Slowing down by 10 miles per hour can reduce your fuel consumption enough to notice, and for most trips the extra time taken is negligible. For example, consider a 60 mile trip. Driving at 60 miles per hour will get you to your destination in 1 hour or 60 minutes. Speeding up to 70 miles per hour will get you there in about 51.5 minutes. You saved only 8.5 minutes, but you burned through a lot of extra gas ( and money ) to do it.