The following is a tutorial showing you how to modify the game's running memory to allow you to purchase anything that costs gold for free. There may be other applications for this but a have not looked into anything else.
Things you will need:
1) The game installed and up to date on your device.
2) GameKiller (my version is 1.90)
3) Some knowledge of hex editing (helps)
4) Float to hexadecimal converter - http://gregstoll.dyndns.org/~gregstoll/floattohex/
I take no responsibility for any bans received as a result from attempting this. I have been doing it for a few days with no such consequence but just be aware.
Alright, let's get to it!
1) Open game and go to the upgrade section. Go to whatever part you would like to upgrade and look at the silver cost to upgrade that part. Mine for instance is the engine for the 2012 Honda Civic which is 3,300 silver. Note that value.
2) Minimize FF6 (Fast and Furious 6) and open GameKiller. In GameKiller select the game you will be modifying which is FF6. Once loaded you will need to search for the value that you noted earlier, the price of the part you wish to upgrade (in my case it will be 3300). Type in 3300 then click the magnifying glass and scroll down on the drop down box and select Float. When it comes back with the results you will notice that they have a decimal point in them, this is what we want. There may be a few extras that are not the exact number. It is ok to delete these. My number will be 3300.0. After sorting through and deleting the junk numbers I come up with 4 results (yours may vary a bit).
3) Click on the first number and edit the value, deleting all numbers so that the value box is blank. Hit ok and you will notice that the value will change to 0.0. Now switch back to FF6 and change to a different part then back to the one you want to modify. You should see that the price for the silver purchase went to 0. If the price did not change, repeat this step with the other values until it does. Once you find the right value, delete all others to clean up the screen so you don't accidentally click the wrong one.
4) With the correct value located, click on it then select edit memory. This will bring up a screen with many hex values. This is where hex editing knowledge will come in handy.
5) Go back to FF6 to the part you are wanting to purchase and find the amount of gold it will take to upgrade it. In my case it is 3. Note this value.
6) Now go to the float to hex converter in the link provided (or any converter you want) and in the float section add the gold value from the item you wanted to edit then select convert to hex. Notice in the float box where you typed it added a decimal point and some zeros. This is a float value. In my case, 3 is 40400000 in hex. Note the hex value.
7) Go back to GameKiller and look at the hex screen. Somewhere next to where it starts, usually right next to it, you will see your hex value but it will be backwards going from right to left. This is the gold value that you want to edit. Start on the left side of the Hex value and edit it to 00 01. In my case 40 40 would change to 00 01.
8) Go back to FF6 and change to a different part then back to the one you want to upgrade. You will notice that the gold value has changed to 0. Simply purchase with gold and you will get it for free. Congratulations if it has on successfully modifying this game!
9) Repeat the above for all other parts. **
**Note - Once you get the hang of this, you will start to know what to look for and notice that the values for the other parts are in the same vicinity usually a few lines of hex above or below the one you edited. Instead of repeating the entire process you can just scroll up or down and look for the gold values of the other upgrades. Be careful as some edits may be wrong and cause the game to crash.
Good Luck and happy hacking!