| Non-linear speedometer error [message #370652] |
Sat, 08 October 2022 18:37  |
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RF_Burns
Messages: 2277
Registered: June 2008
Location: S. Ontario, Canada
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Senior Member |
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I recently changed my final drive from the OEM 3.07 to 3.42. Previously my speedometer read about 10% low. When my speedo read 50mph, my GPS read 55mph. I usually had the GPS turned ON, otherwise I just added 10% in my head.
I expected when I changed the final drive, the error would actually decrease and the speedo would be more accurate. Instead, when the GPS read 55, the speedo is reading 60+ (1.09x). The 1.11 change in the FD caused a 1.2 change in the speedo (was 5mph low, now 5mph high).
We just came back from the first trip with the new FD and I noticed something strange. Seems at 40mph, both the GPS and the speedo read the same. But at faster speeds, the speedo reads much further away as a percentage. I'm going 75mph by the speedo to get to 65mph on the GPS (1.15x).
I've never noticed this before as I am either aiming for 55mph or 65mph.
So it seems to me that a geared reducer will not fix this problem. I'll need to investigate this more as I was almost home before I noticed the 40=40mph thing.
Has anyone else noticed this. I think I'll need to correct it with stickers on the gauge, just like when we went metric back in the '70's here in Canada.
Bruce Hislop
ON Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.
1 ton front end
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
My Staff says I never listen to them, or something like that
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