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--></style></head><body lang=EN-US link=blue vlink="#954F72" style='word-wrap:break-word'><div class=WordSection1><p class=MsoNormal>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:</p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Tires – either Revs per Mile or loaded radius (this is different than the nominal diameter and usually available on the tire manufacturers website)</p><p class=MsoNormal>Rear End Ratio – for a Healey there are limited options usually 43/11 = 3.91, 41/10 = 4.10 or 39/11 = 3.55</p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>If you do the check while driving in 4<sup>th</sup> 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:</p><p class=MsoNormal><o:p> </o:p></p><div><table class=MsoNormalTable border=0 cellspacing=0 cellpadding=0 width=204 style='width:153.0pt;border-collapse:collapse'><tr style='height:12.75pt'><td width=91 nowrap valign=bottom style='width:68.0pt;padding:.75pt .75pt 0in .75pt;height:12.75pt'><p class=MsoNormal align=center style='text-align:center'><span style='font-size:10.0pt;font-family:"Arial",sans-serif'>MPH <o:p></o:p></span></p></td><td width=113 nowrap valign=bottom style='width:85.0pt;padding:.75pt .75pt 0in .75pt;height:12.75pt'><p class=MsoNormal align=center style='text-align:center'><span style='font-size:10.0pt;font-family:"Arial",sans-serif'> RPM <o:p></o:p></span></p></td></tr><tr style='height:12.75pt'><td nowrap valign=bottom style='padding:.75pt .75pt 0in .75pt;height:12.75pt'><p class=MsoNormal align=center style='text-align:center'><span style='font-size:10.0pt;font-family:"Arial",sans-serif'> 30 <o:p></o:p></span></p></td><td nowrap valign=bottom style='padding:.75pt .75pt 0in .75pt;height:12.75pt'><p class=MsoNormal align=center style='text-align:center'><span style='font-size:10.0pt;font-family:"Arial",sans-serif'> 1,670 <o:p></o:p></span></p></td></tr><tr style='height:12.75pt'><td nowrap valign=bottom style='padding:.75pt .75pt 0in .75pt;height:12.75pt'><p class=MsoNormal align=center style='text-align:center'><span style='font-size:10.0pt;font-family:"Arial",sans-serif'> 40 <o:p></o:p></span></p></td><td nowrap valign=bottom style='padding:.75pt .75pt 0in .75pt;height:12.75pt'><p class=MsoNormal align=center style='text-align:center'><span style='font-size:10.0pt;font-family:"Arial",sans-serif'> 2,230 <o:p></o:p></span></p></td></tr><tr style='height:12.75pt'><td nowrap valign=bottom style='padding:.75pt .75pt 0in .75pt;height:12.75pt'><p class=MsoNormal align=center style='text-align:center'><span style='font-size:10.0pt;font-family:"Arial",sans-serif'> 50 <o:p></o:p></span></p></td><td nowrap valign=bottom style='padding:.75pt .75pt 0in .75pt;height:12.75pt'><p class=MsoNormal align=center style='text-align:center'><span style='font-size:10.0pt;font-family:"Arial",sans-serif'> 2,790 <o:p></o:p></span></p></td></tr><tr style='height:12.75pt'><td nowrap valign=bottom style='padding:.75pt .75pt 0in .75pt;height:12.75pt'><p class=MsoNormal align=center style='text-align:center'><span style='font-size:10.0pt;font-family:"Arial",sans-serif'> 60 <o:p></o:p></span></p></td><td nowrap valign=bottom style='padding:.75pt .75pt 0in .75pt;height:12.75pt'><p class=MsoNormal align=center style='text-align:center'><span style='font-size:10.0pt;font-family:"Arial",sans-serif'> 3,350 <o:p></o:p></span></p></td></tr><tr style='height:12.75pt'><td nowrap valign=bottom style='padding:.75pt .75pt 0in .75pt;height:12.75pt'><p class=MsoNormal align=center style='text-align:center'><span style='font-size:10.0pt;font-family:"Arial",sans-serif'> 70 <o:p></o:p></span></p></td><td nowrap valign=bottom style='padding:.75pt .75pt 0in .75pt;height:12.75pt'><p class=MsoNormal align=center style='text-align:center'><span style='font-size:10.0pt;font-family:"Arial",sans-serif'> 3,910 <o:p></o:p></span></p></td></tr></table></div><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>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.