for those, click on the car you want, then click "Export Car", name it, and go into the file, and change the stuff. Line 440 is the multiplyer, and lines 240-272 are the gears, 240 is reverse, 244-268 is gears 1-7 in that order. Simply change the values, then save the file. Get back to the program and click the car you just exported on the program screen. Then click "Import Car", find the file, and select it and you'll see the gears changed on the program...
Hope that helps.