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Has Anyone Run a 3.73:1 Rear End Ratio in Their 122s?

A

AmazonMan

New member
Joined
Jul 17, 2017
Location
Oregon
Hi, I'm rebuilding a 122s wagon to be my daily commuter and I've been thinking about how to get the most fuel economy out of it. I'm planning on the obvious like an m41. I've also heard the ipd torque cam has some mpg gains. Recently I was looking at rebuilding the rear differential. I found that there are many rear end ratio options for the m30 rear end. I'm interested in the 3.73:1 that Vp auto parts and Classic Volvo Restoration sell. Mainly for the mpg gain at highway speeds. I'm interested to see if anyone has run this ratio and what the drivability is like. Thanks
 
How much HP, and how fast are you wanting to go on the highway?

You might end up with a taller gear than a stock B18 can comfortably push at 70+ mph.
 
Considering what actual Oregon freeway speeds are [10mph less than Cali] I'd say the M41 would be plenty of gearing even with the stock ratios.

Unless you have a really well built up B20 then maybe 410's or 391s tops.
 
Thanks for the replies. I have a rebuilt b18 that ill probably pull out of my 122 sedan and put in the wagon. Id like to be able to do 70 relaxed. What do you think the best rpm is for milage is? max torque is around 3500, so maybe around 3000? Based on my math with the 3.73 you should be able to do 75 mph with a j-type and 79 with a d-type at 3000 rpm.
 
I'm running a ford T5 - 3.15 first, 8.8 with 3.73's and a 17" rim. First gear is like starting the m41/4.56 wagon in 2nd, so hills are less fun... the .73 5th leaves me at 2800 at 75 mph.
 
It doesnt help that I'm running an FX400 clutchmastere clutch. Theres not much slipping the clutch...its either break the tires, or stall it, not much in between. Then again its really easy for me to just barely break the tires, so Ive gotten used to it, except when it barks them into 4th, or when it just smokescreens in 3rd when a little damp ;-)
 
When I run my slicks, 15x8, 225/50/15 it’s much more manageable to get it moving. My daily tires are 245/45/17, and I’ll be going up to a 275/45/17 when I can.
 
AmazonMan said:
Thanks for the replies. I have a rebuilt b18 that ill probably pull out of my 122 sedan and put in the wagon. Id like to be able to do 70 relaxed. What do you think the best rpm is for milage is? max torque is around 3500, so maybe around 3000? Based on my math with the 3.73 you should be able to do 75 mph with a j-type and 79 with a d-type at 3000 rpm.
Click to expand...

Efficiency is more speed related, so I'll take that as a trick question and say at or under 60 mph where the cars hit a wall of air.

Strictly speaking of engine rpms I'd guess below 2500 there would be diminishing returns, but the two factors are inextricably intertwined so like all design criteria it's a tradeoff.
 
Well, I'll try out the 3.73 with my m40 and see how it goes (I still need to find an m41) I could always get the stock ratio put back in if it really doesn't work out. I'll post back here when I try it out, might be a while though.
 
Being an economy car Volvo usually got the gearing right, at least up until 1983 when they geared the stick shift cars too high with 331s to meet CO2 emissions numbers at 55mph.
 
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