Skye,
Your method is sound, with one caveat. When checking your RPM vs MPH
make sure your in 4th, with no O/D, or recompute your table taking in to
account a final drive ratio of either 3.135 or 3.205 depending on which
overdrive you have.
I think your 1060 is the turns per mile for the speedo. Somewhere
around the early '70's I believe, someone out there will pipe in with the
correct time I'm sure, the speedo drive ratio from the O/D transmissions
changed. I believe the later one used around 1000 turns/mile and the
earlier ones still used the 1280. However it was (books not available at
the moment) from the numbers you measured for your speedo vs mile markers
test would indicate a mismatch between the speedo ratio and the O/D speedo
drive ratio.
As for the tachometer, I'd verify it against a tach/dwell meter if you
have access to one. Don't forget that most of them require doubling the
tach/dwell reading (8 cylinder scale) for a four cylinder ( at least the
inexpensive one's that I can afford do....).
Harlan.
-----Original Message-----
From: Skye Poier <skye@ffwd.bc.ca>
To: MG Nuts <mgs@autox.team.net>
Date: Friday, October 09, 1998 1:59 AM
Subject: Speed, RPM, fun facts
>Until I get a timing light with a tach, I'm not really sure the tach or the
>speedo are correct on my B. For kicks, I calculated how fast you should
>be going at certain r.p.m.'s...
>
>My tires are 185/70R14s which the online tire calculater
><http://www.mgb.bc.ca/misc/tirecalc.html> gives a diameter of 24.1"
>and using C=2*pi*r = 75.712" circumference. Multiplied by the desired
>r.p.m. and by the rear axle ratio (11/43), then convert inches to miles and
>minutes to hours. ie:
>
>75.712 in 3000 rev 11 1 mi 60 min
>--------- * -------- * -- * -------- * ------ = ~ 55 m.p.h
> 1 rev 1 min 43 63360 in 1 hour
>
>I came up with the following table (rounded to the nearest number):
>
>rpm mph
>---- ---
>2000 37
>3000 55
>4000 73
>5000 92
>
>This is for 4th gear where the trans ratio is 1:1
>Or, simply multiply r.p.m. by about 0.0184 to get m.p.h.
>
>Is my method sound?
>
>Here's some more interesting information. My speedo has the following
>written on it: Jaeger SN 6125/04 1060
>
>Is 1060 the turns/per/whatever? I can only find references to 1280 and
1240
>speedo's in my material. I compared my odometer to mile markers on the I5
>crusing in O/D and found my odometer to be exactly 20% fast (reports 12 mi
>when I have only gone 10 etc).
>
>If the above table is correct, my tach is way off. I timed myself with
>a watch to see what r.p.m. would get me 1 mile in 60 seconds (60 mph) and
>the tach reported around 4700 r.p.m. again in O/D so either my calculations
>are wrong or I have been cruising at a much lower r.p.m. than I thought!
>Which is just as well, because with my tired suspension and not quite
>balanced tires I wouldn't want to go any faster for now. I can dream about
>blowing the doors off SUVs though, can't I? ;)
>
>Math is fun.
>
>Skye
>
>--
>1966 MGB GHN3L-70307 __,__\__
>The MGB Experience http://www.mgb.bc.ca/ (_o____o_)
>
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