CAUTION: I am going for the most-boring-post award with this one.
The Spridget came with three different rear end ratios.
1:4.22 until 1967 I think
1:3.9 from 1967 to 77 or 78 I think (I should look this up)
1:3.7 from 77/78 to the end of production
I have been thinking about swapping out my rear end (no jokes here!) for a
lower ratio one (lower rpm on the highway) but was concerned about
off-the-line response around town. Here are my conclusions.
My car is one of the last to have the 4.22 ratio which made for high rpm
on the highway. I installed the 5-speed conversion which has a 5th gear
ratio of 1:0.821 (stock 4th is 1:1). So the speed that I used to achieve
at 5000rpm (whatever speed that may be), I now achieve at 4100rpm. If I
swap in a 3.9 rear end, I will achieve the speed at 3800rpm. A 3.7 would
yield 3600rpm. Sounds great!
But making this change will also be noticed in a "longer" 1st gear, making
take-off from a stop more sluggish. Fortunately, the 1st gear in the
Datsun trans is "shorter" than the one in the ribcase, partially
offsetting this effect:
1st gear ratio:
Ribcase 3.2:1
Datsun 3.513:1
So I calculate an effective rear end ratio. Using a 3.7 rearend with a
Datsun trans will have the same overall 1st gear ratio as using a rearend
ratio of 4.06 with a ribcase box. Since 4.06 is between 4.22 and 3.9, the
two ratios used with the 1275 cars, I shouldn't have any problem
starting up on steep Seattle hills!
By now, Larry Miller is probably the only one still reading (he wondered
about this too), but for completeness some more facts.
Using a 3.9/Datsun combination yield an effective ratio of 4.28, almost
exactly what I started out with (4.22).
To calculate how much a rearend change would help you (even our ribcased
friends), take the rpm you usually run on the interstate, devide by your
current ratio, then multiply by the 'new' ratio.
You could also calculate how much faster you could go at the same rpm by
taking your current highway speed, times your current ratio, devided by
your new ratio - however, you probably don't know how fast you are going
anyway because your speedo needle is fluttering back and forth, it doesn't
move at all, or it is inaccurate.
One more note of caution, the 3.7 was only used in the 1500 Midgets which
have more torque and a 'shorter' 1st gear of 3.41. So an earlier Spridget
with a ribcase box might not like this 'long' rearend.
OK, so who has a 3.7 for sale (cheap?)...
Ulix __/__,__
.......................................................... (_o____o_)....
'67 Sprite
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