In a message dated 97-05-28 19:24:45 EDT, streeter@sanders.com (Ken Streeter)
writes:
> Philip E. Barnes wrote:
> > Since the speedo in my car doesn't work, or rather doesn't
> > indicate speed in MPH, I need some help from the list.
> > I figured out the speed vs. RPM relationship based on the gear
> > ratios and tire size for my non-O/D car and come up with 64 MPH
> > in 4th gear at 3000 RPM. Does this sound about right?
>
> Yes. A quick rule of thumb for a TR6, and probably as
> accurate as a Smiths speedo, is to double the number shown
> on the tach when in 4th, convert to the right range, and
> read it as MPH. Thus, 2500RPM -> 50MPH, 3000RPM -> 60MPH,
> etc.
Ken and Phil:
For a TR6, as a rule-of-thumb, that is close enough. A universal formula (for
our Spitfire and GT6 friends) for figuring MPH/1000 RPM is:
MPH/1000RPM: (tire diameter X 2.975)/axle ratio
Assuming a 215R65/15 tire (what I have on my TR6), and a rear axle ratio of
3.7 -
(26 X 2.975)/3.70 = 20.90 MPH
Tire diameter: (2 X tire size X aspect ratio)/25.4 + wheel diameter:
(2 X 215 X 0.65)/25.4 + 15 =26"
To arrive at the values for other than 4th gear, just divide by the selected
gear ratio. For example, in 3rd, it would be
20.94/1.33 = 15.75 MPH/1000RPM
With 0.82 overdrive, it would be
20.94/0.82 = 25.54 MPH/1000RPM
The 2.975 is what you get when you factor in all the conversion factors for
inches to feet to miles, and minutes to hours, and factor in the value of Pi.
This formula is as exact as the data you feed it. Using this equation and the
stock TR6 tire data produces a value of 20.84. The owners manual states
20.74. Fairly close? At 3000 RPM, road speed should be 62.52 (or 62.22) MPH.
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
www.sky.net/~boballen/mg/Masters/
'74 MGBGT---3000mile/year driver, original condition
'68 MGBGT---organ donor for the '74
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