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Author Topic: speedo/rpm per mph  (Read 192 times)

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countrywoman

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speedo/rpm per mph
« on: 16 July 2025, 19:44:00 »
I was wondering what the rpm against road speed is for a 3L , my speedo is out 5mph at 25 doing 30. 42 is 50. Checked against sat nav.
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Enceladus

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Re: speedo/rpm per mph
« Reply #1 on: 16 July 2025, 20:06:56 »
Something seems to be wrong. If your actual speed is 30mph then the speedo, in the car, should read 30 mph or over. Not 25mph. The speedo is not allowed to under read/indicate the actual speed.
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Doctor Gollum

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Re: speedo/rpm per mph
« Reply #2 on: 16 July 2025, 20:09:24 »
Plod Speedo should be calibrated but is dependent on the correct rolling radius. 3.0 plod had 15" steel wheels* whereas 3.2 had 16"s with 225/55/16s.

* Never had an Omega with 15s but could well be the same tyre size as the standard spare.
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countrywoman

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Re: speedo/rpm per mph
« Reply #3 on: 16 July 2025, 20:48:43 »
tyre size is correct, are all drive shaft reluctor rings the same as I used 2L drive shafts and never thought to count the teeth.
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Doctor Gollum

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Re: speedo/rpm per mph
« Reply #4 on: Yesterday at 08:52:25 »
In that case, the calibrated speedometer should be correct.
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Doctor Gollum

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Re: speedo/rpm per mph
« Reply #5 on: Yesterday at 09:23:56 »
205/65/15 should give 2,034mm rolling radius.

I suspect you're using 195/65/15 as a basis.
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