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Old 08-02-2012, 06:55 PM   #1
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Gas mileage

We upgraded our tires from P275/60r20 to Michelin LTX/A2 275/65R20.
The odometer is off by 3%
The EVIC on the Ram shows 11.5 mpg
Towing Eagle Super Lite HT 23.5 weight 6900 lbs loaded.
I towed a 5900 lbs TT before that on the old tires and only got 9 mpg
Would the tire change make that difference?
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Old 08-02-2012, 07:36 PM   #2
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I know that we're using the same tires on our F150....and all I know is that we haven't been able to make heads or tails because of the computerized everything in the truck. We got the speedo set correctly (or as close as you can, anyway) but that the odometer, trip computer, and mpg calculators are all way off.
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Old 08-03-2012, 06:47 AM   #3
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I don't know how to calculate manuall. Maybe using the GPS mileage?
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Old 08-03-2012, 09:40 AM   #4
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Quote:
Originally Posted by Boubou View Post
We upgraded our tires from P275/60r20 to Michelin LTX/A2 275/65R20.
The odometer is off by 3%
The EVIC on the Ram shows 11.5 mpg
Towing Eagle Super Lite HT 23.5 weight 6900 lbs loaded.
I towed a 5900 lbs TT before that on the old tires and only got 9 mpg
Would the tire change make that difference?
Our GMC can be recalibrated to correct this type of change- Perhaps your local MECHANIC [not a parts swapper] or the dealership can do this for you- We also changed tire sizes and my MECHANIC changed some settings in the truck computer to rectify the tire change- The odometer reads correctly as does the speedo and changes were made in the shift pattern as well- Good luck on your mission- JMHO- DD
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Old 08-03-2012, 12:05 PM   #5
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I asked the Dodge dealership and was told it couldn't be done. Same guys that told me the LTX AT2 275/65R20 10 ply were to be inflated at 35 psi for towing :O
Kinda lost trust in them
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Old 08-03-2012, 12:06 PM   #6
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But will visit my favorite mechanic and talk to him. (not at dealership)
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Old 08-05-2012, 02:51 PM   #7
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Your vechicle is calibrated to the size tires it came from the factory with. Changing brands of tires can affect mileage +/- due to construction of the tire, less or more rolling resitance ect. Now even if you stay with the same tire size across different brand tires there can be a slight variation in physical size - just like different brand shoes arent all the same fit. Now if you go with a different tire size altogether will affect what your vehicles computer calucualtes unless it is re-calibrated to the new tire size. Altitude, temperature, humidity - brand/quality of gas can all affect mileage as well. (when grocery stores up here started selling fuel I gave them a try and avg'ed 100 to 125 kms per tank less - I no longer bother with grocery store gas.
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Old 08-05-2012, 03:23 PM   #8
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Used to buy cheapest gas I could find no matter where. Now I buy from Shell (Exon?) as much as possibel
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Old 08-05-2012, 05:53 PM   #9
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I believe that going from a 60 series tire to a 65 series tire means that the 65s are a bit taller and narrower, and therefore there should be less rubber in contact with the road and less rolling resistance, for what that's worth.
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Old 08-05-2012, 06:55 PM   #10
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While it is true the 65 series tires are a little bit taller they are not any wider except for a difference in the manufacturer. The width is the same at 275mm. If you check with a GPS you may actually find your speedo to be pretty close to actual speed.

To manually find your fuel mileage fill the truck to full first. Write down the mileage. Now drive it and at the next fillup, fill until full and then write down the mileage and the gallons of gas. Take the miles at this fillup and subtract the miles from the last fillup. You then divide that number of miles by the number of gallons and that is your fuel economy. I do this for all fillups for all vehicles. If I see a drop I know there is something amiss.
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