> Randall, can you explain a little more. When I got my TR6
> with no OD at the time, the speedo did not work and I used
> 21.2 MPH per 1000 rpm at 1:1. For 3800 rpm, that would give
> 80.6 MPH with stock tire diameter.
My TR6 handbook gives engine speed @ 10 mph as 482 rpm in 4th direct. So,
at 3800 rpm, it would be doing (3800 / 482) * 10 = 78.8 mph.
> For the 3 at
> 90.5 mph, 90.5/3.8=23.82 mph per 1000 rpm in OD.
But the TR3 had smaller tires originally than the TR6 did.
> With a
> .78:1 OD that would be 18.6 mph per 1000 rpm at 1:1. Is
> this correct for the 3, or am I off base?
The standard OD ratio on a TR3 is .82, not .78. Since the British always do
things backwards, they talk about the reciprocal of the gear ratio; which is
1.22 (hence 22%). (.78 ODs were found on some British cars, but not, AFAIK,
on any Triumph.)
> What do you mean by 42 revs per mile?
Sorry, my mistake : wrong units. The TR3 handbook gives engine speed @ 10
mph as 420 rpm in 4th OD with radial tires. (The bias-ply Dunlops were
taller, 410 rpm @ 10 mph.) So, with original radials, 3800 rpm in 4th OD
would be (3800 / 420) * 10 = 90.5 mph.
Or, if you prefer, 23.8 mph per 1000 rpm.
Of course, very few of us are running the original Michelin X radials
(although they can be had, for a price). So to get serious about
calculating speed from engine rpm, you need to know the effective diameter
of _your_ tires. (Which varies significantly from what you get from
multiplying out the tire size numbers; and even depends on what rim width
you use.) Most tire makers publish a nominal effective diameter (although
frequently it's expressed in "turns per mile").
Randall
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