3X Small DC motors Work great as Generator

Description

Up for sale are 3 small dc motors.  They can be used as generators.  The motor body is 1.8 X 1.1 dia. inches.  The .09 dia. inch shaft extends from both ends.  One end has a 8 tooth 14 pitch gear, the other is .2 inches long.  There are 2 solder terminals for connection.  The dc resistance is about 3 ohms.  The motor starts to turn at 1.5 volts.  I tested one, assuming the rest would be similar, with the following results.
Input       RPM with
Voltage   no load
2            1500
3            2100
4            2800
5            3750
6            4600
7            5000
8            6000
9            7500
10          8500
11          9200
12          10000
13          11000
14          11100
15          12200
16          14200
17          14600
18          15000
19          15400
20          17100
Two motors connected in a motor and generator configuration
Input voltage Output voltage
2                  1.14
3                  2.11
4                  3.01
5                  4.02
6                  5.01
7                  6.02
8                  7.00
9                  8.00
10                8.94
11                9.94
12               10.91
13               11.94
14               12.89
15               13.84
16               14.77
17               15.7
18               16.52
19               17.45
20               18.5
Please feel free to ask any questions and thanks for looking.