Easier swap: tranny or rear end gears?
 

I've read a lot on the internet about people changing their gear ratios or swapping their 3 speed transmissions for overdrive transmissions in the attempt to lower their rpm's on the highway and to get better mileage. Right now, I'm running about 2300-2400 rpm at 55 mph which kills me at the gas pump. I don't do any off-roading and am not looking to put a high performance package together. I just want to be able to drive a little more to work without having to fill up that 31 gallon tank so much.

So which is the easier and cheaper way to go? A transmission swap or changing the gear ratio in the front and rear axles? Any input would be greatly appreciated.

I would go with the 4 speed transmission, it solves the problem rather than moving it(sluggish acceleration vs poor highway mileage) what vehicle is it? year?

That depends on what it is you are working on and the availability of axle(s) with the ratio you need. If you have a 3-spd transmission (auto or manual), then you are working on something relatively old. There are pros & cons to both. Used axles & transmissions for older vehicles are often plentiful, but you'll likely need to do some freshening up on either to make them last. An overdrive automatic transmission would require a more significant investment up front to bring up to proper performance standards and get swapped over than a gear swap in the rear end would in most cases.

My feeling and the direction I really want to take on my '74 is that I would rather swap in the overdrive automatic and get the benefit of retained performance with lower cruising RPM. But I am changing the gears to offset changes in tire diameter. I have a set of 3.73's for my D44/14BFF combo as well as 4.10's on hand that will go nicely with up to 35's in my truck (currently has 33" tires) and I would like to make room for the eventuality of 38's, but I'll need a D60 front by then and would be going to even lower (numerically higher) gearing for the axles when that eventually comes.

Oops- I forgot to add that its a 79' with 3.40 gears. What would be the steps to take and what other parts would I need if I went with an OD transmission, say a 700R?

2wd or 4wd? Manual or Auto currently? Is this a pickup or K5?

If automatic & 2wd, then going to an OD automatic like the 700R4 will be fairly easy. You would need to either get a conversion crossmember or move yours back & redrill the holes as the 700R4 sits back 2.12" further than the TH350 at the mounting location. The overall length of the 9" tailshaft TH350 is right on with the 700R4 (except the 700R4 that came in the Corvette), but the 6" tailshaft TH350 would be 3" shorter so depending on what you have for a transmission in your truck now, you may need to have your driveshaft shortened to fit. You can find more dimensions on TCI's Transmission Dimension Page.


If you are 4wd, then you will have to swap transfer cases to one compatible with the 700R4 and get the appropriate driveshafts to mate up along with all of the appropriate brackets which may be tricky depending on what you are driving.

It's a completely stock 1979 4wd automatic K5.

should be able to find a mid 80s blazer or suburban as a doner for a 700r4 and transfer case, maybe even driveshafts and shift linkage so its a bolt in setup.