Detect Signal

[skimask]

Which pin do you plan on using to trip the interrupt?

Assuming the pulse goes high (maybe PortB.0?), let the ON INTERRUPT kick you into the interrupt subroutine and stay there until the pin goes low so you don't kick the counter more than once.
Check and reset the pulse counter (if counter > 4000 then = 0 etc.etc.) and increment the mileage counter, and do the displaying and button checking in your main loop. No sense doing that in your interrupt subroutine.
First off though, don't worry about counting mileage...put the whole thing together and whip up a 555 timer or something (maybe even a pushbutton, the switch bounce will give you more than one pulse if the button is old enough!) to simulate the VSS pulse and get the display and everything working without having to go out to the car every 2 minutes to test it out. Just get the thing counting up as you need it (heck you can get both an odometer and speedometer and average speedometer and gas mileage and oil change intervals and all sorts of crap into this), then worry about all the neat stuff for later since you've already got basics down.

[Kman]

I was trying the same thing as you, but I was wanting to have a Point of Sale printer record my daily millage. and use a Real time clock to Give it a daily time stamp!
But my problem was to writing the milliage to the eprom and keeping the pulse count while moving.

[angrysmileyface]

I dont intend to store to the eprom until the car stops and is shut off, but that is later once i get it all built and tested