Somewhat over 62.5 mph but likely under 72.8 mph
A bit of assumption going on but...
When Captain America went for a relaxed run around Washington DC in Captain America: The Winter Soldier Sam remarks that he "ran 13 miles in 30 minutes" that equates to a 26 mile an hour relaxed running speed.
As he was sprinting in Civil War we know that he was moving much faster than this.
Usain Bolt has hit 27.8mph during sprinting in the 100m. As Cap is the peak a human can get it is only fair we compare his top sprinting speed to the current peak sprinter.
The world record for a male London marathon runner (Wilson Kipsang) is 12.85 mph (26.219 / 2.04). This is 2.16 times slower than Usain Bolt's max speed.
The average men's pace was 6.47 mph (26.129 / 4.04) this is 4.29 times slower than Usain Bolt's max speed.
While Cap was running around DC he didn't seem to be exerting himself too much, so probably not WR equivalent effort, he was also probably exerting himself more than the average marathon runner (he was showboating a little). So lets take the median of the two 12.85 + 6.47 / 2 = 9.66 mph and assume Cap's relaxed running speed is the equivalent of this, or 2.8 times slower than Bolt's max.
If the gap between his relaxed running speed and him sprinting is the same 2.8 times gap as Usain Bolt's max speed when sprinting then he can hit about 72.8 mph on full gas (26 * 2.8).
If we work this out for the upper and lower marathon paces mentioned above:
- If Cap was running around DC at WR marathon pace and had the same
2.16 gap then he can sprint at around 56.16 mph
- If Cap was running at average marathon pace and had the same 4.29 gap
then he can sprint around 111.54 mph
In Romania most roads have a speed limit of 100 kmh or 62.5 mph, motorways have 130 kmh or 81 mph and rural roads have 90 kmh or 56.25 mph.
Romania only has 2 motorways both heading out of Bucharest (and not completed) and the road in the tunnel doesn't look like a rural road so it's most likely just a normal road. (cant get the actual Romanian speed limit of this road as it's filmed in Berlin in the Messedamm Underpass).
If the cars were going at the speed limit for a normal road then we can assume that Cap, Buck and T'Challa were all travelling somewhat faster than 62.5 mph, which would be realistic with the assumed 72.8 mph.
They would not be able to outrun the cars if they were running at the lower limit of 56.16 mph and they would be travelling much much faster than the cars if it was at the 111.54 mph upper limit.