</p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Harold<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><div style='mso-element:para-border-div;border:none;border-top:solid #E1E1E1 1.0pt;padding:3.0pt 0in 0in 0in'><p class=MsoNormal style='border:none;padding:0in'><b>From: </b><a href="mailto:banjojohn@cox.net">banjojohn</a><br><b>Sent: </b>Sunday, June 13, 2021 3:51 PM<br><b>To: </b><a href="mailto:ryan@jimryan.com">Jim Ryan</a>; <a href="mailto:eyera3000@gmail.com">i erbs</a><br><b>Cc: </b><a href="mailto:healeys@autox.team.net">Ahealey help</a><br><b>Subject: </b>Re: [Healeys] Speed to RPM ratio?</p></div><p class=MsoNormal><o:p> </o:p></p><div><p class=MsoNormal>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. <o:p></o:p></p></div><div><p class=MsoNormal>Regardless of my actual speed, my speedo and tach should maintain a direct relationship in any given gear.<o:p></o:p></p></div><div><p class=MsoNormal>Thanks again for all the opinions and various charts and facts provided by the list members.<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>John<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div id="composer_signature"><div><p class=MsoNormal><span style='font-size:9.5pt;color:#575757'>Sent from my T-Mobile 4G LTE Device<o:p></o:p></span></p></div></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal><span style='color:black'>-------- Original message --------<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='color:black'>From: Jim Ryan <ryan@jimryan.com> <o:p></o:p></span></p></div><div><p class=MsoNormal><span style='color:black'>Date: 6/13/21 4:17 PM (GMT-06:00) <o:p></o:p></span></p></div><div><p class=MsoNormal><span style='color:black'>To: i erbs <eyera3000@gmail.com> <o:p></o:p></span></p></div><div><p class=MsoNormal><span style='color:black'>Cc: banjojohn <banjojohn@cox.net>, Ahealey help <healeys@autox.team.net> <o:p></o:p></span></p></div><div><p class=MsoNormal><span style='color:black'>Subject: Re: [Healeys] Speed to RPM ratio? <o:p></o:p></span></p></div><div><p class=MsoNormal><span style='color:black'><o:p> </o:p></span></p></div><div><p class=MsoNormal>Am I missing something? I just check accuracy with the GPS on my phone.<o:p></o:p></p></div><p class=MsoNormal><o:p> </o:p></p><div><div><p class=MsoNormal>On Sun, Jun 13, 2021, 3:25 PM i erbs via Healeys <<a href="mailto:healeys@autox.team.net">healeys@autox.team.net</a>> wrote:<o:p></o:p></p></div><blockquote style='border:none;border-left:solid #CCCCCC 1.0pt;padding:0in 0in 0in 6.0pt;margin-left:4.8pt;margin-right:0in'><div><p class=MsoNormal>My tach is driven by my distributor gear<o:p></o:p></p><div><p class=MsoNormal style='margin-bottom:12.0pt'>Speedo comes off my transmission. Not sure about the relationship to accuracy. <o:p></o:p></p><div><p class=MsoNormal>Ira Erbs<br>1959 AH 100-6<br>1967 MGB<br>Milwaukie, OR<o:p></o:p></p></div></div></div><p class=MsoNormal><o:p> </o:p></p><div><div><p class=MsoNormal>On Sun, Jun 13, 2021, 11:50 AM banjojohn <<a href="mailto:banjojohn@cox.net">banjojohn@cox.net</a>> wrote:<o:p></o:p></p></div><blockquote style='border:none;border-left:solid #CCCCCC 1.0pt;padding:0in 0in 0in 6.0pt;margin-left:4.8pt;margin-right:0in'><div><div><p class=MsoNormal>Thanks.<o:p></o:p></p></div><div><p class=MsoNormal>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.<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>John<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div id="m_-8612040742838000386m_-7997059852725315655composer_signature"><div><p class=MsoNormal><span style='font-size:9.5pt;color:#575757'>Sent from my T-Mobile 4G LTE Device<o:p></o:p></span></p></div></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal><span style='color:black'>-------- Original message --------<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='color:black'>From: Bob Spidell via Healeys <<a href="mailto:healeys@autox.team.net">healeys@autox.team.net</a>> <o:p></o:p></span></p></div><div><p class=MsoNormal><span style='color:black'>Date: 6/12/21 12:25 PM (GMT-06:00) <o:p></o:p></span></p></div><div><p class=MsoNormal><span style='color:black'>To: Bob Haskell <<a href="mailto:rchaskell@earthlink.net">rchaskell@earthlink.net</a>>, <a href="mailto:healeys@autox.team.net">healeys@autox.team.net</a> <o:p></o:p></span></p></div><div><p class=MsoNormal><span style='color:black'>Subject: Re: [Healeys] Speed to RPM ratio? <o:p></o:p></span></p></div><div><p class=MsoNormal><span style='color:black'><o:p> </o:p></span></p></div><p class=MsoNormal style='margin-bottom:12.0pt'>That's it!