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Normal refills occur when one fuel bar is remaining.  I do not top off the tank.
Highest Tank ==> 72.2 Lowest Tank ==>  40.3
Averages ==> 55.1 56.9     503 14    75,969 9.18     1,378
Date Tank
MPG
Lifetime
Average
12-Month
Average
Average
Temp. ºF
Delta
MPG
Tank
Miles
Tank
Days
Total
Miles
Refill
Gallons
Total
Gallons
Notes
08/26/04     69.0   3.0                3     Picked up car
09/02/04 44.6 45.1 45.11 67.6 279.4 7          282 6.260 6 Gallons estimated based on Prius MPG
09/09/04 50.4 47.7 47.75 65.9 5.8 467.5 7          750 9.446 16 2 fuel bars remaining
09/10/04 43.4 45.3 45.32 65.0 -7.0 439.4 1       1,189 10.539 26 Complete Interstate driving
09/12/04 45.9 45.7 45.74 64.0 2.5 420.1 2       1,609 8.937 35 About 95% Interstate driving
09/17/04 49.2 46.5 46.50 63.2 3.3 490.6 5       2,100 9.977 45 About half Interstate driving and then back to normal
10/02/04 49.9 46.9 46.94 59.6 0.7 452.0 15       2,552 9.208 54 Starting to get cold - temperature affects mileage
10/11/04 48.9 47.3 47.32 55.0 -1.0 299.0 9       2,851 5.887 60 Good price on gas, refilled early 
10/27/04 50.5 47.2 47.18 50.6 1.6 441.0 16       3,292 9.525 70 Might this be the last 50+ tank of the year?
11/10/04 50.0 47.4 47.39 49.5 -0.5 483.8 14       3,776 9.892 80 Last fuel bar blinking.
11/22/04 44.0 47.2 47.18 44.2 -6.0 313.9 12       4,090 7.016 87 Found the tire pressure to be very low. AARGH!
11/28/04 48.3 47.0 47.04 35.8 4.3 437.1 6       4,527 9.554 96 400 highway miles averaging ~62MPH - outbound
11/30/04 47.5 47.1 47.11 34.0 -0.8 449.5 2       4,976 9.384 106 400 highway miles averaging ~62MPH - return
12/11/04 43.1 46.7 46.66 34.3 -4.4 405.9 11       5,382 9.736 115 The avearge temp fails to capture the bitter mornings
12/22/04 40.4 46.5 46.52 22.3 -2.7 387.6 11       5,770 8.678 124 Fighting to keep the mileage above 40
01/03/05 41.8 46.0 45.97 25.7 1.4 350.4 12       6,120 9.090 133 Little bit of a warm stretch
01/15/05 40.6 45.6 45.61 23.7 -1.2 348.2 12       6,468 8.696 142 Hopefully it gets warmer from here
01/26/05 41.5 45.2 45.24 18.7 0.9 356.8 11       6,825 9.051 151 Even with colder temps I was able to improve MPG
02/03/05 46.2 45.1 45.09 25.4 4.7 381.9 8       7,207 8.952 160 Steller tank performance!
02/16/05 46.0 45.3 45.30 33.4 -0.2 401.6 13       7,609 8.127 168 This warm weather isn't going to last.
03/06/05 44.2 44.9 44.93 27.6 -1.8 363.6 18       7,972 9.470 177 Out of town for a week
03/16/05 44.6 44.9 44.93 29.3 2.4 435.0 10       8,407 9.687 187 Temperature is creeping up!  Hurray!!  :-)
03/30/05 50.0 45.1 45.09 36.4 5.8 503.4 14       8,911 10.501 198 First 50MPG tank of the season.  More to come.
04/13/05 53.0 45.6 45.62 46.5 3.0 540.3 14       9,451 9.550 207 Is this a preview of upcoming mileage?
04/30/05 51.8 45.6 45.62 47.9 -1.2 456.3 17       9,907 9.984 217 Could have gone farther, but decent gas price.
