[Healeys] Speed to RPM ratio?

[i erbs]

My tach is driven by my distributor gear
Speedo comes off my transmission.  Not sure about the relationship to
accuracy.

Ira Erbs
1959 AH 100-6
1967 MGB
Milwaukie, OR

On Sun, Jun 13, 2021, 11:50 AM banjojohn <banjojohn at cox.net> wrote:

> Thanks.
> I am actually just comparing the tach with the speedometer indicated
> speed, which, I believe reads a little high due to tire size.  But the
> relationship between the indicated speed and the tach should be consistant
> since they are both driven by actual engine or output shaft speed.
>
> John
>
>
> Sent from my T-Mobile 4G LTE Device
>
>
> -------- Original message --------
> From: Bob Spidell via Healeys <healeys at autox.team.net>
> Date: 6/12/21 12:25 PM (GMT-06:00)
> To: Bob Haskell <rchaskell at earthlink.net>, healeys at autox.team.net
> Subject: Re: [Healeys] Speed to RPM ratio?
>
> That's it!
>
> On 6/11/2021 4:31 PM, Bob Haskell wrote:
> > http://longstonetyres.co.uk
> >
> > Cheers,
> >
> > Bob Haskell
> > Austin Healey 3000 BN7/BT7 registrar
> >
> > On 6/11/21 7:01 PM, Bob Spidell wrote:
> >> Note there is a not insignificant difference in diameters of tires
> >> with the same nominal size (e.g. '165/80-15'). The English tire
> >> site--can't think of the name offhand--has (had?) a table comparing
> >> the diameters of different brands/models of the same size tires, and
> >> IIRC they could differ by as much as half an inch. I'd say to measure
> >> the actual diameter of your tires, but it's kinda hard when moving;
> >> the 'flat spot' goes away--partially, at least--and the tires 'grow'
> >> some from centrifugal force (anyone seen film of top fuel dragsters
> >> launching; the rear tires must expand a few inches or more?).
> >>
> >>
> >> On 6/11/2021 3:36 PM, John OBrien wrote:
> >>> Thanks to all who replied.  These will give me something to work with.
> >>>
> >>> John
> >>>
> >>> On 6/11/2021 4:26 PM, John OBrien wrote:
> >>>> Hi Guys and Gals
> >>>>
> >>>> I am trying to calibrate my tach.  I installed a Spiyda RVI - RVC
> >>>> conversion board, because my tach was acting wierd. Anyway, I am
> >>>> still trying to get this setup to read correctly.  My question is,
> >>>> on my BJ8 with standard 3.9:1 diff. what should my RPMs be in
> >>>> fourth gear going 50 MPH , without overdrive engaged.  I'm running
> >>>> 165/80R15 tires. I'm just looking for a reference point to compare
> >>>> what should be, and what I am reading.
> >>>>
> >>>> Any help is appreciated.
> >>>>
> >>>> Thanks
> >>>>
> >>>> John
> >>>>
> >>
>
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