[Healeys] Speed to RPM ratio?

richard mayor boyracer466 at gmail.com
Wed Jun 16 09:45:18 MDT 2021 

I have an GPS app on my cell phone that tells me what speed I am traveling.
I then look at my speedo and compare.  Easy Peasy.

On Wed, Jun 16, 2021 at 12:03 AM Roger Grace via Healeys <
healeys at autox.team.net> wrote:

> 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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