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Swapping rear axle gear ratios

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losmorob

losmorob

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I have a '67 1275 stock Sprite which should be a 4.2 to 1 rear axle ratio. I have an opportunity of picking up a '78 axle assembly which should be a 3.7 to 1 ratio. If I swap the pumpkins, it should help with highway cruising speeds. But will it be too much acceleration lost? Anybody do this swap before?
 
From what I hear you should be OK with the 1275. I picked up a 3.9 for my '68, however, I haven't had it on the road since swapping it in so I can't really comment on how much it affected acceleration.
 
I use a 3.7 rear gear and it really helps with lowering
rpms at highway speeds. My 1380cc BE is modified but it still accelerates reasonably well. Living out in the country I
like the reduced rpm at cruising speed. If I lived in a city and didn't see highway speeds I'd stick with the 4.2 however.
 
I have a 3.55 and can still take off well with 175/70 x13s on it. 3rd gear is where 4th was with my 3.9 then I have 4th for normal driving and 5th for the highway. It is much improved on cockpit noise with the top up. About 2700 rpms at 60.
 
Well I pulled my pumpkin and suprise, it's a 3.90. I wonder if it's worth the 125 bucks to put in the 3.7. I live in the country and everything is a highway ride to get there.

So at 60mph, how much less rpm would I experience?
 
Not a lot.
 
losmorob said:
Well I pulled my pumpkin and suprise, it's a 3.90. I wonder if it's worth the 125 bucks to put in the 3.7. I live in the country and everything is a highway ride to get there.

So at 60mph, how much less rpm would I experience?
Click to expand...

$125 for a 3.7 rear gear is a good price. I think you could make some $ on an e-bay sale. Maybe not, but I've seen them sell for quite a bit more just recently. Of course you could get stuck with a gear set ya can't sell and then you have a spare...or....
Maybe somebody here could take the 3.7 off your hands for the $125. :wink:
As for your gains vs a 3.9...I'd agree, not much.
 
If your tach reads 3500 at 60 MpH with a 3.9, then putting in a 3.7 would lower the rpms to about 3325.
 
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