<br><br>On 6/11/2021 4:31 PM, Bob Haskell wrote:<br>> <a href="http://longstonetyres.co.uk">http://longstonetyres.co.uk</a><br>><br>> Cheers,<br>><br>> Bob Haskell<br>> Austin Healey 3000 BN7/BT7 registrar<br>><br>> On 6/11/21 7:01 PM, Bob Spidell wrote:<br>>> Note there is a not insignificant difference in diameters of tires <br>>> with the same nominal size (e.g. '165/80-15'). The English tire <br>>> site--can't think of the name offhand--has (had?) a table comparing <br>>> the diameters of different brands/models of the same size tires, and <br>>> IIRC they could differ by as much as half an inch. I'd say to measure <br>>> the actual diameter of your tires, but it's kinda hard when moving; <br>>> the 'flat spot' goes away--partially, at least--and the tires 'grow' <br>>> some from centrifugal force (anyone seen film of top fuel dragsters <br>>> launching; the rear tires must expand a few inches or more?).<br>>><br>>><br>>> On 6/11/2021 3:36 PM, John OBrien wrote:<br>>>> Thanks to all who replied. These will give me something to work with.<br>>>><br>>>> John<br>>>><br>>>> On 6/11/2021 4:26 PM, John OBrien wrote:<br>>>>> Hi Guys and Gals<br>>>>><br>>>>> I am trying to calibrate my tach. I installed a Spiyda RVI - RVC <br>>>>> conversion board, because my tach was acting wierd. Anyway, I am <br>>>>> still trying to get this setup to read correctly. My question is, <br>>>>> on my BJ8 with standard 3.9:1 diff. what should my RPMs be in <br>>>>> fourth gear going 50 MPH , without overdrive engaged. I'm running <br>>>>> 165/80R15 tires. I'm just looking for a reference point to compare <br>>>>> what should be, and what I am reading.<br>>>>><br>>>>> Any help is appreciated.<br>>>>><br>>>>> Thanks<br>>>>><br>>>>> John<br>>>>><br>>><br><br>_______________________________________________<br>Support Team.Net <a href="http://www.team.net/donate.html">http://www.team.net/donate.html</a><br>Suggested annual donation $12.75<br><br>Archive: <a href="http://www.team.net/pipermail/healeys">http://www.team.net/pipermail/healeys</a> <a href="http://autox.team.net/archive">http://autox.team.net/archive</a><br><br><a href="mailto:Healeys@autox.team.net">Healeys@autox.team.net</a><br><a href="http://autox.team.net/mailman/listinfo/healeys">http://autox.team.net/mailman/listinfo/healeys</a><br><br>Unsubscribe/Manage: <a href="http://autox.team.net/mailman/options/healeys/banjojohn@cox.net">http://autox.team.net/mailman/options/healeys/banjojohn@cox.net</a><o:p></o:p></p></div><p class=MsoNormal style='margin-bottom:12.0pt'>_______________________________________________<br>Support Team.Net <a href="http://www.team.net/donate.html">http://www.team.net/donate.html</a><br>Suggested annual donation $12.75<br><br>Archive: <a href="http://www.team.net/pipermail/healeys">http://www.team.net/pipermail/healeys</a> <a href="http://autox.team.net/archive">http://autox.team.net/archive</a><br><br><a href="mailto:Healeys@autox.team.net">Healeys@autox.team.net</a><br><a href="http://autox.team.net/mailman/listinfo/healeys">http://autox.team.net/mailman/listinfo/healeys</a><br><br>Unsubscribe/Manage: <a href="http://autox.team.net/mailman/options/healeys/eyera3000@gmail.com">http://autox.team.net/mailman/options/healeys/eyera3000@gmail.com</a><o:p></o:p></p></blockquote></div></blockquote></div><p class=MsoNormal style='mso-margin-top-alt:0in;margin-right:0in;margin-bottom:12.0pt;margin-left:4.8pt'>_______________________________________________<br>Support Team.Net <a href="http://www.team.net/donate.html">http://www.team.net/donate.html</a><br>Suggested annual donation $12.75<br><br>Archive: <a href="http://www.team.net/pipermail/healeys">http://www.team.net/pipermail/healeys</a> <a href="http://autox.team.net/archive">http://autox.team.net/archive</a><br><br><a href="mailto:Healeys@autox.team.net">Healeys@autox.team.net</a><br><a href="http://autox.team.net/mailman/listinfo/healeys">http://autox.team.net/mailman/listinfo/healeys</a><br><br>Unsubscribe/Manage: <a href="http://autox.team.net/mailman/options/healeys/ryan@jimryan.com">http://autox.team.net/mailman/options/healeys/ryan@jimryan.com</a><o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p></div></body></html>