My truck has had a rough life... It has 171,*** miles and compression is fading away badly.. She's ready to die.. It was in canada from 96-'00 don't know how it ended up down here... But hopefully I can get out of it while its still worth something.

 

There is a common issue with crossflow between tanks with these trucks and its generally more common in trucks with unused tanks. What happens is the check valve in the FDM inside the tank becomes stuck open and that allows the functioning tank to backfill the unused tank and overflow it out onto the ground as you drive. Needless to say your fuel milage will suck badly if you're dumping half of every tank on the ground so you want to figure out if you have this problem or not.

 

^x2

Thankfully both of mine work and before the tune-up I got 13 mpg on a 250 mile trip uphill (4,300 ft - 7,220 ft). 90, 5.0, AOD, 3.55, 150k+ miles. Haven't tested it since.

 

Ive got a 95 ford f150 302 auto 4x4 with 35s on it and b4 i put the big tires on it i was getting 8mpg and after i put the tires on it im still getting 8mpg its got 3.55 running gears.

 

Should i drive in overdrive or drive

 

I have the same issue only the rear tank on mine shows full all the time...crap, maybe it is full! Thanks for the advice, with gas at 4.20 a gallon i can't be dumping any of it...

 

How would you check this cross flow problem

 

Empty the tank that is not in use or that you suspect has the problem and then drive the truck on the other tank for a while, the tank you previously emptied should still be empty of course and if it's not you have the problem.

 

Then thats not my problem cause its still empty....my old truck had a leak in it ..im wondering if this might to

 

That's not possible.. with larger tires the truck travels further so if nothing else changed(it never does but for the sake of simplicity in this example) your milage should have increased with the bigger tires. Bottom line is you aren't calculating fuel milage correctly and the main device used to calculate milage(the odometer) is no longer accurate, so you need to recalibrate your speedo for those tires and then divide the distrance travelled between fillups(using the trip meter) by the gallons needed to refill the tank, that is your MPG. Do this at every fillup and you'll get a average of what milage the vehicle really gets over various conditions and driving styles.

 

31's are a bit too large IMO for a 200k basically stock 302....especially if you have 3.08 gears. Does your truck constantly shift in and out of o/d? If fuel mileage was your concern I would go back to a tire closer to stock height, and also be sure to run full gasoline where you can....if you can find stations not selling 10% ethanol blends. E10 gas will reduce your fuel mileage. Also, have you checked for any engine codes? An analysis from a shop with a good wideband a/f controller could help get your tune in check as well.

One last thing that people often overlook....anything that makes your truck heavier will also effect your mileage. If you've added heavy bumpers, step bars, nerf bars, tool boxes that are slam full, tonneau, and camper covers. All of these things make the power to weight ratio go down significantly. You can easily add another 1,000 lbs. of weight to an already heavy vehicle if your not careful.

5500#'s with a 160hp 302 =

I average around 15-17mpg on a 40% ethanol blend in mixed driving. But I have tuned my motor to run this combination. I like to run full e85 (13-15mpg), but they haven't figured out how to price the stuff around here so it's not cost effective at this point due to the fuel mileage drop. For days when I want full out performance I change my tune and run the e85. On regular e10 87 octane my mileage goes to 17-19. I haven't had the chance to test full 100% gasoline mileage due to the lack of stations with 100% gas.

 

im running 3.55 gears...i usually drive in overdrive but iv been switching between them to see which gets the best mpg. But even when i ran stock tires i still got the same mpg..my old truck had a 351 in it and that thing ran like **** but got better mpg then the 302 i got now.

 

I'm curious as to how you got this tune. Did you do a write up? I would be interested in trying it out if I can find any stations with an over 10% blend. Your full e85 mileage is close to my current 10% mileage on the highway.

Overdrive is good if you can stay in it, if it constantly shifts out to keep up to speed or it's barely going fast enough for overdrive to engage then you're better off in drive.

 

I adjust the tune through a program call Binary Editor and a Moates Quarterhorse chip that is installed in my J3 port of the truck computer. I started another thread a couple months ago concerning EEC tuning and Quarterhorse tuning. To make sure you are running Ethanol correctly you have to know what the mixture and once you know that you can figure what the stoichmetric afr is for that particular mixture. When you have the correct stoich figured you adjust the calibration in the Moates quarterhorse chip through the Binary Editor software to make sure the EEC is commanding the correct afr for the fuel used. Stoich on 100% gasoline is 14.64 to 1, E10 is 14.08 to 1, E40 is 12.38 to 1, and summer blend e85 is 9.77 to 1. To get my 40% mixture I pump x amount of e10, and then x amount of e85. I have an excel program I made to help get the ratio as close as possible. If you're within 5% you're good. This calculation is a manual process....on a flex fuel vehicle, they have an ethanol sensor that reads the mixture that the eec uses to know what fuel tables to command the correct "needed" afr.

My motor specifically if also heavily modified. It has ported GT40 heads with extensive valve work, shorty headers with custom 2.5 inch y pipe, no cats, no smog pump. It has also been stroked to 352 c.i....so airflow is much different than stock 5.0. I also have 24# Bosch injectors with the EV6 pintle design which atomizes fuel better than the EV1 design. I run the stock distributor with MSD 6al ignition, MSD blaster coil, and 9mm plug wires. Motorcraft copper plugs gapped to .055. '03 4r70w with 2400 stall and hayden cooler with 3.08 gears in back.

On straight highway running with e10 87 octane I have averaged as high as 21. I could probably command the eec to target a slightly higher afr, but I am happy with the mileage I am getting without risking anything by running lean. I keep a close watch on everything by datalogging occasionally and watching my Innovate wideband afr. Command and the eec and motor performing the command is two different things. To know if you're motor is tuned, you need a wideband, and preferrably some way to datalog.

 

I'll have to find your other thread and bookmark it for future reference. I've always been interested in widebands and tuning, but never really had a vehicle to experiment on.