05/15/05 51.6 45.9 45.89 50.7 -0.2 518.9 15      10,426 10.044 227 Second half of the last tank and this one, A/C on
05/30/05 55.5 46.3 46.33 56.4 3.9 514.0 15      10,940 8.943 236 First ever 55MPG tank!!
06/17/05 56.0 46.6 46.59 68.6 0.5 553.5 18      11,494 10.550 247 Was fighting at the end to keep it above 56
07/07/05 58.2 47.2 47.17 70.0 2.2 612.9 20      12,107 10.000 257 Some of the time was carpooling
08/03/05 60.0 47.5 47.47 74.2 1.8 589.3 27      12,696 10.773 267 A whole month on a single tank!
08/24/05 58.5 48.0 48.01 72.0 -1.5 624.1 21      13,320 10.014 277 Some A/C
09/14/05 58.3 48.4 48.66 71.5 -0.2 573.2 21      13,893 9.834 287 Getting gas every three weeks.  I could get used to this.
09/22/05 55.4 48.4 48.80 62.3 -2.9 282.5 8      14,176 5.366 293 Decided to short-tank for fear of rising costs (Hurricane Rita)
10/15/05 55.2 48.6 48.95 60.0 -0.2 526.3 23      14,702 9.896 303 Glad I short-tanked the last one.  Prices spiked and dropped.
10/23/05 53.2 48.7 49.01 49.3 -2.0 369.9 8      15,072 7.047 310 Decided to short-tank because the price was quite attractive.
11/15/05 50.8 48.7 49.07 45.4 -2.4 500.9 23      15,573 10.431 320 Most likely the last 50+MPG tank for quite some time.
11/30/05 44.4 48.7 49.32 33.0 -6.4 427.1 15      16,000 8.830 329 Mileage dropping like a stone.
12/14/05 40.3 48.4 48.97 18.8 -4.1 362.1 14      16,362 9.270 338 Dissapointing Mileage; but accounted for with dropping temps
12/30/05 45.0 48.1 48.83 25.9 4.7 390.4 16      16,752 10.071 348 A nice turn in the weather helped with this one.
01/22/06 48.6 48.1 49.79 34.7 3.6 458.4 23      17,211 9.354 358 First refill of the new year.
02/09/06 46.8 48.1 50.18 30.7 -1.8 429.0 18      17,640 8.836 366 OEM EV Button Installed during this tank
03/03/06 48.2 48.0 50.22 25.1 1.4 455.0 22      18,095 10.372 377 MPG went up while temp went down?  EV button?
03/26/06 46.7 48.0 50.80 34.1 -1.5 425.3 23      18,520 9.409 386 Dissapointed in the mileage drop.  May be using the EV too much
04/19/06 54.5 48.2 51.27 47.3 7.8 489.1 24      19,009 8.405 395 Man how I love warm weather.
05/05/06 54.0 48.3 51.01 51.9 -0.5 553.4 16      19,563 10.789 405 5 MPG over last year at this time.
05/16/06 54.6 48.3 51.28 51.0 0.6 534.3 11      20,097 10.452 416 It rained 9 of the 11 days.   Still better than last year. 
06/13/06 57.9 48.5 51.27 61.0 3.3 618.8 28      20,716 10.980 427 I was really hoping to break 60mpg with this one.
07/01/06 58.9 48.8 51.67 68.0 1.0 597.2 18      21,313 9.912 437 Used the Air conditioner on this one.  Notable hit to mileage.
07/20/06 60.5 49.1 52.17 72.0 1.6 601.0 19      21,914 9.696 446 First 150 miles was at 53.4 MPG.  Brought it up
08/02/06 61.7 49.2 51.77 78.0 1.2 574.2 13      22,488 10.782 457 New personal record; I suspect 10.782 is from stretching the bladder
08/21/06 62.1 49.5 52.14 70.0 0.4 640.0 19      23,128 10.326 468 New personal record! I triple-checked that temperature drop.
09/14/06 63.0 49.9 53.04 66.8 0.9 686.2 24      23,814 9.555 477 Another Personal Record!   Again, temperature down, MPG up.
