Increasing Vd for narrow frequency range

[SaintK]

I'm building a combination of shift light and a tach(like everyone else), but am running into a problem. I'm making the shift light start lighting up at 6.4k rpm and all the lights on (10 LEDs using LM3914) at 6.9k rpm.

This translates to 17Hz difference and the LM3914 seems to want a fairly large RefHi and RefLo difference. With traditional methods I can only get about 0.5v difference for the 17Hz difference. I'm afraid that this isn't enough to have an accurate separation for the LEDs and that they'll "bleed" into each other.

Only way I can seem to fix this is by increasing the value of C1 or R1 (or both), but again I can only get about 1V difference.

So my question is is there anyway to expand the voltage difference with a narrow frequency band? Also is it safe to input the rpm signal straight into the signal pin without scaling down the voltage? It's a 13.5V digital signal, so no spikes, and from what I read so far it seems like it'll handle up to 28V max?

I'm working with a 14pin LM2917 and LM3914. My rpm signal is 13.5V digital signal with a frequency of 33.333Hz at 1k rpm from the ECM.

Thanks for the help guys!