[dsy1]

I have a 700R4 with speedometer gears selected to match my gear ratio and tire size in my '69. When I check it with a GPS it reads almost dead-on perfect when I drive to work in the morning, but it is about 6+ MPH off in the afternoon when I drive home on the same freeway (going north instead of south). Can anybody think of an explanation for this? I've checked a couple of times and it is this way each time.

The gauge indicates 65 or 66 in the morning when the GPS says 65, and the gauge shows about 71 or 72 in the afternoon when the GPS says 65.

 

[mrennie]

Temperature decrease causing the tires pressure to drop resulting in a slighly smalller diameter?

If you install shorter tires your speedo reads higher in relation to your actual road speed. If the GPS is 100% accurate this would seem to make sense.

Doppler effect is my backup theory although doubtful....

 

[dsy1]

^^^thanks, but it would be the opposite in this case, since it is warmer in the afternoon than in the morning. Anything in the speedo head itself that could be temperature sensitive?

 

[richietables]

Even if it was a tire pressure issue, it would have to be a pretty substantial pressure change for that percentage, I would imagine. None of the gearing can change mechanically and you have to assume the GPS signal is consistent. Besides temperature, I can't see how the time of day could have anything to do with it. Likewise north or south.

All else being equal, that question is a huge head scratcher to me.

 

[slingslingbinks]

Hey dsy1, I'd like to know what's up with this one? Please do this little test some day. Test it in the morning then take a short trip the other way and see if it's directional or if it's the temperature related. Do the same in the afternoon. I use a gps to set the spedo's on Kenworth truck's all the time. Nothing like a trucker that think's there not going fast enough or loosing miles. C-ya Scott

 

[Brian_d]

How about temperature related - speedo cable dragging slightly in the AM but does'nt drag once it's warm in the afternoon?

 

[The Old Reliable]

Brian narrows it to its probable origin(s) in the speedometer assembly!! :hurray:

And, if the cable is the culprit in the discrepancy as stated, with the (afternoon/warm) variance with the GPS being a higher mph, a new
and different speedo trans gear is needed to get in better calibration.

:beers:

 

[dsy1]

I'll try some more tests with the Chevelle when I get a chance. I can also use a different vehicle and see if I get the same results, so I can narrow down the problem to see if it is GPS-related or vehicle-specific. I'm in San Diego, so the temperature differences are not huge. Maybe 50-55 degrees in the morning when it's cold, 75 in the afternoon.

 

[Verle]

Its the Coriolis effect.