09/26/06 61.2 50.2 52.81 59.0 -1.8 546.0 12      24,360 8.439 486 The last bip was blinking but only needed 8.5 gallons.
10/13/06 60.8 50.2 52.72 52.5 -0.4 566.1 17      24,926 11.054 497 I’m sure this will be my last 60+ tank of the year.
10/30/06 56.3 50.3 52.96 43.5 -4.5 544.7 17      25,471 9.338 506 First 1/2 Interstate.  Severe temp drop and very windy
11/15/06 56.5 50.5 53.38 41.6 0.2 480.3 16      25,951 7.522 513 Surprised at maintaining MPG.
11/30/06 55.3 50.5 53.38 41.4 -1.2 414.5 15      26,366 8.806 522 Getting Cold.  Got gas prior to Winter Storm.
12/15/06 50.2 50.6 53.92 27.1 -5.1 456.2 15      26,822 7.333 530 First time I used the heater this winter.
12/29/06 53.9 50.5 53.42 37.0 3.7 363.5 14      27,186 8.699 538 First full tank with a blocked grill.  Temperature increase.
01/11/07 54.5 50.5 54.55 37.8 0.6 459.4 13      27,645 9.610 548 Nice uptick in temperatures.
01/30/07 50.1 50.5 54.37 24.5 -4.4 462.2 19      28,107 9.193 557 Cold.  Very cold.
02/08/07 44.0 50.3 54.15 5.6 -6.1 393.8 9      28,501 9.001 566 Flat tire, dead battery, record cold; I shouldn't complain.
02/22/07 52.0 50.4 54.05 16.3 8.0 439.2 14      28,940 8.500 575 Mileage increase outpaced the temperature increase
03/06/07 52.5 50.4 54.20 27.3 0.5 410.5 12      29,351 7.897 582 Would have liked better but the snow and rain kept falling
03/23/07 57.0 50.3 54.39 38.6 4.5 495.3 17      29,846 10.343 593 Looking forward to summer weather.
04/07/07 58.3 50.6 55.29 46.5 1.3 576.6 15      30,423 8.661 601 Got off to a warm start then turned cold again.
04/22/07 57.5 50.6 55.12 41.9 -0.8 503.0 15      30,926 9.780 611 Had another cold snap.  Hopefully the last of the season.
05/02/07 61.2 50.8 55.02 54.2 3.7 587.9 10      31,514 9.387 621 1. First 60+ tank of the season! I think I'll start counting them.
05/15/07 62.6 51.0 55.69 58.9 1.4 591.5 13      32,105 9.093 630 2. Almost a record tank, but last-minute rain ruined it. 
05/31/07 64.1 51.1 56.00 64.4 1.5 583.7 16      32,689 10.056 640 3. New personal record.  56 MPG here I come!
06/16/07 66.0 51.3 56.67 69.8 1.9 693.5 16      33,382 10.483 650 4. New record. Kept dropping below 66 had to fight to keep it up.
07/02/07 66.0 51.6 57.17 70.3 0.0 664.1 16      34,046 9.719 660 5. Was hoping to exceed 66.0.  Hopefully next time.
07/20/07 63.7 51.7 57.14 73.8 -2.3 622.4 18      34,669 10.483 670 6. It was a bad tank anyway ending with 110 miles of highway.
08/08/07 66.7 51.9 57.26 75.4 3.0 676.8 19      35,346 10.515 681 7. New Record!  First 110 miles was highway @ ~56MPG.
08/28/07 66.7 52.1 57.70 71.3 0.0 678.8 20      36,024 10.810 692 8. Saw 67MPG for a few days then the rain came.
09/15/07 65.1 52.3 58.28 67.5 -1.6 660.8 18      36,685 9.001 701 9. Maybe I can squeak out one more 60+ tank this year.
10/02/07 68.0 52.5 58.36 67.5 2.9 612.4 17      37,298 9.500 710 10. New record!  In October!  What's up with that?
10/08/07 63.0 52.6 57.90 71.3 -5.0 564.9 6      37,862 9.384 720 11. This tank was more than 75% highway miles.
