50 toyo mt's on XD monster 18's Sold.
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post #2 of 10 (permalink) Old 10-26-2019, 11:33 AM
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Never believe the DIC for fuel economy. Do the calc.
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post #3 of 10 (permalink) Old 10-26-2019, 11:36 AM Thread Starter
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That’s why I was asking what others have had with similar set ups. Oddly I didn’t adjust my speedometer, I cross checked it with gps and was either right on at 50 and 70 or it flattered back and forth.

2012 Chevy CCLB 3500HD LTZ LML Delivered 12/28/2011

2004 Chevy CCLB LB7
EFI Live, 5in magna flow straight pipe, Air dog II 165, Banks CAI and scoop, Banks big head waste gate, Intake horn mod, Spek gauges, 33x12.50 toyo mt's on XD monster 18's Sold.
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post #4 of 10 (permalink) Old 10-26-2019, 11:49 AM
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Easy go here. Simple calculation this it does for you.

https://www.tacomaworld.com/tirecalc

If your mileage is empty on a flat road that is pretty bad. Contrary to what others have said my DIC MPG meter is accurate. Have calculated it out after filling to the top several times.

2020 GMC Denali 2500

Last edited by Busa Dave; 10-26-2019 at 11:52 AM.
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post #5 of 10 (permalink) Old 10-26-2019, 11:56 AM Thread Starter
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It seems like it’s been sucking down fuel for the last 5 months before I did new tires. I can’t figure out why.

2012 Chevy CCLB 3500HD LTZ LML Delivered 12/28/2011

2004 Chevy CCLB LB7
EFI Live, 5in magna flow straight pipe, Air dog II 165, Banks CAI and scoop, Banks big head waste gate, Intake horn mod, Spek gauges, 33x12.50 toyo mt's on XD monster 18's Sold.
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post #6 of 10 (permalink) Old 10-26-2019, 11:59 AM Thread Starter
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I’m down about 2.5 mpg on the dic over the last 5 months.

2012 Chevy CCLB 3500HD LTZ LML Delivered 12/28/2011

2004 Chevy CCLB LB7
EFI Live, 5in magna flow straight pipe, Air dog II 165, Banks CAI and scoop, Banks big head waste gate, Intake horn mod, Spek gauges, 33x12.50 toyo mt's on XD monster 18's Sold.
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post #7 of 10 (permalink) Old 10-26-2019, 12:28 PM
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I have a 2020 with 35x12.50x20 Nitto ridge Grapplers on factory wheels. GPS confirms about 2% MPG impact to the good so it is actually 2% better than this. Have no idea why it get this good of mileage. I add the Ford additive because I had a case left from my 2018 6.7, use every other tank some of the Lucas Diesel Deep Clean like I have used on my other trucks.

Cannot explain this but it is the norm for 67 on a straight with no wind. Go to 80 then falls to 18.
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post #8 of 10 (permalink) Old 10-27-2019, 04:15 PM
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If you went from a shorter tire to a taller tire and havenít recalibrated your speedo for the taller tires, your mileage will be affected, usually lower.

If you was always driving 50/70 mph then your theory might be close.
Use a gps, drive 10 miles compare your miles driven to the gps miles driven, then figure a % difference, to figure in for true approx mpg.
Going by the dic is not a very good way to figure especially when your tires are probably different in size to what you now have.

Here enter your previous tire size and your new tire size to get a much closer approx figure.
https://www.discounttire.com/learn/tire-size-calculator

If your new tire is wider than your old ones, thatíll affect your mileage in a negative way.

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post #9 of 10 (permalink) Old 10-27-2019, 04:39 PM Thread Starter
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I went from a 295/55/20 to the 305/55/20. My mileage decreased before the tire change. The tuning service I use wanted to know what the difference between the Speedometer and gps at those speeds. I understand the dic isn’t accurate it has just been crazy to see it drop over those when it stayed constantly at 18.5 for so long. Sounds like part of it could be due to the lower cetane in diesel now. I also found all four tires at different pressures today ranging from 62 to 80 psi.

2012 Chevy CCLB 3500HD LTZ LML Delivered 12/28/2011

2004 Chevy CCLB LB7
EFI Live, 5in magna flow straight pipe, Air dog II 165, Banks CAI and scoop, Banks big head waste gate, Intake horn mod, Spek gauges, 33x12.50 toyo mt's on XD monster 18's Sold.
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post #10 of 10 (permalink) Old 10-28-2019, 08:25 AM
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Quote:
Originally Posted by sleeperlml View Post
I went from a 295/55/20 to the 305/55/20. My mileage decreased before the tire change. The tuning service I use wanted to know what the difference between the Speedometer and gps at those speeds. I understand the dic isnít accurate it has just been crazy to see it drop over those when it stayed constantly at 18.5 for so long. Sounds like part of it could be due to the lower cetane in diesel now. I also found all four tires at different pressures today ranging from 62 to 80 psi.
You have provided some good data points here. One is the fuel comment. I buy mine at just a few stations here in DFW and a couple on the road to the ranch in NM. YES there is a difference in fuel and it will impact mileage. The other is TP--run all of mine at 70 Cold (adjusted just last week for cooler temps had to add 4 lbs) when running empty and yes they go up a few lbs when hot but no issues even if they are at 80 psi cold and go to 84 or more they are designed to do that.

I have a GPS app that tells me speeds so it is very easy to check been using it my many years because my tires and sometimes wheels are always changed the day after I get the truck because they will not last one trip to the ranch. I ruined one last week in fact 6mm stick in the edge of the tread. Took 3 plugs to get back to Texas.

Try fuel and this is a controversial subject but the additives I use do make a difference. Seen it in RAM, Ford and GM diesels. GM gets the best MPG but the 2020 is the first year in many where GM got their act together so I could buy one that would not get torn up(DEF tank) at the ranch.

2020 GMC Denali 2500
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