Well I'm a new excursion owner and thought I did my homework and ya..........pretty sure I screwed up. So I was looking for a monster to do some overlanding with and build up, but also something that could tow my 6,500lb camper. After researching for a couple months I figured a nicely modded v10 (Really didn't want the diesel noise and hassles of a 7.3. Yes it is a great motor though) would work out great and maybe get around 8 mpg towing. I found a 2000 X that was super clean with 190k on it and nicely lifted on 37s. I pulled the trigger and had it shipped up to me. I absolutely love it and have spent the last couple months doing little repairs and prepping it for the summer. Here is the issue though. Driving it this winter around town I can't get it above 6.5mpg. Yes I'm letting it warm up but damn 6.5mpg???? My lifted tundra got 12 on 35s with stock gears. I feel like I am missing something so here is what I've done to it and what I know and maybe someone can chime in and tell me if I'm missing something.

2000 Excursion v10 6.8 4r100 4x4
190k miles
Labeled as 4.30 gear ratio, but likely has a gear swap due to rotation count( haven't had a chance to open the pumpkin yet) possibly 4.88 by count.
Unknown lift but clears the 37x13.5x17 Toyo RTS petty well with only a little rubbing at full lock
K&N FIPK2 Cold air intake with outer wear (brand new filter)
Exhaust y pipe fix from SPD Performance
5 Star tuning on SCT X4 (running the 91 ECO tune right now)
New pcv valve
Removed and deep cleaned the throttle body (unreal dirty)
Seafoamed the engine
Amsoil injector cleaner
New forward o2 sensors
Replaced 2 bad coils (need to order a full set still)

It runs like a top but just can't get any mileage and it seems like my fuel gauge might be off. Last time I filled it up it was at a quarter tank, but only took 28 gallons. Does this seem correct? if it will get 8mpg while towing I will be happy, but when it only gets 6.5 around town and averaging 225ish miles a tank that's not going to work if I'm way up in the mountains. Any thoughts

 

Was the speedometer ever corrected? If it has not it will be a ways off and your mileage calculations will be off also.

 

If that K&N filter is the oiled type, be sure to clean your MAF sensor, the oil can really gunk up the sensor which could be hurting the Economy.
Verify your Speedo/odometer via a GPS, any differences will affect the mileage calculations.
Are you dividing the miles driven by the gallons needed to fill? That's the only accurate method to determine MPGs.
Excessive warm up idle times will drive the mileage down, cold engine calls for higher fuel delivery and at idle it takes longer to warm up out of that cold enrichment.
How heavy is your right foot when driving around town getting 6.5 MPG.
With our 44 gallon tanks a true 1/4 tank would have needed 33 gallons to fill it up, but the gauge isn't perfect and they all have their unique personalities. Have you driven it until the low fuel light comes on yet? How many gallons did it take to fill then. You just have to learn how your gauge relates to actual tank level.
Are you running 91 Octane gas to match that tune? Why a premium grade fuel economy tune? I have the 87 Economy tune but don't use it anymore, it gets the same mileage on the 89 Performance tune and is more fun to drive. Most of its miles are logged towing the 12K TT where it gets 8/9 MPG on highway trips.

 

verify the gears, if the are still 4.30 that will affect you, verify alignment, verify speedo is correct,

swap it down to 35s I lowerd my 6.0 down from 6 inch lift to 4 and went to 35s gained 5 mpg, new 4.10s are coming

 

Thanks for the responses guys. Ill try to respond to all your points

1. Speedo adjustment - I was thinking about that right after I posted this. I went ahead yesterday and reflashed the ecm with the 91 Eco tune and changed the revs per mile to 560 which should match 37s. Threw it up on GPS and still the same issue at 35mph on the speedo I am doing 40. I figured it may need a gear adjustment then so I reflashed it again and changed the gears to 4.88 and left the revs per mile at 560. Ran it again and it stayed exactly the same. Still shows 35 on the speedo and 40 on gps. I was thinking of calling 5star to see if I'm missing something in the programming.
2. MAF and oiled filter - I cleaned the MAF before replacing the destroyed K&N with a brand new one so everything should be squeaky clean. I have been thinking of replacing it just to cover my bases
3.Tune - I was trying out the 91 eco tune to see if it made a difference vs the 87 tune. Yes I am running 91 in it this tank to try it out. What tune have you guys found to be the best???
4. Fuel Tank Level - Sounds like im right on where I need to be then. The first time I filled it up it took 32 gallons from a quarter tank.
5. Gears - I think my next course of action will be to get into the diffs and really see what gears are in it. On gps running 65 I am at about 2200rpm roughly. I ran all this through a gear train calculator and cam up with somewhere around 4.88-5.13s for gears. I am starting to suspect this thing really has the stock 4.30s in it. Any thoughts on this.
6. 35's - I have been contemplating this and I think I may just make the move and sell the 37s. I love how it looks, but I really need some function as well. Did it really make that big of a difference when you guys switched???

Can't thank you guys enough for the help and advice. I think I need this snow storm to pass so I can open up these diffs and really see whats in there.

 

Try entering the revs per mile for 35 tires and see if that helps. 2200 at 65 is not going to help fuel mileage any. That will give power, but not mileage. The 2 valve modular engines I've had start sucking gas above 1700-1800 rpm.

 

to correct the speedo on my 2010 f350 i actually had to buy a module that plugged in line with the harness behind the gauges, not sure if a 2005 would have to do the same, the tuner from 5 star could not make the correction.

 

the 35s made a huge difference, the rig looked nice when i got it, but the suspension and the tires made it ride like crap, i took about 4 inches out of it between suspension ans tires, makes it handle much better

 

Ya I think a set of 35s might be in my future. I will verify the gears first to see if its possible to downsize even. If it does have 4.88 or 5.13s it might be tough to go down without a regear.

 

My EX is rolling on true 35"s (305/70R18 Nitto Duras 35.28"X12.8") being spun through 4.88 gears, that gives an effective ratio of 4.39, right in the sweet spot for V-10 performance and towing power. It gets between 7 and 9.5 MPG towing a giant 12K TT and can still knock down 14.5 MPG running unloaded with the cruise set on 65 MPH.

 

Awesome! If I didn't just get 30" of snow i'd be opening up the pumpkin to see what I have, but I really think you guys are right. 35s is a great setup that would be a nice compromise for me.

 

No need to open it up to see. Just jack it up and spin the wheel by hand, counting the number of times the driveshaft goes around for each full revolution of the tire. A chalk or paint mark on the driveshaft helps.

 

On an open differential, it takes two full revolutions of the wheel to get an accurate count of the rear ratio. One wheel on the ground, with the other used to turn the two complete revolutions. With a mark and a reference point on the wheel and drive shaft, and knowing the available ratios for the given rear, you can be very accurate with this method.