10/15/07 64.9 52.8 57.94 55.2 1.9 596.5 7      38,459 9.175 729 12. First 375 miles were highway miles.  The rest as normal.
11/02/07 59.5 52.9 58.39 53.5 -5.4 556.9 18      39,016 9.021 738 Finally dropped below 60mpg.  Temps and short trips contributed.
11/17/07 59.5 52.9 58.36 41.8 0.0 543.6 15      39,559 9.430 747 Such a high tank surprised me given average temperature.
12/04/07 55.3 53.0 58.18 32.6 -4.2 478.4 17      40,038 8.255 756 Cold temps and short trips brought it down.  Was holding high 50s.
12/15/07 53.4 53.0 58.28 23.4 -1.9 450.5 11      40,488 9.003 765 This tank was Chicagoland gas.  Next tank from Wisconsin.
01/04/08 55.8 53.0 58.58 28.8 2.4 485.0 20      40,973 9.053 774 Temp and Mileage up.  Wisconsin gas.
01/18/08 57.8 53.0 58.49 33.1 2.0 509.6 14      41,483 9.068 783 Surprised at the MPG.  Was in 58 until right at end.
01/31/08 50.1 52.9 58.52 14.7 -7.7 438.1 13      41,921 9.144 792 Temperature was a major factor as were a couple snow storms.
02/14/08 53.6 53.0 58.70 22.6 3.5 461.0 14      42,382 8.409 800 Major snow storms during this tank.  Temperatures improving.
02/27/08 51.9 53.0 59.11 19.1 -1.7 458.2 13      42,840 8.409 809 Cold weather and snow storms and short trips.
03/10/08 56.2 52.9 58.91 24.7 4.3 493.6 12      43,334 10.248 819 Hopefully the temps are on the way up.
03/25/08 59.3 53.0 59.27 34.3 3.1 571.4 15      43,905 9.132 828 So frustrated.  450 miles at 60+ MPG and then major snow storm.
04/14/08 58.8 53.1 59.79 40.7 -0.5 561.5 20      44,467 9.304 837 With higher temps I expected better mileage.  Just didn't happen.
04/28/08 66.9 53.2 59.77 51.2 8.1 601.3 14      45,068 9.175 847 1. First 60+ tank of the season.  Was at 67.8 but then bad weather.
05/19/08 62.7 53.3 60.13 49.8 -4.2 584.4 21      45,652 9.289 856 2. Was hoping to get higher but will take another 60+ mpg tank.
06/06/08 65.9 53.4 60.08 56.7 3.2 624.2 18      46,277 10.161 866 3. Relatively chilly weather so far this season
07/02/08 71.2 53.7 60.37 68.6 5.3 700.2 26      46,977 9.509 875 4. First 70+ tank!! Yippee
07/23/08 72.2 53.8 60.27 70.6 1.0 663.6 21      47,640 10.091 886 5. New personal record. 
08/05/08 71.3 54.0 60.76 72.0 -0.9 492.5 13      48,133 6.500 892 6. Rotated tires, bad gas?, short tanked prior to trip.
08/10/08 63.8 54.0 60.43 69.7 -7.5 467.4 5      48,600 8.254 900 7. Almost 100% highway miles.
08/20/08 64.9 54.1 60.56 68.1 1.1 664.6 10      49,265 10.504 911 8. About 75% highway.  Disappointed with mileage.
09/08/08 71.6 54.3 61.14 68.4 6.7 693.5 19      49,958 8.874 920 9. Aimed for a new high but ended with a temp drop and rain.
09/21/08 68.3 54.4 60.92 63.7 -3.3 580.0 13      50,538 8.500 928 10. Temperature is dropping.
10/06/08 65.7 54.5 60.71 58.2 -2.6 575.2 15      51,114 9.680 938 11. Might be my last 60+ tank of the year.
10/20/08 65.0 54.6 60.81 55.7 -0.7 624.1 14      51,738 10.001 948 12. Temperature's dropping.
