Has anybody used a hand-held GPS to get an accurate idea of speed?
What is the length of straight line necessary to have to get a good
reading at constant speed?
Mark
1972 TR6 (and a new J-type O/D I want to test)
-----Original Message-----
From: owner-6pack@autox.team.net [mailto:owner-6pack@autox.team.net] On
Behalf Of Navarrette, Vance
Sent: May 1, 2006 11:22 AM
To: SamuelsMA@aol.com; 6pack@autox.team.net
Subject: RE: Speedometer/Odometer
Michael:
The odometer is gear driven, and has no provision for
calibration. You didn't say what size tires you are running on your car,
but I am betting that at least some of the difference in the odometer
reading is your tire size, the rest being production tolerances. 4%
isn't huge, but I would hope that the odometer could be within 2%. The
stock tires were 185-15, which were approximately 82 profile, or
185/82-R15 in today's nomenclature. So compare the difference in rolling
radius of your tires with the stock tires.
As for the difference in the odometer and speedometer, remember
that the speedometer CAN be calibrated even if the odometer cannot. The
speedometer needle is not directly connected to the drive cable, rather
there is a magnetic clutch (called a "drag cup"). The amount of slippage
can be adjusted, so that the speedometer is correct even if the odometer
is not, and vice versa. There is a spring that provides the force
pushing the needle towards 0, while the drag cup tries to drag the
needle towards 140MPH. The speedo is calibrated by adjusting the force
on the return spring for the needle.
The mileage sounds decent, but is a bit on the low side (IMHO).
What color are your plugs? If they are sooty black, that is wasted fuel
going out the tailpipe - so get your fuel mixture correct. Check your
ignition timing, and make sure your dizzy is in good shape; vacuum
retard is working, mechanical advance is functional, etc. Check your
valve lash too.
The biggest variable is, of course, the driver. If you are like me, you
hot foot it at every stop light just to hear that gorgeous engine sound.
Short shifting instead will improve your mileage noticeably.
If you are hell bent on improving your mileage, an earlier
distributor with a vacuum advance will really help your highway mileage.
You can retrofit it and hook up the advance to the proper vacuum port to
get a mileage pop.
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