[Healeys] Speed to RPM ratio?

[Bob Spidell]

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
>>