Recently I had my transmission rebuilt again. It has been running great. Shortly after that I replaced my speed sensor, twice. The one I had in (which was a replacement) bounced a little around 65 to 70 mph. So this time I got a Standard Ignition from O'Reilly's. It would not go in all the way, which later caused a leak, so I got another which fit well. No leak. But since I put in the Standard Ignition sensor, my MPG has gone up 1-2 miles per gallon. Using my Actron Scanner, I found that the computer and the speedometer registered the same speed until you got to 65-70, where the speedometer was about 2 miles faster. Anyway, I just found out my rear tank is 18 gallon, not the 16 I thought it was. And the front is 19. Two fill ups ago, I ran my front tank down until it sputtered and only got 255.5 miles out of it. That is about 13.44 miles to the gallon. This last fill up, even though I did not run the rear tank all the way down, I did put in about 17.75 gallons in. I went 292.2 miles, which makes that tank about 16.46 miles to the gallon.

I ran my truck somewhat the same both times, 200 miles on rear tank, 255 on front, and then got gas on front tank test. 200 miles on rear tank, 180 miles on front, 92 on rear, and then got gas. Also used same gas station for both. What do you think is causing the discrepancy? Could fuel pumps give you better MPG? The truck seems to be running just as well on either tank. And the routes to work have also been the same with little change.

What do you think? I would love to get the same milage out of my front tank.

 

Given the numbers you've calculated, suggests about a 20% difference between tanks. Whew!

Suggest the leading probability is a measurement error somewhere, such as where 'full' is when you started as compared to when you refueled - relating to the gas pump clicking-off point and how much further you may fill after that each time, or perhaps using a different pump at that station where the truck may be tilted differently either front to back or side to side due to pavement sloping for rain run-off or such..

Suppose it could be related to the fuel pumps. One way would be to measure and compare the fuel pressures while running off each tank. IIRC for your model year and engine would like to see in the 45psi range. Offhand, to be the cause of a 20% fuel economy error, figure the truck would be running funky or smelling really rich or cobbling up for a short period after switching tanks or something different between tanks, so not holding much hope on a pump being a culprit, but not completely throwing this out.

 

There may be a situation where gas is being channeled from one tank to another due to a faulty pump check valve.

 

wde3477- Thanks for the reply. I know what you are saying about the math and fill up issues. My rear tank needs to be on a slight incline away from the pump to get a good fill. That's why I kept track of my milage per tank to near or empty. The rear tanked did not empty completely, but I did manage to get over 17.5 gallons in on an 18 gallon tank. I also work the pump until I can't pump any more without overflow. The front tank was dying on me so I assumed it was right at empty. Maybe I should do it again and check to see how much gas goes in. Maybe the pump is short.

raski- Thanks for the reply. I thought that might be happening, that's why I ran the tanks down to see if the fill amount was correct for the tank. I haven't eliminated it.

How about a faulty regulator? Lets say the pumps are not at the same pressure and the regulator is not controlling it? What do you think?

 

In case anyone cares, I tested the fuel pumps and they are fine. They tested within 2lbs of each other, 48 and 46. Also seems the regulator is fine. So I don't know what the difference is. I ran another test to empty with the same pump, gas, and fill. Rear tank stayed the same 292 miles, front went up to 276 miles. I guess it is the way I am driving at the time or weather/road conditions. Anyway I'm averaging 15 mpg, better then the 13 I was getting. I'm going to call that a win and move on.