Something

proc format_speed {value section} {
	global speedmeasure speedthreshold theme

	if {![string is double $value]} {
		return $value
	}

	switch -exact -- [string tolower $speedmeasure] {
		"mb" {
			set value [format "%.2f" [expr {$value / 1024.0}]]
			set type $theme(MB)
		}
		"kbit" {
			set value [expr {$value * 8}]
			set type $theme(KBIT)
		}
		"mbit" {
			set value [format "%.1f" [expr {$value * 8 / 1000.0}]]
			set type $theme(MBIT)
		}
		"autobit" {
			if {$value > $speedthreshold} {
				set value [format "%.1f" [expr {$value * 8 / 1000.0}]]
				set type $theme(MBIT)
			} else {
				set value [expr {$value * 8}]
				set type $theme(KBIT)
			}
		}
		"autobyte" {
			if {$value > $speedthreshold} {
				set value [format "%.2f" [expr {$value / 1024.0}]]
				set type $theme(MB)
			} else {
				set type $theme(KB)
			}
		}
		"disabled" {
			return $value
		}
		default {
			set type $theme(KB)
		}
	}

	return [themereplace [replacevar $type "%value" $value] $section]
}