[Healeys] Speed to RPM ratio?

Roger Grace roggrace at telus.net
Sun Jun 13 19:17:06 MDT 2021 

I usually check by ensuring rear tires are at correct pressure, then making
a mark on pavement at valve and counting say 8-10 turns then measuring
distance carefully with a tape measure. Gives good results when doing the
usual calculations. This takes the mystery out of data that the tire people
publish.
rg

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On Sun, Jun 13, 2021 at 5:29 PM Harold Manifold via Healeys <
healeys at autox.team.net> wrote:

> The Tach is an accurate way to determine vehicle speed and can be more
> accurate than the speedometer but you need to know these variables:
>
>
>
> Tires – either Revs per Mile or loaded radius (this is different than the
> nominal diameter and usually available on the tire manufacturers website)
>
> Rear End Ratio – for a Healey there are limited options usually 43/11 =
> 3.91, 41/10 = 4.10 or 39/11 = 3.55
>
>
>
> If you do the check while driving in 4th without the overdrive engaged
> the math is easier. One complication is the tire manufactures do not report
> Revs per Mile consistently and sometimes a conversion is required. Assuming
>  Vredestein Sprint Classic      165HR15 and 4.10 rear end ratio this table
> will work:
>
>
>
> MPH
>
>  RPM
>
>                 30
>
>                   1,670
>
>                 40
>
>                   2,230
>
>                 50
>
>                   2,790
>
>                 60
>
>                   3,350
>
>                 70
>
>                   3,910
>
>
>
> if it would help anyone send me your tire make and size and rear end ratio
> and I will sent a chart like the above.
>
>
>
> Harold
>
>
>
> *From: *banjojohn <banjojohn at cox.net>
> *Sent: *Sunday, June 13, 2021 3:51 PM
> *To: *Jim Ryan <ryan at jimryan.com>; i erbs <eyera3000 at gmail.com>
> *Cc: *Ahealey help <healeys at autox.team.net>
> *Subject: *Re: [Healeys] Speed to RPM ratio?
>
>
>
> Jim, you are right.  I have used GPS and my speedo is not accurate, but
> close enough for me.  My point was that the speedo is mechanically linked,
> from the tranny, to the engine, and my tach gets its signal from the coil,
> which also is directly linked to engine speed through the distributor.
>
> Regardless of my actual speed, my speedo and tach should maintain a direct
> relationship in any given gear.
>
> Thanks again for all the opinions and various charts and facts provided by
> the list members.
>
>
>
> John
>
>
>
>
>
> Sent from my T-Mobile 4G LTE Device
>
>
>
>
>
> -------- Original message --------
>
> From: Jim Ryan <ryan at jimryan.com>
>
> Date: 6/13/21 4:17 PM (GMT-06:00)
>
> To: i erbs <eyera3000 at gmail.com>
>
> Cc: banjojohn <banjojohn at cox.net>, Ahealey help <healeys at autox.team.net>
>
> Subject: Re: [Healeys] Speed to RPM ratio?
>
>
>
> Am I missing something?  I just check accuracy with the GPS on my phone.
>
>
>
> On Sun, Jun 13, 2021, 3:25 PM i erbs via Healeys <healeys at autox.team.net>
> wrote:
>
> 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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