I went through my Excel spreadsheet of fuel purchases and copied out some "before / after" results for use of different additives. I took care to pick only cases where the driving conditions before/after were similar (extended trips etc). For the cases where mpg is below 20, I was pulling small utility trailer.
The comments indicate what went into the tank at THAT fillup, the numbers on the same line therefore indicate performance of the PREVIOUS tank of fuel. Example: on Feb 4th 2002, I had returned 18. 6 mpg on the previous tank of plain diesel, and decided to put in 16 oz. of PS to see if it would improve. The next tank I filled with plain diesel, and the 18. 8 mpg showed PS didn't do much.
Since my truck has returned average of 20. 1 mpg over its life with a 0. 8 mpg standard deviation, an improvement / reduction of 0. 5 mpg is essentially in the noise and can't be relied upon as evidence of the additive's performance. Interestingly, only with MMO have I seen a statistically signficant increase. My results have indicated that, at least for my truck and driving style, there isn't much difference between PS and Stanadyne performance-wise, and MMO appears to give better results overall. YMMV
Fillup Date Trip Main Gals MPG Comments
02/04/02 544. 8 70728 29. 273 18. 6 Add 16 oz. Power Service
02/07/02 576. 2 71304 30. 577 18. 8 Plain diesel fillup
08/15/02 432 79088 21. 803 19. 8 Added 8 oz. Stanadyne
08/16/02 305. 4 79393 16. 167 18. 9 Added 8 oz. Stanadyne
08/17/02 409. 3 79803 21. 945 18. 7 Plain diesel fillup
08/21/02 602. 8 80405 31. 376 19. 2 Plain diesel fillup
09/10/02 634. 3 81040 31. 767 20. 0 Plain diesel fillup
10/11/02 391. 4 82510 18. 912 20. 7 Add 16 oz. MMO
10/26/02 440. 1 82950 20. 246 21. 7 Plain diesel fillup
11/17/02 541. 1 84081 26. 302 20. 6 Add 8 oz. Stanadyne
12/01/02 479. 9 84561 25. 125 19. 1 Plain diesel fillup
01/13/03 550. 7 86305 29. 056 19. 0 Add 32 oz. Power Service
01/21/03 537. 1 86842 29. 119 18. 4 Plain diesel fillup
10/20/03 575. 5 101724 28. 645 20. 1 Add 16 oz. MMO
10/23/03 527. 1 102251 24. 297 21. 7 Plain diesel fillup