Why are my Mpgs so low?

[dsberman94]

I agree that your neighbor may not be telling you the whole story. I have an 03 7.3 on 35s and one on 37s... neither gets 16-17 or 22 hwy, even hand calculated and corrected for tire size.

Maybe on the worlds longest 10% downhill grade. Haha.

 

[ckrueg]

Heck my dad barely gets 20+ in his 7.3 going 55 on 31 inch michelins. No way you neighbor really gets 22

 

[Lipka101]

I've always thought a mpg question with a lifted truck was comical. But I will try not to derail the thread.

I've got a 2.5" lift running 37s, and I'm deleted. I see about 13.5 city and that's hand calculated. Highway I get about 18 at 70mph.

I get the same numbers with no lift and stock size Goodyear wrangler at/s. I'm deleted too.

 

[ncollins64]

When I first bought mine it was bone stock. On the way back from Texas I was getting 11.5 mpg. Deleted it and tuned it and I went to 17 mpg highway. I was leveled on 35s. Now with all my engine work I'll get around 14 in town and I averaged 17.5 mpg on the way back from IL cruising 75 at 1800 rpm. Also truck has 3:73 gears. All this was hand calculated.

 

[Kedstrokin]

I'd like to know what y'all people do to get higher than 13. My truck on 37s got at best 13.5 highway. Now it sits on 33s and gets 14 highway. Y'all may need to get y'all calculator fixed.

 

[AZPSD64]

I'd like to know what y'all people do to get higher than 13. My truck on 37s got at best 13.5 highway. Now it sits on 33s and gets 14 highway. Y'all may need to get y'all calculator fixed.

Try a lower gear. I'm running 37s 8" lift with 3:73s freeway mileage at 75 mph is at 18mpg. hand calculated. Filled up last night.

 

[ncollins64]

All my calculations was by hand. I don't go off the computer mpg

 

[footlong70]

Well for starters make sure your truck is calibrated to the right tire size. And double check the speedo with a GPS. Even my Blackmaxx set at 38.5" tires which is what I'm running is out to lunch. I have to put it at 37.00 which puts it perfect within one km/hr. If the computer is registering smaller tires than actual, the odometer is always going to show less miles traveled.
Next pressure test and be sure you have zero boost leaks. I've fixed countless friend's 6.4s who complained about low fuel mileage. Usually multiple small leaks, or a big leak. Holes in boots, loose clamps, holes in the intercooler... Also check up pipes.
One thing I've learned with the 6.4 is small tires or high gearing kills mileage big time. Big tires actually help. Low cruising RPM makes for good mileage since it makes monster torque down low. I hand calculate 17-18MPG regularly. 38.5" rubber, 8" lift, 3.55 rearend, mostly empty. All the first stage mods, on the HD300 tune always. Highway speed 110km/hr puts me at 1500RPM. Pulling my travel trailer @ 9000lb usually gets me about 11-12mpg but never single digits.
Anything over 10k weight I wouldn't try though without re gearing or smaller rubber.