Fixed reference speed offset instead of percentage

🚧 THIS IS A TICKET MIGRATED FROM UPVOTY. The original ticket and comments will be available for a limited time over at:
https://abrp.upvoty.com/b/request-a-feature/fixed-reference-speed-offset-instead-of-percentage/
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It would be handy to have a fixed amount we can offset the planned speed to drive instead of a percentage. I generally go 5mph above the speed limit on all roads, but right now you can only enter a percentage for the reference speed. I use a reference speed percentage that gets my highway driving to the approximate correct speed, but lower speed limit roads will not be correct, as my 5mph above would need a higher percentage to be correct.