Oh man, you must think we're geniuses! All right, I'll take a stab at it.
A shorter, slower route should take less fuel regardless of hills. Yes, you use more fuel climbing the hill, but then you coast down the other side. Now, there are other factors involved like wind. A stiff headwind will kill your mileage, but it will be worse the faster you go.
Quote: 2) Travel Hwy 90 to 82 to 84 to 15 - shortest and would cost less fuel but take more time.
From I-15 take I-70 east to
US-89 south to Flagstaff.
1408 miles from Gig Harbor. Your first option is 1496 miles
This would cut off several miles and is also very scenic.
Another option is to take US-93/US-6/NV-318/US93 south from Twin Falls to
I-40 east to Flagstaff. 1390 miles. This is the route I would probably take.
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2004 Ford 350 Super Duty 2W Dirve Long Bed PU
2008 327RLT Montana Mountaineer 35 ft. 5th wheel 2005 Alaska and other Trip Logs
Personally I avoid I-5, I travelled it from Bakersfield to Oregon in a Class A, it was miserable. I would think gas prices are lower in Idaho and Utah as well.
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1999 GMC Sierra 2500 6.0 V8 Extended Cab 4x4 4.10
2002 Forest River Salem Lite 25' w/slide
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All right, I'll take a stab at it. 
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