I use Truckersadvantage (Fleetone) and TCH most of the time. If I stop at Flying J, I get the cash price but pay a fee each time I use the card. I believe the fee is about $2. With Truckers Advantage, I can get some fuel discounts and can use it most anywhere buy Flying J. On occasion I will use my American Express Card. I also have a Comdata card through Landstar's Savings Plus program. I cannot fund the Landstar card. Only Landstar can do that. When I broker a Landstar load they put my money on the Comdata card and I can either use it to purchase fuel, tires, etc., or take it off and move the money into my own bank account. I usually put it into my bank. If I use the Landstar Savings Plus Comcard, I can get the same discounts on their fuel stops as if I were leased to them. I am sure that won't make most BCO's happy, but that is what I have been told by Landstar. I find that I can do about as well using my own fuel cards. I don't use Comdata because their fees are too high, just like T-check. The Truckers Advantage card doesn't have a fee attached.
There are some bank and American Express cards which offer discounts on fuel by using them. Some only discount on gas, not diesel. I was told that American Express doesn't give discounts on diesel, but I can still get my 5% points. Since I must pay about $0.06/gallon at the chains to use the card, I rarely pull it out unless I fuel at an independent truck stop.
The best way to save on fuel is check prices before going to a particular area. I think that I posted these numbers in another post, but I stopped at a Pilot north of Mobile a couple of days ago. Fuel was $2.85/cash, $2.91/credit card. South of Montgomery there is a Flying J which had fuel for $2.75/gallon for cash. $2.81/credit. I bought fuel at Flying J and used my TCH card. I saved $0.10/gallon. When you buy 100+ gallons with each fill up, that is a lot of money. The truck I was in holds 300 gallons of fuel, although I rarely wait until I need much over 200 gallons. I rarely buy fuel at TA because their prices are usually higher than the other truck stops. They seem to keep their prices higher than most of their competitors so that it appears that the owner operators are getting a higher discount or better price. Sometimes they are less. I can often beat their prices by going to a different fuel stop. If you are running hard and can save $0.10/gallon, you have saved a lot of money over a year.
I save where I can. One other thing I do is keep spare lights in my truck. That can also save a lot of money. For instance, I can buy a marker light for about $2 at a local parts store. On the road, I have seen the same marker light sell for around $5 or more. I also keep tools in my truck to do minor repairs. Just changing my own marker light can save $35 or more in labor costs plus the wait time. It only takes a few minutes to change out a marker light. I can spend 5 or 10 minutes and be on my way or wait for 1-4 hours to get a mechanic to do the same job and charge at least $35. I know that wasn't part of your question, but it is another way to put money back in your pocket. I had a headlight go out this week. I stopped at a dealer to get a headlight. After dinner I replace the old headlight. It only took a few minutes and I was on my way.