OK,
I will grant that you are correct in what you say about the speedometer
reading. But stop right there.
This thread started because someone asked speed of the car relative to the tach
reading.
Bear in mind that in this scenario, you have no working speedometer so you want
to know how fast you are going by looking at the tach.
The Speedometer is immaterial. I don't know how everybody got sidetracked on
the speedometer issue, but the issue is of speed of the
car not what the speedo says. It was the absence of a working speedometer that
started this thread in the first place.
See where I'm coming from?
Joe
Dean Dashwood wrote:
>
> Joe,
>
> It's very rare that I disagree with you (in fact, I don't think I ever have),
> and I'm not sure if I'm disagreeing in this case, or just misunderstanding
>you.
>
> If you change your tyres, then, at a given engine speed and in a given gear,
> your road speed will change (faster for bigger tyres, slower for smaller
>tyres).
> However, the speed shown on your speedo will *not* change (i.e. it will be
> wrong). Therefore, I disagree with the sentence below, "changing the tyre
> diameter will vary the speedometer reading" - it will *not* vary the
>speedometer
> reading if the engine speed and gear selection are the same; it will, however,
> vary the actual road speed.
>
> If this is what you meant, then hopefully I've cleared things up a little bit,
> because this is not how I read your e-mail.
>
> If you really did mean that at the same engine speed, the speedo will read a
> different value depending on tyre size (and diff ratio) then could you please
> explain how this is the case, because it is not my understanding?
>
> Cheers,
>
> Dean
> ------------
>
> Date: Tue, 23 May 2000 07:01:59 -0700
> From: Joe Curry <spitlist@gte.net>
> Subject: Re: speedos and rotating diameter
>
> Let me weigh in on this one.
>
> There is definately a difference in the tach to speed ratio if everything
>else s
> not equal. Changing tire diameter will vary the
> speedometer reading , as any tire shop will tell you. Also varying the ratio
>of
> the differential will change the speedometer reading.
> While you can calculate the speed by doing the proper math, all the elements
> must be taken into consideration. the drive line does not
> end at the back of the transmission where the speedometer cable is connected.
>
> Joe
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