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2017 F350 FX4 Lariat Crew Cab short box
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Discussion Starter #1
I thought I would ask around here, I am driving a 2017 F350 with the 6.7, I just took off my stock rims and tires and installed some Falkin Wildpeak A/T3W, 325 60R20 on Moto Metal +18 offset rims, my fuel mileage went from 17+ with the stock tires to 13 with these, I have tried adjusting the air pressure in the tires so I get the correct tread patch but it didn't change anything
I am just wondering if anyone else has seen this. I get the feeling like this isn't right, just wondering if there is something I'm missing or need to check.
Thanks for any info in advance.
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Would the offset and aggressive tread lower mileage. I did experience lower mileage when I went to Nitto tires with aggressive tread from all-weather.
 

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17 average over the life of the truck? Or tank to tank? Winter fuel kills mpg so seasonal fuel blends will have an affect.
Aside from that, yeah. The additional wind drag will cost ya. Believe it or not, a 2" increase in ride height is significantly noticeable. I rode around for a bit with a 2.5" level and took a hit. Then, adding another (actual) 1.1" tire diameter cost me more. For me, towing the same trailer 1600 miles on the same route at the same time of year, the difference was a -3mpg hit with a lift and bigger tire.
Aside from all that, a bigger tire will rotate less per mile. So the truck miles per tank are less than actual traveled.
 
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As @The Evil Twin said, larger tire rotate fewer times per mile, so the truck records fewer miles. If you put the same amount of fuel in a fill up before and after the tire/wheel change, you'd see lower fuel mileage. A couple of ways around that. One, get a speedo adjusting programmer to change the tire size to make the miles come out right -or- Two, find the difference between the two tire sizes and add the difference to your miles, then calculate your MPG.
 
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I forgot to mention that you can correct the tire difference in Forscan also. At least I'm pretty sure you can with the 17‐19s. In the 11-16 it is a hex value for the metric tire size.
 
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2017 F350 FX4 Lariat Crew Cab short box
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Discussion Starter #6
Thanks for all the info, I have yet to get it on the weight loss plan and tuners, I am still completely stock, engine and ride height (NO LIFT, NO TUNER). I just ordered up the Forscan adapter and put it on my computer so once I get it I will have to play around with it. I have compared it to the GPS and I'm just over 1.5 MPH over what my speedo reads, as far as I'm concerned that's close enough for me.
I will be doing a full weight reduction plan this spring.

Forscan question... Can the shift points be adjusted through the scanner??? I'm thinking a few more RPM might help the MPG and I don't want to beat on it to get the trany to learn some harder shift points.
 

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you need to get your tire size adjusted in the computer. I think Forscan will only adjust to certain tire sizes (from factory basically).

Shift points are not adjustable from what I've seen in there.

Hopefully someone else can chime in with more detail on the tire size but you may also want to talk to your local stealership as they can help you out.
 

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In the 11-16s you could enter the tire size as a hex value. So any size can be used. With the correct size, it very well could change the shift points but I don't know that for sure.
You went from a 34.1" tall tire at 9" tread width to a 35.6" tall/ 12ish wide tread. That significantly increased your rolling resistance and frontal area. If you have 3.31 gears then your city took a real hit. Your MPG is pretty spot on. Your real might is probably 14ish though. I lost about 3 daily driving and 1-2 towing.
 

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You went from a 34.1" tall tire at 9" tread width to a 35.6" tall/ 12ish wide tread. That significantly increased your rolling resistance and frontal area. If you have 3.31 gears then your city took a real hit. Your MPG is pretty spot on. Your real might is probably 14ish though. I lost about 3 daily driving and 1-2 towing.
 
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