The problem is that the car is front of you. There's no way for you to know their speed. The only solution is to drive next to them and match their speed.
There might not be enough lanes though. The only guaranteed way is to get close enough that you can look in through their rear window to see their speedometer.
The best strategy is to run into their back bumper at a low speed… If you do it just right your bumper will either get stuck underneath or on top of their bumper, then they can just pull you along for a while. If they don’t notice you will save a lot on gas.