86-88 5KCSTQ tachometer

[Paul Meyers]

Recent trip to California from Utah in my 87 5Kcstqa. Speedo reads high.
Don't want tickets (family in car, very embarassing). Don't want to go
slower than necessary either (truckers get testy when you doddle on the
interstate). Decide to calibrate 3000 rpm to see where to set cruise control
to avoid highway patrol. Using my trusty wrist-based stopwatch (nifty
Citizen titanium eco drive, but that's another story), 3000 rpms gets me
exactly 48 seconds per mile (gracias to the thoughtfully placed mile markers
on the interstate in Utah, Arizona & Nevada -- but not California (why
not?)). Turns out 48 seconds/mile is 75 MPH. So if 3k RPM is 75 MPH, what is
70, 65, etc.? Well, it turns out that, with 48 sec/mile equal to 75 MPH at
3000 RPM, you devide RPM by exactly 40 to get miles per hour (trust me, this
works out).

So, 70 is 2800 RPM, 65 is 2600 RPM, 60 is 2400 RPM, corresponding tidily to
the indicated markings on the tach. Wow.

Of course, this depends some on the tires and car, right? Well, this morning
I used the same technique on my Pearl 88 and amazingly, 3000 RPM is again 48
seconds/mile (you would expect the final drive ratio to be the same, but
expectations are often wrong). Both cars use the standard 205/60-15 tires.

So, to those with unreliable speedos, calibrate your tach. It's much better
than guessing. And it just might line up with the markings as mine did.

Of course, your experience may vary.

Paul Meyers 
Gray 87 5KCSTQ 1.8bar eurolights fuchs 145 kM, replacing electronic ignition
loom 
Pearl 88 5KCSTQ 1.8bar eurolights fuchs bilsteins 126 kM 2pc-em k24 turbo 
Repainted Pearl 87 5KTQA 1.8bar eurolights TT wheels bilsteins 196 kM daily
driver
Blue 86 5KCSTQ parts car