I punched up some numbers for my bicycle and it looks like your got to do some figuring to decide how you are going to use your timers.

I computed non-metric, miles per hour, for a 27in diameter tire. I computed speeds for 0mph to 99mph maintaining the following as variables:

(1) Processor speed in Mhz
(2) Tire diameter in inches
(3) Timer/Clock divisor
(4) Prescaler (8051 mode dependent)
(5) Timer size in bits

The first significant note is that your 12Mhz clock will be overflowing many times between revolution pulses which you can handle various ways.

If this were for a bicycle, battery power could be extended by slowing the clock rate down. By changing the spreadsheet variables above, you can explore the best solution before your start your schematics.

I still recommend looking at a minimalist PIC uP for this. An 8051 is overkill.

-Jay C. Box