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37 mpg is the best I have done so far...oh wait, nevermind...that was in my 5000 lb SUV. My ISF now gets about 16 mpg with I/H/E and mostly backroad driving.
I just did a 2 hour trip on highway and got 8.6 litres per 100km which is approximately 33 mpg. I avtually took a picture with my phone as i couldn't believe it myself. Around town I average 12- 14 litres per 100 or 20-24 mpg. I only use 98 octane fuel which may help
C5 and C6 Vettes get quite a bit better gas mileage than my IS-F ever thought of getting.
Smaller frontal area, similar Cd, and 500 lbs lighter do have their advantages. Not to mention the millions and millions of man-hours over decades put in the SBC's development and tuning.
My current mileage has gone to crap. I only drive about 3 miles and never get off cold cycle. I'm lucky to see 14 mpg. I'm also very concerned about my oil's service life with almost nothing but short trips - I'm pretty sure 12k OCI is a thing of the past unless my driving patterns change again.
90% Chicago streets 10% Highway. Since cities vary, I thought I'd be specific. City avg is 10 and highway avg is 20 with rush hour and 30 without rush hour. Don't want to calculate track avg.