i know with my 4.10s my engine is turning 2000 rpm at 60 mph with 275/70/17 tires

 

I've got an '01 with a 5.9, 4.10 gears and 33X12.50X17 pro comps and I am averaging about 14.5 mpg keeping foot out of it, interstate and highway, and then it drops significantly in town, probably around 11 or so.

 

mines gets around 12 to 13 around the city when i drive it right

 

Went up a little bit. I drive city mostly. Wont know how much it really went up till this tank is done.

 

4.56 with 33X12.50 tires

9 mpg average

 

When you swapped to 4.10 gears did you also change the stock 32 tooth speedo gear in the trans tailshaft to the 36 tooth gear to correspond with 4.10 gears?
If you didnt then your speedo/mileage is reading high by about 12%

 

No I have the stock gear but I went from 225/75/16 to 265/75/16

 

IIRC gearing is based on a 30" tire
Your getting mid/hi 14s

265/75 are oem tires for my V10 w/3.55
275/60 are oem tires for my Indy w/3.55 (swapped to 3.92s & 35t speedo)

 

I have 4.11's in my pickup and if I keep my foot out of it on economy highway I averaged 17.4mpg then city I avg. 13-14 depending. Going 70 on the freeway I sit at 2160rpms. I didn't change any gearing in the trans and my speedometer is 100% accurate. Tested it against my GPS in my phone in a straight away with cruise set at exactly 70 and the GPS reported the same speed. Are you talking about the pinion?! Still is a gas hog though. Speaking of mileage, I need to replace my O2 sensors. What do you use in your truck? I'm stumped on what brand to get. I have 2 O2's pre cat and after cat.

 

What sized tires?

98 gets speedo signal off a tone ring in the diff
Prior to 98 it gets it from the trans speedo gears in the tailshaft(pic below of mine).

I have 3 o2s for my 96 also (stk is 2) The rear o2 is a Bosch n/b, my front o2s are both widebands, ones a Bosch & ones a FAST