Here's what I did:
I saved a tune that I did nothing to for one of the starting cars, I then signed out and on a second profile downloaded the tune and did this:
Top gear west
Circuit race 1 lap
I did 49 races, hitting restart at the end. 24 hours later I received 19cr on my profile unlocking the first two achievements.
So I repeated the process but only did 45 races and this morning received 14cr and unlocking the achievement.
So secondary account bang out 100 races using your tune should be just over 2 hours and in the morning you'll unlock all 3 tuning achievements.
Hope it helps, I found this to be the quickest way
TheSaucyGibbon adds some great tips/alternative solution. Thanks and credit goes to him for the following:
(This assumes that you have created your design and tune and have previously applied them to your boost account vehicle)
1. Switch off your Kinect sensor, it will try to recognise you and log you in on your normal profile.
2. Be sure that your normal profile is logged out.
3. Log in as your boosting profile.
4. Grab a pen and paper.
5. Start Forza, go to Free Play.
6. Select track : Top Gear / West Circuit
7. In Game Options, change max drivatars to 1, laps to 1. All else leave alone.
8. In Load Tuning Setup, search for then 'download and install' your tune again (just in case).
9. Start Race, track will load. Watch that loading thingy go round and around in circles for some time.
10. At Pre-race, start and FULLY RACE your first race. Sorry about the shouting there.
11. At Post-race, scroll across and select 'Restart'.
12. At Pre-race, select 'Start race'. While the race is loading, grab your pen and draw a short vertical line on your piece of paper. This is your record of having done one race.
13. As soon as you see 'GO', press your 'start button' to pause the game. Scroll all the way across and select 'Quit Race'.*
14. If you have 120 pen marks on your piece of paper, go to Step 15. If you have less than 120 pen marks, go to Step 12.
15. Congratulations, you can stop now until your next session tomorrow