At 12:51 AM 10/9/98 -0700, Skye Poier wrote:
>.... not really sure the tach or the speedo are correct on my [1966] B.
For kicks, I calculated ....
>
>My tires are 185/70R14s .... rear axle ratio (11/43), .... ie:
75.712 in 3000 rev 11 1 mi 60 min
--------- * -------- * -- * -------- * ------ = ~ 55 m.p.h
1 rev 1 min 43 63360 in 1 hour
>....
>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?
Yes, as long as you're using actual engine speed and NOT reading the
tachometer. I find it much easier to use 60 mph for reference, as 1 mile
then is exactly 1 minute, and engine speed comes out directly in rev's per
minute. Adjust your speed on the highway until your time between mile
markers is exactly 1 minute per mile, and that's 60 mph. Then when you
work your formula backwards you get:
63360 in 1 rev 43
-------- * --------- * -- = 3271 rpm (at 60 mph, direct drive)
1 mi 75.712 in 11
>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?
Yes, 1060 turns per mile, and probably the wrong speedometer for your
application.
>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).
Get a 1280 speedometer and you'll be correct within 1% on miles. Your 1060
speedo likely also reads 20% too high an speed as well, so when it says 60
you're likely only going about 50.
>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!
Yup, the tach is reading way too high (fairly common for that electric
tach), almost double, unless you have some ungodly high final drive ratio
in the rear end, like 7.8:1. I don't know the exact ratio for your OD, but
as I recall, the engine should run about 2400 rpm at 60 mph in overdrive.
If you remove the back from the tach you will find a small potentiometer
inside that is used to adjust the output reading on the face. If you have
enough hands and eyes you could set it to read 3270 while cruising at 60
mph on the freeway in direct drive (4th gear, no OD). Otherwise you could
set it by using some known good tachometer for a speed reference.
Happy motoring,
Barney Gaylord
1958 MGA with an attitude (and tach & speedo in precise alignment)
|