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3.73 vs 4.10

6K views 19 replies 13 participants last post by  foggybottombob 
#1 ·
I'm looking at replacing my pretty useless 2.72 gears in my Dana 30. I'm not sure if I'm better with 3.73 or 4.10s. I have a I6, T5 and 33" tires. I will be on pavement 99.9% of the time and I live in Fl where it's pretty flat. I want to drive on hwy (70 mph) and have the best MPG I can get.

What do you think and why?
 
#4 ·
I'm looking at replacing my pretty useless 2.72 gears in my Dana 30. I'm not sure if I'm better with 3.73 or 4.10s. I have a I6, T5 and 33" tires. I will be on pavement 99.9% of the time and I live in Fl where it's pretty flat. I want to drive on hwy (70 mph) and have the best MPG I can get.

What do you think and why?
4.10 :2thumbsup:
 
#11 ·
Thanks for the replies this far... I don't understand why my local buddies are telling me 3.73. Can you guys give me some more data?

If I run 4.10 what will my RPMs be at 70MPH in 5th gear??
i have 4:10's with 33's and a T-5. RPM's @ 70 MPH will be 2,350 to 2,400
 
#9 ·
There are some tools you can use to calculate what speed you would travel for a specified rpm, based on your transmission, axle ratio, tire size, etc... I have not used them, but they are here: http://www.offroaders.com/info/tech-corner/reading/calculators.htm. I am sure there are other online tools like this, if you do a web search you are bound to find several.
 
#10 ·
Ok now I'm more confused... According to the calculator, if I run 32's my RPMs run higher then if I run 33" tires. How is that possible??

I'm also running lower RPMs with 3.73 then I would with 4.10 but everyone here is telling me to go with 4.10??

My jeep will be a DD with a lot of flat highway driving.
 
#12 ·
your rpms are higher with smaller tires because they have to rotate more to do the same distance, same with the deeper gears 4.10s will give you higher rpms because your driveshaft will have to spin 4.1 times each time your tires spin once, vs 3.73 times each time your wheel spins once. the reason everyone is saying 4.10 is because it will give you a good balance of power and mpg on 33" tires. its all about finding a good balance of what you want. 3.73s will most likely give you better mpgs on the highway, but 4.10s will give you more power to get to highway speed
 
#13 ·
Thanks everyone for all the good info. So I believe it comes down to a few things... If I go with 4.10 gears I will get a little more power to reach my desired speed a little easier, but in return I'd run higher RPMs, which might cast me more fuel. If I go with 3.73 gears I might be a little sluggish reaching desired hwy speeds but run at lower RPMs which should same on the fuel.

Does this summarizes it correctly?
 
#14 ·
Thanks everyone for all the good info. So I believe it comes down to a few things... If I go with 4.10 gears I will get a little more power to reach my desired speed a little easier, but in return I'd run higher RPMs, which might cast me more fuel. If I go with 3.73 gears I might be a little sluggish reaching desired hwy speeds but run at lower RPMs which should same on the fuel.

Does this summarizes it correctly?
Yep. Depending on where your engine gets its power though 4.10s could potentially give you better MPGs. The engine has a sweet spot that it likes to run at
 
#15 ·
#17 ·
You'd be spinning 2500 rpm @ 70 mph with 4.10 gears. With 3.73 gears it would be 2250 rpm. Not alot of difference there and not drastic enough to affect that sweet spot for cruising. But thats just my $0.02.
 
#18 ·
I'm running 4:10, automatic, 4.2L, and unfortunately, lockers front and rear, I do off road, but don't care for lockers, rather have regular posi.. anyway, 33" tires get me right down the road, not sure of the RPM at any given speed, but.... I just bought a TH700RE that will give me a lower first gear and overdrive last gear..
 
#19 ·
I just installed 4.10's and have 31's. Power is great. no pushing the throttle to go up hills. at 75 at 2800 rpm. I do go off road but 90% is on road.... since I love my CJ so much. So I do like the 4.10s I would recommend them for future upgrades. I havent noticed any real change in fuel economy since i went from a 3.08 to the 4.10's If you got to 33's, it would run perfect
 
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