Ummmm . . . no.  In the first place, your mustang wasn't properly equipped.
If it had been, you wouldn't have felt you needed to use the speedometer.
But manual tranny drivers don't use the tachometer to shift anyway.  They
generally go by the sound of the engine, when deciding when to upshift or
downshift.  So you shouldn't have been substituting anything for an
instrument that you shouldn't have been using.  -Dave

I used to have a 64 1/2 Mustang with a factory four speed manual, and
no tach.  The speedometer was handy for being able to determine and hit
the shift points where horsepower had reached it's peak. I'd say that
has to do with driving.

I've said many times that speedometers have NOTHING to do with driving.
Nobody is able to refute that statement without mentioning OTHER things that
ALSO have NOTHING to do with DRIVING.  (example, cops, speed limits, traffic
tickets, etc.)  -Dave

I think speedometers are pretty useless, since quite frankly most sane
people drive the speed they are comfortable for road and weather
conditions (the one usenet troll this is geared to will invariable fair
to understand that).  Ideally, lose all speed limits, lose the
speedometers, lose the false science and ad-hoc revenue generation,
clearly mark areas where caution is required (and hey, if there are
inherent dangers, use zig-zag road markings or rumble strips or some
equivalent as approipiate - but no freaking traffic blocking...I mean
traffic 'calming' ), and use the space on the dashboard for larger turn
signal indicators (so the ol'  and not so ol' folks won't be driving
2km signalling for a turn).
Cops could still give out their tickets for reckless driving, and
'agressive driving' wouldn't be a crock offense based on speed + one
other offense, but would actually mean something (like weaving in and
out without signalling and tailgating agressively whenever they can't
weave).  And instead of wasting time hiding in speed traps, cops could
actually patrol for real safety violations and... Eh, we all know this
already (except for one troll, who refuses to accept reality).
"Speed kills nobody, Stupidity kills millions"

Hey, I think you've got something there.  I say all speedometers should read
25MPH, maximum.  Then you would have an iron-clad defense against roadside
tax collection activity.  You can't be charged for defective equipment if
the factory installed it.  You can't be charged for speeding if you honestly
have no idea how fast you are going.  :)  Oh, and speed limits below 35MPH
are rarely (like, almost never) enforced.  -Dave

Seeing as there are roads with 70 MPH speed limits within a few hours'
drive from my house, how would I be able to tell whether or not I was
speeding?

nate

It's insane that they read anything over 60.  Someday GM is gonna get
sued over this.  Somebody will get killed by a speeder doing 80 and
the victim's family will sue GM for selling cars that encourage
speeding.

In the late 70s and 80s, most (all?) cars had speedometers that would only
go up to 85.  Were they mandated by the government?  What year did they come
out?  Why 85?  At the time the maximum speed limit was 55.