Mpgs
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4wd High
Joined: Sep 2021
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Mpgs
I’m sure it’s been brought up numerous times but any idea as to why my 16 6.7 gets if I’m lucky 14-15 highway, 8-10 towing roughly 8k TT ,& 10-12 city. It is fully deleted have ran 75,90,&120 tunes neither make a difference minus the power. According to the door plaque I have 331 gears with 37s, yes I know it’s a big tire to turn but will it kill my mileage that bad?
Hotshot
Joined: Oct 2006
Posts: 10,187
Likes: 8,069
From: Texas
I’m sure it’s been brought up numerous times but any idea as to why my 16 6.7 gets if I’m lucky 14-15 highway, 8-10 towing roughly 8k TT ,& 10-12 city. It is fully deleted have ran 75,90,&120 tunes neither make a difference minus the power. According to the door plaque I have 331 gears with 37s, yes I know it’s a big tire to turn but will it kill my mileage that bad?
All you can likely do is go back to smaller tire and exert less pressure on the skinny pedal.
FTE Legend
Joined: Mar 2017
Posts: 31,962
Likes: 9,194
From: Western NY
What these guys said. 37s are too big a tire for those 3.31s. You took the truck out of it's powerband by going with bigger tires. If you put your stock LT275/65R20s back on, you should see an improvement. If you slow down, you will see more. That's a big jump in tire size with gears like 3.31s. You should have re-geared your diffs to lower gears.
Laughing Gas
Joined: Nov 2020
Posts: 995
Likes: 367
From: Republic of Texas
If you re gear your truck to about 4.56 range or deeper you may see some improvements if you insist on being possibly lifted and run big tires like that.
Also..... I have been guilty of mis-using the power that if have.
With great power comes greater responsibility to the pocket... Choose the right foot application wisely.
Also..... I have been guilty of mis-using the power that if have.
With great power comes greater responsibility to the pocket... Choose the right foot application wisely.
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4wd High
Joined: Sep 2021
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Ok thank you all for reply’s! 37s on 20s we’re on when I bought it just have a lot of tread left wanted to use them before buying new but mpgs are killing me so might make the change sooner! Front is leveled only no true lift. Would keeping the 20s with a 33-35 improve greatly or would it be best to go to 18s with 33-35s?
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Hotshot
Joined: Oct 2006
Posts: 10,187
Likes: 8,069
From: Texas
Ok thank you all for reply’s! 37s on 20s we’re on when I bought it just have a lot of tread left wanted to use them before buying new but mpgs are killing me so might make the change sooner! Front is leveled only no true lift. Would keeping the 20s with a 33-35 improve greatly or would it be best to go to 18s with 33-35s?
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FTE Legend
Joined: Mar 2017
Posts: 31,962
Likes: 9,194
From: Western NY
The wheel size isn't what's important, it's the size (diameter) of the tire. My 18 inch steel wheel/spare tire has the 275/70R18 which is a tad shorter than the stock 275/65R20s that are on there. Depending on what your sidewall height is, you may be bigger than the stock size... Or maybe not.
4wd High
Joined: Sep 2018
Posts: 9
Likes: 1
From: Richmond, Virginia
I mean, if she’s tuned then it has all the difference in how they wrote the tune is what I’ve seen. BUT I’m still running stock suspension with stock 18s and 3.55
Last edited by BigTim804; Jun 12, 2022 at 02:42 AM.
Logistics Pro
Joined: May 2011
Posts: 4,832
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From: Maine
Don't post when you don't know what you're taking about.
Cross-Country
Joined: May 2022
Posts: 60
Likes: 14
One of my rigs came with 20's too my man. and I was told I'd see better mileage with 17 or 18 inch wheels and the 3.31 gearing. I chose to heed the advice. Now I regularly get anywhere from 750-850 miles per tank provided Im not towing.
Logistics Pro
Joined: May 2011
Posts: 4,832
Likes: 668
From: Maine
if you're running factory 33s on 18" wheels of course you're going to get better mileage than 34.5s on 20" wheels.
But it's not the 20s, nor the 18s. It's the overall diameter, rolling resistance, and weight of the tires.
More Turbo
Joined: Feb 2013
Posts: 689
Likes: 44
From: British Columbia
You guys are killing me. My truck came with 20's too. I clip along at 70 mph and just barely hit 1500rpm. I get 15-18mpg most days, but I'm not complaining. I think that's better than my '79 F-350 with a 460 that can only get 6-8 mpg! Is it possible it's your set up and the way you drive? Are we forgetting these are trucks and not mini vans??... 
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4wd High
Joined: Sep 2021
Posts: 9
Likes: 0
You guys are killing me. My truck came with 20's too. I clip along at 70 mph and just barely hit 1500rpm. I get 15-18mpg most days, but I'm not complaining. I think that's better than my '79 F-350 with a 460 that can only get 6-8 mpg! Is it possible it's your set up and the way you drive? Are we forgetting these are trucks and not mini vans??...