Mike Chaffee writes >
> (Keep in mind that it's a 3-main engine and the final drive translates
> to about 3700-4000 RPM at 65 MPH.
This sounds pretty close to right, see table below. I don't know how hard it
is on the car, but it drives the occupants nuts after a bit.
Paul Gilders writes +
+ My '66 Midget seems to indicate rpm of nearly 5000 at 65mph.
Should be closer to 4000.
My dear departed '66 Midget (yes I am still missing it) had a wildly optimistic
tachometer and a speedometer that ran pretty close. My readings were similar
to Paul's. I was afraid I had a 4.56/1 final drive out of a Morris Minor or
something. I counted the revolutions of the driveshaft with one wheel raised
and confirmed that it had the 4.22/1 standard gears in it. Later cars
(beginning about 1970? help me out here, people) had a ratio of 3.9/1 in an
attempt to improve cruising. I am an autocross freak, so I liked the deeper
gears, but freeway driving is really noisy and boring.
Here is a table I made up showing true RPM at true 60 MPH for both Midget
ratios. The formulae are all taken from BF Goodrich figures. I have a set of
instructions for you to create a spreadsheet that will show wheel and tire
dimensions and speed for any tire size, transmission and final drive
combination according to the BFG formulae. I can Email them on request or post
if there is interest.
Tire size 4.22/1 3.9/1
185-80-13 3450.8 3189.2
175-80-13 3541.3 3272.8
165-80-13 3636.7 3360.9
185-70-13 3667.5 3389.4
155-80-13 3737.3 3453.9
175-70-13 3756.8 3471.9
145-80-13 3843.7 3552.2 stock size for factory calibration
185-60-13 3913.3 3616.5
Fourth gear on a Spridget (A-block cars) is straight through, so for highway
cruising the only gearing factor you need is final drive.
First, jack up one rear wheel on the Midget and count the bolts on the
driveshaft going past as you turn the wheel. I'll let you do the math. This
method should tell you whether you have a 4.22/1 or 3.9/1 ratio. (If you feel
like it, you can measure your tire's circumference to make your own
calculations. But I warn you that BFG figures show about a 3% "loss" for tire
flat spot. I am not going to debate this theory, as we have been all through
it before on the lists. I assume that BFG tested for this, and I accept their
formulae.
Now go on a road with good mile markers. Pick a steady speed, note the RPM and
Speedometer and drive this constant speed for several miles and time the mile
markers with a stopwatch.
By comparing all the figures you get with the spreadsheet, you can determine
the relative accuracy of the speedometer and tachometer.
Phil Ethier, THE RIGHT LINE, 672 Orleans Street, Saint Paul, MN 55107-2676
h (612) 224-3105 w (612) 298-5324 phile@pwcs.stpaul.gov
It's still hip to be octagonal. What shape is a Lotus emblem?
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