Should I go to 4:10s?

[scottveazie]

Currently running 4:56 with granny low transmission, 283 with quadrajet. Would going to 4:10 make much difference in mpgs?
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[Captainfab]

Probably not much of a difference in mpg. I am guessing your rpm's will drop 3-400 rpms @60 mph.

[markeb01]

The only improvement I can see is possibly raising your top speed from 45-50 mph to 55-60 mph, depending on rear tire diameter.

[scottveazie]

Well I'm running 16.5 rims all the way around which made a difference over the 15's and top speed is around 70. I would like to bring the rpms down. I've found someone with the 4:10 3rd member, is that hard to change out?
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[kieth]

Originally Posted by scottveazie

Well I'm running 16.5 rims all the way around which made a difference over the 15's and top speed is around 70. I would like to bring the rpms down. I've found someone with the 4:10 3rd member, is that hard to change out?
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go to 3:73's a set of gears is not that hard to find......Kieth

[scottveazie]

I would love to go 3:73 but I'd have to change out the whole axle and since I have coil spring suspension, finding 3:73 eaton axles seems to be pretty tough.

[Rich 5150 69]

Just keep watching CL they pop up pretty often, too bad your not in calif, there are three on there now...all those guys are looking for 3.42 and 3.08, maybe consider buying a gear set and install...

[Captainfab]

I myself have never seen a 3.73 geared 3rd member for a Eaton HO52/72. The tallest I have seen are the 4.10's

Originally Posted by kieth

go to 3:73's a set of gears is not that hard to find......Kieth

Originally Posted by Rich 5150 69

Just keep watching CL they pop up pretty often, too bad your not in calif, there are three on there now...all those guys are looking for 3.42 and 3.08, maybe consider buying a gear set and install...

To change the 3rd member, you will need to remove the bolts holding the axles in, and then slide the axles out. I would remove the rear cover, to let most of the oil out. Then you can remove and replace the 3rd member.

[luvbowties]

Originally Posted by scottveazie

I would love to go 3:73 but I'd have to change out the whole axle and since I have coil spring suspension, finding 3:73 eaton axles seems to be pretty tough.

Just a couple of figures: 4.10 divided by 4.56 gives .8999, or we can say .90. So, going down to 4.10 gear will change things, like rpm's at a certain speed by 10% (1.00-.90=.10). It should raise top speed by about 10%. Running 31" tires rather than 28" tires: changes things as follows: 28/31= .90; therefore, those 3" taller tires should drop rpm's at a given speed by 10%; or raise top speed by 10%. Remember, we are assuming a perfectly linear relationship, which is really a little off in reality--but close.

Cheapest, quickest, & easiest thing to do to drop rpm's would be to run taller tires. Figure it using ratio-and-proportion like above & it'll get you close to what you want.

Should you desire to change tranny to automatic, w/overdrive, a 700R4 will make it cruise in od where the engine feels that the rear end is now: 4.56 X .70(% gear reduction from overdrive) = 3.19:1. And would give one heckuva 1st-gear launch off the line! Would that be the best of both worlds? This changes rpm's at given speed by 30%; increases top speed by 30%; raises mpg by 30%--and if all this theory is correct, it would be great! Nonetheless, the rpm-drop should be as calculated, where mpg & top speed may be overrated.

[scottveazie]

Thanks so much for all the information, it really helps. Maybe a 10% gain isnt worth the trouble, I don't know really. Since this truck is all original I've tried to keep almost everything that way but an automatic would be nice. If there was a way to do it without majorly changing appearance or to make it look stock, I'd certainly consider it. I guess my theory with the 4:10 is then I wouldn't have to change much, although the guy also has a 3:30 complete axle too for sale.
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