08 F-350 Spartan 275 @55 mph got 23 mpg with 32 inch tire
Now this is winter fuel and canadian gallons that are 1.2 times bigger than u.s gallons
Increase this to 75 mph and it drops to 17.5
Outside temp about 40F
If you drive your truck like a smart person, you can run the 275 tune. Our 300 tune is the best daily driver tune though....at least in my opinion. I love that tune.
You will be fine with the 275. When I had stock tires I saw 16.5 MPG mix driving and I do not drive slow. On the hwy I got close to 20 MPG. This is with the 275. Hope this helps.
MPH and RPM's are huge with these trucks. MPG's really suffer when you are running 2100-2200 rpms running 80 down the interstate. That being said thats the only way I drive and I see 17 MPG at 80 MPH, it goes up to 20 if I drive 60-65 on the interstate. This is in the 250 tune
My 08 FX4 with factory 20s and spartan 250 would not do better then 15mpg at 75mph on the highway. I have heard many 20+ mpg claims with spartan tuned 6.4Ls but I have never came close to it. I did get 18.5mpg one tank and that was winter blend and 90% 60mph driving.
Yes as with any car, the lower the RPM's are the better the mileage, but it really seems to effect these trucks. Just the difference in 300 rpms can mean a good 2 mpg increase.
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