We took the Accord instead since the HHR needed an oil change and I did not have time to do it. So if you do not read this I will not be offended.

The original post:

Since I have a very analytical mind here are some basic calculations.

First the assumptions:
Gas price - $4.00 per gallon
MPG at a given speed - Taken from my observations and others posted here on the site (This will be the most debated)
The additional amounts are incremental to the previous calculation.

33 mpg
1600 miles round trip-- $4.00 $ per gallon
65 miles per hour-- 48.48 gallons needed
24.62 hours -- $193.94 $ spent

29 mpg
1600 miles round trip-- $4.00 $ per gallon
70 miles per hour-- 55.17 gallons needed
22.86 hours -- $220.69 $ spent

Additional time 1:45-- Additional dollars $26.75

25 mpg
1600 miles round trip-- $4.00 $ per gallon
75 miles per hour-- 64.00 gallons needed
21.33 hours -- $256.00 $ spent

Additional time 1:31-- Additional dollars $35.31

Total Additional Time 3:16 -- dollars $62.06

So is it worth it to go faster that 65 miles per hour?
If I go 70 MPH, I save 1 hour and 45 minutes but spend an additional $26.75, which is $15.28 per hour.
If I go 75 MPH, I save 3 hours and 16 minutes and spend an additional $62.06, which is $19.09 per hour.
My time is worth more than $19.09 per hour so I would spend the additional $62 to save 3 hours.

The Results:
28.8 MPG
1,547.9 miles round trip--$3.99 per gallon avg
70 MPH average--53.61 gallons used
22.11 hours round trip--$214.10 spent