What's the best transmission setting for each race?
Race 1 - Fuji Speedway - 261mph
High gearing for the long S/F straight; long gears also help to not punch you too fast out of each of the tight turns in the complex.
Race 2 - Indy Road Course - 249mph
Depending on your entry onto the banked section, you *MIGHT* hit the limiter just as you cross the S/F line - especially if you are slipstreaming. If that happens regularly, consider upping to 255 or 261. But that will slow you down on the mid-section straight.
Race 3 - Cote d'Azur - 199mph
Toughest race of the championship here.
You run the gearing low almost solely to try and get into 1st place at the first corner on the first lap. Once you're in first, the AI is pretty cagey about overtaking, but they will stay on your backside the whole race, so try not to slip up.
You shouldn't hit the limiter with your transmission at 199mph, unless you're slipstreaming - and you really *DO* want to be in 1st place for most/all of the race.
You could try dropping the transmission to 193mph if you're struggling to leap into 1st place at the beginning of the race; but then you will hit the limiter in the tunnel.
Race 4 - Nurburgring GP/D - 236mph
You might hit the limiter on the S/F straight... But if you are gazing at your top speed, then you'll probably forget to brake for the hairpin, so let the limiter do its job.
Lower gearing helps you get through the Hammerhead at the beginning of the race, but can make it a bit twitchy for the off-camber right turn out of the stadium section.
Race 5 - Monza - 249mph
You win this race through the chicanes, so high gearing isn't essential. The placement of the chicanes and the long right at the end of all three straights means that slipstreaming isn't a great strategy, since you'll likely out brake yourself.
You should be able to get into first place by the end of the first or second lap, so slipstreaming isn't an issue - 'cos you won't be lapping anyone.
Race 6 - Suzuka Circuit - 249mph
You *CAN* use a slipstreaming strategy at Suzuka, because you'll be able to cope better than the AI with the corner and chicane at the ends of the two longest straights. But elsewhere, you'll be balancing the the throttle in 3rd and 4th gear, so acceleration is more important than non-slipstreamed top-speed.
...
Acknowledgement:
I picked these gearing settings up before Christmas in a post by Sirtokealot:
https://www.gtplanet.net/forum/showthread.php?t=142091&page=4
Good luck!