In a message dated 97-07-29 03:07:50 EDT, jw@mail09.mitre.org (Joe Whalley)
writes:
> I have my Spitfire on the road after the PO kept it in the grass for 3
years.
> I think it runs amazingly well given its age and my expertise... It
> indicates
> 3500RPM at 55 MPH. Is this about right?
Joe:
Use the formulas below to figure the correct speed for a given rpm (all
example calculations assume a 195/70 R14 tire on a car with a 3.91 rear axle
ratio):
Tire diameter: (2 X tire size X aspect ratio)/25.4 + wheel diameter:
(2 X 195 X 0.7)/25.4 + 14 = 24.7"
MPH/1000RPM: (tire diameter X 2.975)/axle ratio
(24.7 X 2.975)/3.91 = 18.8 MPH
RPM @ 65MPH (65000 X axle ratio)/(2.975 X tire diameter)
(65000 X 3.91)/(2.975 X 24.7) = 3452
To determine speed at a different RPM, multiply by the appropriate factor,
ie, for 2500 RPM, multiply the results above by 2.5.
To find the RPM at a different speed, substitute the correct factor in the
formula above, ie, for 45 MPH, use 45000, instead of 65000.
If you want to calculate for an overdrive unit, multiply the axle ratio by
the OD ratio, and use that value in place of the axle ratio in the formulas.
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
http://www.sky.net/~boballen/mg/Masters/
'74 MGBGT---3000mile/year driver, original condition
'68 MGBGT---organ donor for the '74
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