How to halve the pulse coming from a three wire speed sensor (on a remote control car)?
Note that by limiting speed to 40 kph with half diameter wheels you are achieving twice the motor RPM as previously. This may be acceptable or a problem depending on the application. 

If the digital speed signal is of sufficient magnitude (which it probably is) you can probably add a divide by two "flip flop"  to produce one output pulse for every two in. I say "probably" as it is possible but not overly likely that the following circuitry needs a series of short pulses rather than a half frequency square wave. This too can be accomplished if necessary.
Two flipflop toggle' divide by 2 arrangements are shown below. 

Removal of every 2nd pulse can be achieved by eg having a flipflop toggle circuit as shown below and passing the signal through an eg AND gate with the gate enabled when a FF output is high and disabled when it is low. This has the advantage of preserving the pulse width of the signal. 

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FLIPFLOP TOGGLE DIVIDE BY TWO CIRCUITS:

In both cases Vout is a square wave which changes either from low to high or high to low on active transition of the clock signal. (I will not detail what that means specifically at this stage - clocks can be arranged to be +ve edge or -ve edge triggered or in some cases level triggered.)

A "D flipflop" will provide divide by two functionality when connected as shown below



The above diagram is from this webpage which also provides a good introductory tutorial into digital divide by N circuits and covers use of both D and JK fipflops.
Also covered here - same organisation. 

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A JK flipflop ahieves divide by 2 when connected as shown here



From here



eg MC14013B &amp; friends: