Listers:
I have an interesting conundrum for the list relating to
speedometer/odometer function. First, the story starts with the fact that I
calculated my gas
mileage for the first time a few days ago. The car is a '76 with twin Z-S
carburetors and some high-performance modifications including a 9.5 to 1
compression, a cam, headers, porting, lightened flywheel, electronic ignition,
etc.
It takes premium unleaded at $3.30/gallon. With combined highway/city
driving, I calculated 18 mpg, which is just about exactly what my 2002 Jaguar
XJ
Sport (XJ8) gets!
Anyway, it occurred to me that I didn't know if either my speedo or odometer
was accurate, which would affect my calculation. So I had my son drive his
2005 Jetta on the highway and set a speed of 60 mph on a straight stretch. I
stayed behind him, neither gaining nor losing ground. My speed read 59.5,
which is close enough for me. However, we set our trip odometers at the
beginning and end of the trip, and mine read 4% low compared to his.
My question is, why would our speedometers agree almost perfectly, but our
odometers be different by 4%? Also, is 18 mpg (or really 17, when you take
the odometer inaccuracy into account) in mixed driving (but plenty of highway
at
about 65 mph) similar to others with similarly modified, non-overdrive cars?
Thanks!
Michael
'76 Tahiti blue
CF 57044U
|