The Formula De yahoo group has "endurance" gear box tables in the files section. This was an alternate dice approach for American LeMans type of races with GT class cars and GTS class cars.
IIRC, the GT gear tables went something like this:
1st Standard 1st gear die
2nd 4 minus the Standard 1st gear die result of 2-3
3rd a die combination resulting in a range of 3-6
4th (The standard 3rd gear die) range 4-8
5th (The standard 4th gear die) range 7-12
6th die combination resulting in range of 10-12
I may not have the ranges exactly right, but I think you get the idea. The alternate gear tables made use of the existing FD dice. I don't recall the combinations but some involved a die MINUS another die, others had you roll two dice and combine them and other combinations were a constant plus or minus a die.
The GT dice are pretty tight, the GTS dice had a bit more spread but still tighter then the official "gears".
Okay, I just looked it up...
the GTS 5th gear is the Green (4th gear die) PLUS the Orange (2nd gear die), giving you a range of 9-16 with the '9' and '16' results less common then the 10-15 results. You might try that as an alternative for your 5th gear die and leave gears 1-4 standard.