I filled my car up the other day with 93 octane. I have put 124 miles since my fill up and I am already at just above 1/4 of a tank.......
It's very doubtful that your mileage is as bad as your figures show. The only way to calculate mileage is to, well, you know, I am sure.
Below are figures for both my '09 CTS-V & my '87 turbo Buick. Cadillac figures reflect the actual calculated mileage and that from the computer calculations.
Mileage comparisons (started 06-22-2011):
09 Cadillac CTS-V: start: mileage: 3344:
---date------miles---miles-----gals-----price----cost----ampg--cmpg.--spd---range
06-02-11---3516---172.0---11.430---4.199---48.00---15.0----N/A----00.0---000
06-14-11---3694---178.0---11.222---4.099---46.00---15.9----N/A----00.0---000
06-18-11---3823---129.0---13.220---4.099---40.00---13.2----N/A----00.0---000
06-22-11---3965---142.0---08.294---4.099---34.00---17.1----15.2---00.0---000
07-06-11---4092---126.4---08.335---3.959---33.00---15.1----15.3---00.0---000
07-12-11---4252---160.3---11.141---4.039---45.00---14.4----15.1---00.0---264
07-18-11---4392---139.4---08.913---4.039---36.00---15.6----15.4---31.6---266
07-21-11---4509---117.8---08.170---4.039---33.00---14.4----15.5---32.3---263
07-25-11---4656---146.0---09.752---3.999---39.00---15.0----15.4---33.0---278
07-30-11---4849---193.1---12.753---3.999---51.00---15.1----15.2---31.1---283
08-03-11---4989---140.7---08.501---3.999---34.00---16.6----17.3---39.0---278
08-09-11---5150---160.1---10.053---3.979---40.00---15.9----16.3---34.7---283
183.30 (sample total)/12 (sample quantity)= 15.2 average calculated (ampg)
140.70 (sample total)/09 (sample quantity)= 15.6 average computer (cmpg)
87 turbo Buick Regal:
----date------miles-----miles----miles-----gals-------price----cost-------mpg----cmpg
06-10-11---29303---29527---223.7---13.705----415.9----57.00----16.323---N/A
06-25-11---29527---29722---194.3---12.379----403.9----50.00----15.700---N/A
06-28-11---29722---29912---189.7---10.647----403.9----43.00----17.817---N/A 07-02-11---29912---30139---226.9---13.754----3.999----55.00----16.497---N/A
07-05-11---30139---30287---147.8---08.840----3.959----35.00----16.720---N/A
83.057/5= 16.6114