Author Topic: Speedo errors and correct wheels  (Read 66 times)

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Offline cad425

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Speedo errors and correct wheels
« on: January 04, 2020, 04:13:05 PM »
I'm confused.
Ive used my TomTom GPS to check speedo and odo error - 10% high for speedo, and 13% low for odo - very high errors. Normally, I'd expect this to be caused by larger than original tyres. So I checked wheel sizes.
Lucy has new tyres installed  by PO (well - new in 2015, but only 450 miles since) which are Mastercraft P215 75 R15 white walls. Original tyres should have been GR78x15 B for which equivalence is 205R15. As far as I can tell from Mr Google while the 75 series tyre is 'larger' than original, it has about the same rolling diameter: original 27.7", replaced with 27.56". OK, so no great difference there, and probably a good choice of replacement.
So why then the big errors? Do other 79 Coupe deVille owners have similar errors? Has someone fiddled with the speedo gearing?

Is there something I'm missing?
1979 Coupe DeVille 425 - project since Jan 16
2006 Fairmont V8 - gone to a good home
1996 E220 Cabriolet - for sale, replaced  by...
2013 E250 Cabriolet

Offline Bruce Reynolds

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Re: Speedo errors and correct wheels
« Reply #1 on: January 18, 2020, 03:17:18 PM »
G'day John,

Again, just saw your question.

Automobile Speedos were designed by the manufacturers to read at least 10% faster than true speed.

BUT, since the advent of computers in cars became the norm, the driving public has demanded that the readouts like MPG and other stuff has meant that they be more correct.

What you have seen is normal.

When I converted the Speedo in my '72 eldo to Metric, I corrected the hair spring so the needle read as close to correct as possible, but still left with the vagaries of the odometer, which is moved by actual gearing.

Bruce. :devil:
Bruce Reynolds, Lindisfarne, TASMANIA
'72 Eldorado Convertible (LHD)
'70 Ranchero Squire (RHD)
'74 Chris Craft Gull Wing (SH)
(Past President Modified Chapter)