reaches a plateau of injection at 60 deg C and above, so whether the engine temp is 70, 75, 80, 85, 90, 95, or above 100 deg C has to do more with Heater Core comfort in winter more than fuel efficiency.
From the Charles Prost Book on Bosch Fuel Injection, fuel efficiency plateaus above 70 deg C.

I also have a Honda Odyssey van (I know it is not a BMW) but the stock thermostat is 75 deg C, taking a long time to warm the van in winter.
I changed to 82 deg C (and still lower than my 88 deg C BMW thermostat) thermostat and it is now warm in winter.

Back to my 528i, 88 deg C: when fully warmed up, temp gauge sits at 1 mm to the LEFT of midline, so it is not far off the factory 92 deg C thermostat. Gas mileage is the same, unchanged.

cn

• ENTIRE COOLING SYSTEM REPLACEMENT Tips and Tricks (Very Long, Enjoy!) (pic) - CNN-090 2006-05-14 04:27:25 (47604 views)
  • The censorship at roadlfy is 'low-class' and disgusting - CNN-090 2006-06-16 23:32:13 (1761 views)
  • HEPU Water Pump holding up nicely 1 month/1K miles later..... (url) - CNN-090 2006-06-02 19:31:04 (1477 views)
    • HEPU Water Pump Info for those who want to know... (url) - CNN-090 2006-06-02 19:47:54 (1979 views)
  • Re: ENTIRE COOLING SYSTEM REPLACEMENT Tips and Tricks (Very Long, Enjoy!) - yrwei52 2006-05-19 12:14:09 (1987 views)
    • My answers to your questions (url) - CNN-090 2006-05-20 15:44:05 (1798 views)
      • Re: My answers to your questions - yrwei52 2006-05-23 23:24:04 (873 views)
        • Re: ENTIRE COOLING SYSTEM REPLACEMENT Tips and Tricks - yrwei52 2006-06-14 03:46:05 (1188 views)
  • The mORE rEASONS TO kEEP YOUR Cooling System in Top Shape: The HEAD GASKET (url) - CNN-090 2006-05-18 12:20:25 (2052 views)
  • That is an outstanding example of documenting your work. Definite e38.org material. (nt) - davidc 2006-05-17 02:32:42 (810 views)
    • It had 'only' 550+ viewings in 2 days! Must have been useful to people! (nt) - CNN-090 2006-05-17 09:32:50 (740 views)
  • 88C thermostat gauge temp pic (pic) - CNN-090 2006-05-16 09:32:55 (1957 views)
    • BMW and VDO temp gauge (pic) (url) - CNN-090 2006-05-17 15:18:27 (1686 views)
    • The gauge is dead center ... - srr2 2006-05-16 12:37:06 (1005 views)
      • Re: The gauge is dead center ... - jimcash 2006-05-16 12:47:15 (847 views)
        • Jim, I may be wrong but VDO makes gauges for BMW (url) - CNN-090 2006-05-16 12:53:47 (745 views)
          • Re: Jim, I may be wrong but VDO makes gauges for BMW - jimcash 2006-05-17 07:53:17 (694 views)
            • Ideas for Testing gauge readings (pic) - CNN-090 2006-05-17 09:51:47 (1159 views)
        • Jim, For my info what temp does it have to be - CNN-090 2006-05-16 12:49:19 (720 views)
  • My Observations - dan 2006-05-14 23:04:28 (1370 views)
    • Great input, the more input the better...... - CNN-090 2006-05-14 23:21:05 (891 views)
  • Forgot a few pictures (pic) - CNN-090 2006-05-14 20:13:54 (3901 views)
    • MANUAL TRANS FOLKS! When Raising the Front - CNN-090 2006-05-17 10:34:34 (1086 views)
  • Thanks - This should be very helpful (nt) - Roderick 2006-05-14 19:04:52 (692 views)
  • Re: ENTIRE COOLING SYSTEM REPLACEMENT Tips and Tricks (Very Long, Enjoy!) - jimcash 2006-05-14 07:11:02 (2098 views)
    • From what I understand, Bosch Motronic FI system - CNN-090 2006-05-14 13:01:49 (1054 views)