You should do what the manual says. Accelerate smoothly and shift at the point where your shift will allow you to maintain speed after the shift. You should get into overdrive by 45mph--this will give you the greatest fuel economy.

I have an E (fuel injected) and typically stay in first to about 20mph,second to 30, third to 40 and OD at 45. I just did nearly 900 miles round trip to Niagara Falls and averaged between 28 and 30 mpg--average speed about 48mph, with about 50/50 highway and local miles.

Have a great trip!
Rolfe '71E