That sounds a lot more difficult and tedious than just increasing your bank amount.
This takes the fun out of the game since you can buy whatever, whenever you want......but here goes:
Download "ifile" from Cydia, you will use this to edit the game file.
Important! open ifile, go settings and turn both "hidden files" and "application names" to on. Otherwise you will just see a bunch of letters/numbers that won't make sense to you.
home should be /var/mobile if not, navigate to it. now follow "applications", "Diesel Pulling Challenge", "library", "application support" and here you will see savegame.data
Click and open savegame.data with "text viewer". At the bottom left is the search option. search for "money" and you will see your bank amount beside it. At the top left, choose "edit" and add a few zeros to your dollar amount. Make sure to just change the number here and not delete the code it is surrounded by. Click SAVE and then DONE.
now you are the richest diesel puller around! lol :thumb: