Why not get the reverse gauges, then buy an aftermarket Tach, that way you won't have to worry about the numbers matching, and you have a cool aftermarket gauge, and reverse gauges at the same time.
Becuase aftermarket tachs use the same wire tap as the dash tach, and look silly.
No jeff was wondering what he could do if the reverse tach numbers didn't match the stock ones, so that is why i suggested the aftermarket Tach, as it would still give an accurate reading even if the reverse gauges were off.
I may be wrong but I think both tachs use the same form of measurement, one just goes higher than the other, for show.
my aftermarket AutoMeter tach is wired straight... it doesnt go through the dash at all... and if you look at the tachs while holding costant revs.. (lets say 3000rpm) the tach will say 3000rpm and the stock one will say 3500rpm... the aftermarket tach is more accurate as I figured out through testing..... the stock one is usually around 500rpm higher.....
You're probably right about the stock gauge lagging a bit, but I'm fairly sure both the dash tach and the autometer tach tap into the same signal, so you are in essance monitoring the same thing with 2 gauges.
I do like the idea of a shift light where my hazards button is though 
I think its because the dash tap has to go through so much other shit before it recieves the signal, where as the tach tap is wired straight... thats why the tach doesnt lag but the dash tach lags.... I think....
