Hello have a 93 f150 with a 351 in it. Been working on it and trying to figure out what would cause a huge differance in MPG from the back tank to the front? I recently did a trip that was 700 miles and was able to figure out that my front tank get about 7 more MPG then on the back tank consistantly any one know what can cause this?

The truck runs great no miss at all on either tank and no power differance just on the highway on cruise the differance is great.

 

The fuel return line check valve in the front fuel delivery module could be leaking. This could cause, when switched to the rear tank, some of the fuel returning to the tank to go to the front tank rather than the rear. When you're on the rear tank, does the front tank seem to get fuller?

 

Thanks for teh reply. To answer your question I don't think so if it is it would have to be very little because I didn't see any notice I drained my front tank while driving then went to rear tank and after a while when getting more fuel my front was still near the empty mark.

 

Your not figuring your mileage by using your gauge are you?

 

No I used my front tank from full until it ran out while driving or atleast starting to run out then changed to rear tank. I then went until it was close but not out obviously before getting more fuel. When i filled it I put in 30 gallons total for both tanks to shut off the gas and both were on E basicly so must have about 16 gallon on each maybe 15 but no more than 17 I know for sure. I went 275 miles on my front tank non stop. I only made it another 150 miles on my rear tank before I stopped for more so I wouldnt run out. I was about at 1/8 left on my rear tank when i stopped.

 

I have a 1991 f150 and one of my tanks holds less gallons than the other. This could be true for your vehicle. Look at your manual and I bet one tank holds less fuel than the other.

 

Sounds like bad math to be honest.

The tank sizes differ from one another. Don't quote me, but I THINK everyone has the same size rear tank but the front varies depending on whether you have a short bed or long bed. Check your Owner's Manual, or I can check mine for you later when I get off of work.

IMO, the best way to check mileage on dual tanks:

1. Fill both tanks, preferably at the same station (better yet, the same pump) as you'll be pumping from the next time. If you want to be REALLY ****, have a standard method for when you know it's full. For example, I let the nozzle click off once, wait 5 seconds, then go until it clicks again. Method doesn't matter as long as you're consistent.

2. Reset the trip meter.

3. Drive on one tank (doesn't matter which) until you're ready to switch over. To make the math easy and to make it easy to remember the number, I usually switch right at 200 miles. This usually leaves me with about a 1/4 tank indicated on the gauge (REALLY good idea to not let the tank get too empty, btw, but that's another topic)

4. Switch tanks and reset the trip meter.

5. Drive until ready to fill (for me, that's at about 1/4 tank left)

6. Fill the tank first driven on first. Note the number of gallons until 'full', then compute MPG by dividing the first trip meter reading (using my example, that would be 200) by the number of gallons pumped.

7. Fill the second tank. Divide the current trip meter reading by the difference between the total gallons pumped and the number of gallons that went into the first tank.

And no, I don't do this in my head. That's why God created smartphones with calculators in them.

Mike

 

I just looked at my specs and it shows my front tank holds 18 gallons and the rear 15.

 

That's odd. My front holds 16 and my rear tank holds 19.

 

Not to insult you, but I'll have to agree with Whiteboyslo on the math. Sounds like your also being mislead by the notoriously inaccurate gauges. You need to figure up both tanks separately using only gallons used and miles driven. Forget the gauge. Check your owners manual for the tank sizes. If you don't have one, give us the specs on the truck and we will give you the info.

 

critterf1, you are right, It is the other way around. My front holds less. Sorry.

 

Short beds hold 16 in the front long beds hold 19. Both have 19 gallon rear tanks. LMC I think has a 38 gallon rear tank for them and some companies have up to 40 or so.

 

Hey guys I have a short bed extended cab and I changed the rear tank from another ford truck when both tanks are empty they both hold approximately 15 to 16 gallons I have filled both at same time many times and both take about 15 gallons to fill when thay are close to empty(on red) so must be close to 16 each. On my recent trip I was on freeway the entire time and I was right on mileage it is right at 275 miles from my house to my last job and it took my front tank to get there and it was from full to on the red. when i switched to my rear tank I was on same road driving from hotel to workplace and got about 150 miles and needed to fill both tanks again. After i filled both tanks i switched to my front tank and drove the 275 miles home on the 1 tank any time i use my rear tank I get alot less MPG. I took an old rear tank of a 1994 f150 that had a 302 in it because the rear on this 93 rusted out.

 

Why not fill both tanks and run on the rear first, if the rear is filling the front your front tank will start leaking gas. Then you'll know for sure.

 

Not meaning to hijack this thread so I'll also start another post, but does anyone use an app on their favorite brand of phone to track milage and maintenance etc?