10/31/08 63.3 54.7 60.73 46.0 -1.7 507.4 11      52,245 8.004 956 13. Yet another 60+ tank.  Getting new tires tomorrow.
11/14/08 58.8 54.7 60.76 47.1 -4.5 538.9 14      52,784 8.587 964 Finally dropped below 60mpg.  New tires, perhaps?
12/03/08 51.4 54.6 60.25 31.0 -7.4 430.9 19      53,215 9.501 974 Dropped like a stone.  I honestly think I got bad gas.
12/13/08 50.4 54.6 59.99 20.9 -1.0 468.5 10      53,683 9.065 983 Cold and miserable weather
12/22/08 46.2 54.5 59.90 18.1 -4.2 400.8 9      54,084 8.503 992 4 winter storms in one week.  Record cold and a foot of snow.
12/27/08 55.4 54.5 59.51 23.6 9.2 496.4 5      54,581 9.857 1001 100% highway miles.  Half with strong tailwind.
01/08/09 52.4 54.6 59.85 24.8 -3.0 461.7 12      55,042 7.315 1009 Threatening to be a long cold winter
01/27/09 45.2 54.5 59.39 11.3 -7.2 342.0 19      55,384 8.067 1017 Average temp 11.3ºF.  No wonder mpg was so low.
02/07/09 49.6 54.4 59.36 18.2 4.4 393.8 11      55,778 8.501 1025 Still cold but getting warmer.
02/20/09 53.1 54.4 59.23 30.9 3.5 464.4 13      56,243 9.000 1034 Glad to see mileage back above 50.
03/06/09 52.9 54.3 59.03 28.9 -0.2 447.3 14      56,690 8.999 1043 Had a good tank going until three days of snow.
03/19/09 55.6 54.3 59.33 36.0 2.7 503.0 13      57,193 9.228 1053 Nice bump.  Good to see it heading back up.
04/01/09 56.0 54.4 59.22 38.1 0.4 490.4 13      57,683 8.695 1061 This was a very hard 56.  Kept hovering in high 55s.
04/14/09 55.5 54.3 58.67 39.3 -0.5 469.9 13      58,153 9.695 1071 Can't explain why this one isn't higher.
04/30/09 56.8 54.3 58.57 48.7 1.3 517.2 16      58,670 8.962 1080 Hopefully the start of higher mileage.
05/15/09 59.6 54.4 58.32 54.9 2.8 542.3 15      59,213 9.198 1089 Saw 60 a couple times but hit construction and bad weather.
05/29/09 60.9 54.4 58.23 57.2 1.3 517.8 14      59,730 8.503 1098 This relatively low mileage is really starting to get on my nerves.
06/18/09 61.0 54.5 58.48 59.1 0.1 557.4 20      60,288 8.500 1106 I can't believe how hard I had to work for a 61mpg tank.
06/30/09 64.1 54.6 58.42 70.2 3.1 543.4 12      60,831 9.032 1115 Was above 65 for a while but then drove interstate for a while.
07/17/09 62.5 54.6 58.05 65.5 -1.6 581.4 17      61,413 9.044 1124 Refill before Hybridfest. Weirdly cold.
07/26/09 57.4 54.6 57.76 66.6 -5.1 498.8 9      61,911 9.914 1134 Mostly highway mileage and then a trip to the dealership.
08/07/09 69.0 54.7 57.75 68.8 11.6 483.9 12      62,395 6.999 1141 Short-tanked this one in anticipation of the upcoming trip.
08/10/09 55.5 54.7 57.34 74.5 -13.5 486.7 3      62,882 8.252 1149 100% highway miles.
08/12/09 58.4 54.7 57.15 68.8 2.9 522.4 2      63,404 9.998 1159 About 95% Interstate driving
08/26/09 66.1 54.8 57.43 68.6 7.7 602.3 14      64,007 9.293 1169 Getting back into the groove of things.  Probably no 70+ this year.
09/12/09 63.7 54.9 57.49 62.3 -2.4 597.4 17      64,604 9.034 1178 I fear the cold is coming.  
