This should help you figure it out! All of the times are in seconds!

     
   KPH     MPH   1k Time 250 Lap 333 Lap 400 Lap
   29     18.1    2:04   31.03   41.38   49.66
   30     18.8    2:00   30.00   40.00   48.00
   31     19.4    1:56   29.03   38.71   46.45
   32     20.0    1:53   28.13   37.50   45.00
   33     20.6    1:49   27.27   36.36   43.64
   34     21.3    1:46   26.47   35.29   42.35
   35     21.9    1:43   25.71   34.29   41.14
   36     22.5    1:40   25.00   33.33   40.00
   37     23.1    1:37   24.32   32.43   38.92
   38     23.8    1:35   23.68   31.58   37.89
   39     24.4    1:32   23.08   30.77   36.92
   40     25.0    1:30   22.50   30.00   36.00
   41     25.6    1:28   21.95   29.27   35.12
   42     26.3    1:26   21.43   28.57   34.29
   43     26.9    1:24   20.93   27.91   33.49
   44     27.5    1:22   20.45   27.27   32.73
   45     28.1    1:20   20.00   26.67   32.00
   46     28.8    1:18   19.57   26.09   31.30
   47     29.4    1:17   19.15   25.53   30.64
   48     30.0    1:15   18.75   25.00   30.00
   49     30.6    1:13   18.37   24.49   29.39
   50     31.3    1:12   18.00   24.00   28.80
   51     31.9    1:11   17.65   23.53   28.24
   52     32.5    1:09   17.31   23.08   27.69
   53     33.1    1:08   16.98   22.64   27.17
   54     33.8    1:07   16.67   22.22   26.67
   55     34.4    1:05   16.36   21.82   26.18
   56     35.0    1:04   16.07   21.43   25.71
   57     35.6    1:03   15.79   21.05   25.26
   58     36.3    1:02   15.52   20.69   24.83
   59     36.9    1:01   15.25   20.34   24.41
   60     37.5    1:00   15.00   20.00   24.00
   61     38.1    0:59   14.75   19.67   23.61
   62     38.8    0:58   14.52   19.35   23.23
   63     39.4    0:57   14.29   19.05   22.86
   64     40.0    0:56   14.06   18.75   22.50
   65     40.6    0:55   13.85   18.46   22.15
   66     41.3    0:55   13.64   18.18   21.82
   67     41.9    0:54   13.43   17.91   21.49
   68     42.5    0:53   13.24   17.65   21.18
   69     43.1    0:52   13.04   17.39   20.87
   70     43.8    0:51   12.86   17.14   20.57
      
      

While we could tell you, it would be easier to check out the Gear and Speed Spreadsheet in the downloads section!

Doing this saves us a lot of typing!

-fgf

I believe the chart below will help you out!

  Ring    Cog      Gear        
   56      13      116.3    
   55      13      114.2    
   54      13      112.2    
   53      13      110.1    
   52      13      108.0    
   56      14      108.0    
   55      14      106.1    
   51      13      105.9    
   54      14      104.1    
   50      13      103.8    
   53      14      102.2    
   49      13      101.8    
   56      15      100.8    
   52      14      100.3    
   48      13      99.7     
   55      15      99.0     
   51      14      98.4     
   47      13      97.6     
   54      15      97.2     
   50      14      96.4     
   46      13      95.5     
   53      15      95.4     
   49      14      94.5     
   56      16      94.5     
   52      15      93.6     
   45      13      93.5     
   55      16      92.8     
   48      14      92.6     
   51      15      91.8     
   44      13      91.4     
   54      16      91.1     
   47      14      90.6     
   50      15      90.0     
   53      16      89.4     
   46      14      88.7     
   49      15      88.2     
   52      16      87.8     
   45      14      86.8     
   48      15      86.4     
   51      16      86.1     
   44      14      84.9     
   47      15      84.6     
   50      16      84.4     
   46      15      82.8     
   49      16      82.7     
   45      15      81.0     
   48      16      81.0     
   47      16      79.3     
   44      15      79.2     
   46      16      77.6     
   45      16      75.9     
   44      16      74.3     
      

Here is a pretty full gear chart! (based on a 27" wheel)

 	 38	 39	 40	 41	 42	 43	 44
12	85.5	87.8	90.0	92.3	94.5	96.8	99.0
13	78.9	81.0	83.1	85.2	87.2	89.3	91.4
14	73.3	75.2	77.1	79.1	81.0	82.9	84.9
15	68.4	70.2	72.0	73.8	75.6	77.4	79.2
16	64.1	65.8	67.5	69.2	70.9	72.6	74.3
17	60.4	61.9	63.5	65.1	66.7	68.3	69.9
18	57.0	58.5	60.0	61.5	63.0	64.5	66.0
19	54.0	55.4	56.8	58.3	59.7	61.1	62.5
20	51.3	52.7	54.0	55.4	56.7	58.1	59.4

	 45	 46	 47	 48	 49	 50	 51
12	101.3	103.5	105.8	108.0	110.3	112.5	114.8
13	93.5	95.5	97.6	99.7	101.8	103.8	105.9
14	86.8	88.7	90.6	92.6	94.5	96.4	98.4
15	81.0	82.8	84.6	86.4	88.2	90.0	91.8
16	75.9	77.6	79.3	81.0	82.7	84.4	86.1
17	71.5	73.1	74.6	76.2	77.8	79.4	81.0
18	67.5	69.0	70.5	72.0	73.5	75.0	76.5
19	63.9	65.4	66.8	68.2	69.6	71.1	72.5
20	60.8	62.1	63.5	64.8	66.2	67.5	68.9

	 52	 53	 54	 55	 56	 57	 58
12	117.0	119.3	121.5	123.8	126.0	128.3	130.5
13	108.0	110.1	112.2	114.2	116.3	118.4	120.5
14	100.3	102.2	104.1	106.1	108.0	109.9	111.9
15	93.6	95.4	97.2	99.0	100.8	102.6	104.4
16	87.8	89.4	91.1	92.8	94.5	96.2	97.9
17	82.6	84.2	85.8	87.4	88.9	90.5	92.1
18	78.0	79.5	81.0	82.5	84.0	85.5	87.0
19	73.9	75.3	76.7	78.2	79.6	81.0	82.4
20	70.2	71.6	72.9	74.3	75.6	77.0	78.3