Re: tyre sizes

[Jonathan Zerihan <J.D.C.Zerihan@domain.elided>]

Richard Hudson wrote:
> My Stag is now fitted with standard Alloys again but the tyres are 185
> 14/70's these are as near to the standard tyre size as my monthly
> allowance from my wife would allow. (Free!!!!) the rest of the
> drivetrain is standard but I have noticed that the speedo is out by
> about 10mph (on the optimistic side) i.e. 50 reads as 60mph etc etc.
> 
> Does anybody know roughly how many mph/1000 rpm I am likely to get from
> this configuration. Standard tyres get 19/20mph per 1000. According to
> original press releases

OK, here goes (apologies for maths):

Outer diameter, d = 14 + (2*185*0.7)/25.4 = 24.2 inches
Circumference of tyre = PI * d = 76 inches
Revolutions of tyre per mile = 1760*3*12/circumference = 833

mph/1000rpm = 60*1000/revs per mile
mph/1000rpm = 60*1000/833 = 72 at the wheels

Is the diff 3.7?  (I don't know - I don't even own a Stag.  Why am I
even on this mailing list????!!!)

Assuming it is, then your final mph/1000rpm=72/3.7 = 19.5

From this, I would assume that you have the standard sized wheels and
tyres!! Or, your speedo is out :-)

Note that you will only get this mph/1000rpm at a slight downhill
incline, where there is no torque convertor loss (or gain).

Jonathan