On Mon, 16 May 1994, Mr. Mike Passaretti x7-4485 wrote:
> Ok, folks, my math-lexia is showing its ugly head again.
> available, and Sonja has the 3.7 rear end. If my numbers are
> correct, on any reasonable tyre, that gives me either around 16
> or 20 miles/Krev. With the 4.1 rear end, the numbers are perhaps
> as high as 18 and 22. My local britcar mechanic seems to think
I don't think so, Mike. If you know the tire rolling radius, you can
figure how far it travels in one rear axle revolution (C=2*Pi*r). If you
know the rear axle ratio and the 4th gear ratio or OD ratio, you can get
at miles/1000 engine revolutions. But your math has to have come adrift
somewhere, because 16 miles/krev is extremely high. Look at it this way,
as you drive down the road at 3000 rpm, you would be going 48
miles/minute. Not likely.
Then too, your numbers go the wrong way. Changing from a 3.7 to 4.1 axle
ratio will decrease the number of miles per 1000 revs.
> that my car would be much happier with the 4.1 rear. At a "normal"
> cruise (let's say 70 mph) that's either 4400rpm/3500rpm or
> 3800/3200. Does the extra 300rpm _really_ matter that much, or is
> it more the 600 in non-od he's likely thinking about? Or are my
> numbers totally screwball??
Does your car have OD? Suppose it has OD and turns 3800 in 4th and 3200
in OD at 70 mph. If you change from a 3.7 to a 4.1 rear axle, the engine
will turn faster at any given speed by a factor of (4.1/3.7)=1.11, thus
after installation of the 4.1 your rpm at 70 mph would be 3800*1.11=4211
in 4th and 3200*1.11=3552 in OD. That would be an extra 411 rpm in 4th or
an extra 352 in OD.
I think that would be noticable. There would be a tradeoff--better
acceleration vs. higher cruising rpm--more noise and wear.
If you are satisfied with your acceleration with the 3.7 rear, the extra
commotion on the highway may not be worthwhile. If the car seems
sluggish now, then the higher numerical ratio may be a good idea. My
general impression is that 3.7 is a lower ratio than I'd expect with OD,
if the OD transmission ratio is less than 1.
Ray (this is easier than worrying about torque wrenches) Gibbons
Ray Gibbons Dept. of Molecular Physiology & Biophysics
Univ. of Vermont College of Medicine, Burlington, VT
gibbons@northpole.med.uvm.edu (802) 656-8910
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