Depends on your vehicle. The defaults are fine for my road car with road tires. For a race car use you need to use higher values, and for a bicycle much lower. The limits are there to make the full braking look bright red and full acceleration look bright green. If limits are too high, you see pale colors. If limits are too low, you always see bright colors everywhere.
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What is the ideal setting for high/low speed? the higher the better or the lower?