[AMC-list] engine rpm versus mph

["Armand Eshleman" <aje1@xxxxxxxxxxxxxxx>]

found a site on the web that will calculate engine rpm if given the 
variables. So I entered some data and came up with some interesting info.

axle ratios were chosen to determine engine speed at 70 mph.

using a 26.1 inch diameter tire  ( 255/45- R17 ) Kelly Fierce radials

the other chosen constant was the T5 transmission's .63 overdrive 5th gear

2,87 axle ratio = 1630 rpm

2.72 axle ratio = 1545 rpm

3.54 axle ratio = 2010 rpm

the curious thing about this information is the tach (factory in dash) for 
the white 70 Javelin reads about 2100 rpm at 70 mph using the 2.87 axle and 
T5 in 5th gear.

Apparently there are issues with the tachometer. That becomes very real 
when noticing the tach with bounce up 300 to 500 rpm without using 
additional throttle.

I'm using a MSD Pro Billet ready to run distributor that needs no translator 
box for the tach.

Makes me think the electrics for the tach are a bit unstable or the system 
ground is intermittently or always a bit resistive.

Got any ideas?

Thanks,

Armand 

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