09/30/09 64.1 54.9 56.96 62.3 0.4 573.5 18      65,178 9.279 1187 Honestly surprised by the 64mpg.
10/14/09 58.8 54.9 56.59 46.9 -5.3 536.6 14      65,714 9.480 1196 Started with 150 miles of interstate, finished with cold temps.
11/02/09 59.5 55.0 56.52 48.0 0.7 554.9 19      66,269 8.602 1205 Several days of rain but otherwise relatively warm weather.
11/06/09 50.7 54.9 55.88 44.2 -8.8 472.7 4      66,742 10.333 1215 About 80% was interstate with staedy 15mph headwinds
11/11/09 59.5 55.0 56.22 58.7 8.8 554.0 5      67,296 8.288 1224 Mostly interstate driving but this time with a 10mph tailwind.
11/25/09 59.0 55.0 55.77 45.2 -0.5 484.3 14      67,780 9.733 1233 Cold weather is coming.  This will be the last respectable tank.
12/09/09 55.3 54.9 55.98 31.2 -3.7 479.3 14      68,259 9.355 1243 Average driving; dropping temps.  Using the EBH
12/23/09 50.0 54.9 55.84 24.5 -5.3 426.8 14      68,686 9.000 1252 Almost entirely highway; light snow with some wind
12/27/09 49.7 54.9 56.11 27.4 -0.3 379.4 4      69,066 6.844 1259 100% highway miles, about half through a snowstorm
01/08/10 51.4 54.8 56.02 16.0 1.7 424.4 12      69,490 8.990 1268 So far holding its own in the cold weather.
01/21/10 55.2 54.8 55.76 24.6 3.8 486.3 13      69,976 9.012 1277 Unnaturally warm winter so far
02/01/10 53.5 54.8 56.00 23.6 -1.7 473.4 11      70,450 9.249 1286 Surprised that I'm still holing above 50.  Could I go the whole winter?
02/16/10 55.4 54.8 56.21 23.5 1.9 477.9 15      70,928 9.002 1295 Hoping the temperatures go up from here.
03/02/10 56.9 54.8 56.46 29.2 1.5 495.1 14      71,423 8.354 1303 The warmer weather is coming!
03/17/10 60.4 54.8 56.57 36.7 3.5 534.2 15      71,957 10.000 1313 First 60+ tank of the year! Earliest 60+ ever!
04/01/10 60.9 54.8 56.76 43.9 0.5 547.0 15      72,504 9.093 1322 Was hovering at 60 for a while.  Final days of warm temps helped.
04/14/10 60.4 54.9 56.94 53.2 -0.5 555.6 13      73,060 9.000 1331 Was hoping for higher.  Still a full 5mpg over this time last year.
04/27/10 59.7 54.9 57.41 48.5 -0.7 556.3 13      73,616 8.999 1340 Disapointed by the drop in mileage but it got cold again.
05/12/10 63.9 55.0 57.67 50.7 4.2 573.3 15      74,189 8.700 1349 Hopefully I'll stay above 60mpg for the rest of the summer.
05/26/10 64.2 55.0 57.71 59.5 0.3 568.9 14      74,758 9.501 1358 Keeping up with two years ago when I eventually topped 70mpg.
06/09/10 63.7 55.1 57.75 65.7 -0.5 573.1 14      75,331 9.274 1368 Ended the tank with mileage-lowering interstate driving.
07/08/10 66.1 55.1 57.64 71.0 2.4 638.3 29      75,969 10.004 1378 Was out of town for two weeks.  Nice overall tank.
MPG is tracked on the left axis and temperature is tracked on the right axis.
Chart
Important Dates
Aug, '06 Overinflated tires from 35f/33r to 42f/40r.  Noticeable mileage bump.
Dec, '06 Blocked the grill with piping insulation for the first time.
Aug, '07 Overinflated tires from 42f/40r to 52f/50r.  Questionable mileage improvement.
Nov, 08 New Goodyear Integrity tires.
July, '09 Installed and started using Engine Block Heater.