Shade tree gear change

[Obwonkonobe]

I put in an overdrive trans, and then proceeded to get 10 mpg on the highway. I’m sure the motor isn’t tuned perfect but it runs too good to get 10mpg. It’s a 350 with an eddy carb and some misc bolt ons, a 700r4, and a stock truck 12 bolt with open 3.07 gears and 31 inch tires. I’d like to put 3.73s in to be a good middle of the road gear, should get me around on the street when I daily it, and take my on trips every month or so without screaming at 3 grand.

My quiestion is about the carrier, I’ve heard that 3.07 has the 3 series carrier, but I’ve heard that 3.73 is both 3 and 4 series from different sources. And that I need a “thick” gear set to put a 3.73 gear in my 3 series carrier. I’m not too sure what to buy or what to look for, I can’t get a straight answer. Also I’m not too farmilliar with how the pinion gear gets installed, I’ve had the diff apart with the carrier just about out but I’ve never touched the pinion gear, is it something I can replace with my standard toolset over the weekend? Thanks in advance

 

[carfanatic63]

Going to a 3.73 will only worsen your mileage on the freeway as the 3.73 is a lower gear than your current 3.07 so RPM's will go up when driving.

 

[Obwonkonobe]

Going to a 3.73 will only worsen your mileage on the freeway as the 3.73 is a lower gear than your current 3.07 so RPM's will go up when driving.

I know but I drive on the street every day as opposed to the highway once a month, I figure the 3.73 will be a good middle ground

 

[roundhouse]

What was your mpg before the OD trans ?

You install a 700r4?

Diff gear changes are the automotive equivalent of brain surgery

If you have never done it before and don’t have all the right tools
It’s almost guaranteed to not work

3.73 is pretty common
Shouldn’t be hard to find an axle with that ratio and swap the whole axle

That’s a lot easier than trying to swap gear sets

And a good used 10 bolt rear axle is almost free
Usually around $150

But don’t trust the seller is telling the truth about the ratio

Check it for yourself

Pull the cover and count the teeth on the ring and count the teeth on the pinion

And use a super bright light to inspect the gear set for wear and missing teeth

Also check your speedometer with a gps and make sure your mpg is accurate

 

[4WDKC]

Going to a 3.73 will only worsen your mileage on the freeway as the 3.73 is a lower gear than your current 3.07 so RPM's will go up when driving.

not true, the od of the trans and the low gear makes the engine lug with a load on it cause it to need more gas to go slow or downshift to maintain speed. Where as the same combination with 373s will create less load by increase the engines rpm in the power band and make it work less.

 

[4WDKC]

Also if you installed a 700r4 make sure you have the lockup wired up and working along with the tv cable adjusted correctly.

 

[Obwonkonobe]

Also if you installed a 700r4 make sure you have the lockup wired up and working along with the tv cable adjusted correctly.

All that is squared away

 

[Obwonkonobe]

What was your mpg before the OD trans ?

You install a 700r4?

Diff gear changes are the automotive equivalent of brain surgery

If you have never done it before and don’t have all the right tools
It’s almost guaranteed to not work

3.73 is pretty common
Shouldn’t be hard to find an axle with that ratio and swap the whole axle

That’s a lot easier than trying to swap gear sets

And a good used 10 bolt rear axle is almost free
Usually around $150

But don’t trust the seller is telling the truth about the ratio

Check it for yourself

Pull the cover and count the teeth on the ring and count the teeth on the pinion

And use a super bright light to inspect the gear set for wear and missing teeth

Also check your speedometer with a gps and make sure your mpg is accurate

Would I have to find a 12 bolt truck axle? What car axles should I look for? I don’t want to have to reweld spring pads or shock brackets

 

[4WDKC]

Would I have to find a 12 bolt truck axle? What car axles should I look for? I don’t want to have to reweld spring pads or shock brackets

IF you find an axle from one of our trucks you wont have to change anything but a u joint and maybe shorten the drive shaft.

 

[Obwonkonobe]

Why

IF you find an axle from one of our trucks you wont have to change anything but a u joint and maybe shorten the drive shaft.

would I have to shorten a driveshaft?

 

[4WDKC]

Why

would I have to shorten a driveshaft?

possibly, depends what axle you get vs already have and suspension mods.

 

[Obwonkonobe]

possibly, depends what axle you get vs already have and suspension mods.

Like a 14 bolt vs my current 12 bolt or something?

 

[4WDKC]

Like a 14 bolt vs my current 12 bolt or something?

yes but it is also possible you have a 10 bolt and going from the 10-12 bolt may cause an issue or going from 12-10 would need a different drive shaft. I didnt have to change mine on my truck when I went to a 14b but it is lifted so I got away with it.

 

[78C10BigTen]

Sounds like a discussion @Honky Kong jr should be apart of...

 

[Obwonkonobe]

I will add that I have access to a dial indicator, and I’m not afraid to use it!