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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