My truck has went from avg 6.0 to 5.2. Nothing has changed since the start at 6.0 till now? I always keep a check on each fill up and the first tank was 5.9 then 6.0, 6.0, 5.7 5.5 and now 5.2.

I am the only driver, same place to deliver, nothing has changed except when I had the transmission worked on and new clutch put in. Since then it has not gotten as good.

Any suggestions on this??

Forgot this:

One thing I did notice different yesterday when I filled up, it did not even out in the tanks, only drawed off of the Left side until it got down to halfway and then started drawing off other side. It did not do this before.

You might check to see if they did anything to your computer when they worked on the transmission. You may also check your air filter. A clogged air filter can dramatically affect your fuel mileage. One other thing is that if you have an injector that is having a problem, it could also affect your fuel mileage. I would start with the air filter. That is the least expensive thing to fix. A call to the garage who worked on the transmission would also let you know if they did anything to the computer.

1 other thing I forgot to mention is that I always buy fuel at one place until the last 3 times, which were bought at a different place. Could this have made a big difference? They sell quite a bit of diesel there.

It is possible. I have noticed some difference in mpg with some vendors. It should not make a difference, but could. A good way to tell is to go back to the fuel stop where you were buying fuel and see if your mpg goes back up.

I'd look underneath and make sure they didn't leave a valve shut half off or smash a fuel line under there while working on the transmission.

Looks to me like your problem started when you switched fuel stops. I don't know when most retailers switch to winter blend in your area, but maybe the new stop has already switched.

Maybe the difference between LSD and ULSD??? I notice a difference in mileage with my pickup. about 2 mpg, sometimes more.

Yea, ULSD makes a difference. I use it in NY, but when I fuel up in NJ with LSD(no I don't do drugs)Ha Ha, I notice about a 1 MPG difference.