I know the 01 CL-S carried a 17.2 gallon tank. The gas light came on so I filled 10 bucks putting me at half a tank. Did 70 miles within a day and a half and the gas light came back on. Is it supposed to drain this much gas?

 

It would be a lot easier if you gave some more concrete numbers. But working with what you've given:

Assuming gas is $2.50 for premium. That would be 4 gallons. 70 / 4 is about 17.5 miles per gallon. That's a little low but not way off base depending on driving conditions and style. Driving a manual CL Type S in about 30/70 city/highway, I get on average 24 mpg. I would expect an auto to get maybe 21 in real world numbers.

 

My CLS 6 gets between 22 and 23 mpg. Usually I drive it pretty tame. I do open it up getting onto the highway. Might I suggest you fill it up, reset trip meter and drive it until it gets down to a quarter tank, then fill it again to determine what your actual gas mileage is. Also it its probably better to fill up prior to the low fuel light coming on as the fuel pump uses the gasoline in the tank for lubrication.

 

I have had my car 10 years and 140K miles. Around town I get 21 on average and on the HWY I get 30 gas mileage has been very consistent over the life of the car.. car is rated 17 and 27 respectively. it is not that hard to get good gas mileage... it's called keeping your foot off of the throttle.

17mpg * 4 gals would result in 68 miles so yeah it is possible

 

My 2nd gen 2003 CL-S has 199k miles and gets about 19-21 MPG around town and about 29-30 MPG highway. That's with Michelin AS Sport tires. With the stock Michelin MXM tires I got typically 2 MPG more. My highest was 35.4 MPG at ~55 MPH very mellow without the AC on.

Driving behavior sporty but not hard and fast cornering with coil-over suspension.