700R4 swap - gearing for mpg?

[OldGray]

I have a 305 w/TH350 trans.  I'm not sure but I've been told I most likely have 3.07 rear end gears.

Getting ready to do a swap for a 700R4 to improve highway driving and mpgs.

With my 29.1" tires, the info I got from the RPM calculator is below.

The guy doing the swap for me says I want to be around 2000rpm at 70mph.

With 3.55's, 70mph puts me at 2946rpm / 2008rpm w/OD.

With 3.73's 70mph puts me at 3096rpm / 2110rpm w/OD. (1960rpm at 65mph w/OD)

Based on what my mechanic said, it looks like 3.73's would work well...right?

If not, opinions please.

Thanks

[MrFiveOh]

I have a 305 w/TH350 trans.  I'm not sure but I've been told I most likely have 3.07 rear end gears.

Getting ready to do a swap for a 700R4 to improve highway driving and mpgs.

With my 29.1" tires, the info I got from the RPM calculator is below.

The guy doing the swap for me says I want to be around 2000rpm at 70mph.

With 3.55's, 70mph puts me at 2946rpm / 2008rpm w/OD.

With 3.73's 70mph puts me at 3096rpm / 2110rpm w/OD. (1960rpm at 65mph w/OD)

Based on what my mechanic said, it looks like 3.73's would work well...right?

If not, opinions please.

Thanks

Either one of those will work great. I recently swapped from a 350/350 combo to a 350/700r4 with 3.73 gears. Im pushing 70 around 2k and 80 around 2300rpm i believe. Well worth the swap.

[OldGray]

Thanks.  Did your MPGs improve?  I currently average around 11-12mpg hwy.   

I rarely drive with a heavy foot.  So 3.55 or 3.73 better?

[Rattler12]

I have an NV 4500 5spd manual in my 83 with 3:73 gears so the OD ratio is pretty much the same as the OD on the 700R4.  I get 10 mpg with a pretty healthy 454.......you should get much better than that.....although 305's are not known for their fuel efficiency......or their power. Prolly be better off with a 350/700R4

[OldGray]

It is basically a truck that I use when I need to haul something, or tow a small open trailer (which is rarely).  I would definitely drive it more if the mileage was better so that is what this is about.

The 305 was recently rebuilt before I bought it, so (unless someone gives me a 350) I probably will live with it.  But who knows... maybe... someday...

[MrFiveOh]

Thanks.  Did your MPGs improve?  I currently average around 11-12mpg hwy.   

I rarely drive with a heavy foot.  So 3.55 or 3.73 better?

Either will work comparably. When you look up how to calibrate your speedometer (according to hear ratio) they suggest the same drive and driven gear.)

I'm waiting to get my correct driven gear and gear housing that goes on the trans itself. I know I'm getting better MPG just not sure cause my Speedo is running 10mph faster than I actually am.

[Irish_Alley]

i would go for the higher ratio. this was you have the tq factor to help you get up to speed and then when cruising youll have OD to give you a much lower ratio

[OldGray]

Yeah, after mulling it over for a while I am now leaning towards 3.42

[Irish_Alley]

i would go 3.73 or 4.10, you dont want your rpms to be too low while cruising or you might not even be able to use OD. but im not 100% on all this. i know my 94 with a 4.3 and 4l60e is all the time kicking in and out of OD while going 70. i really cant keep it at 70 or it will just lug the engine and my speed will drop, so i have to be going around 73 to be able to keep the speed up

[OldGray]

With the 3.42s and my tires:

1935rpm @ 70mph / 1797rpm @ 65mph

Is that not high enough?

[Irish_Alley]

But what about cruising at 55? You'll be 1500 at 50 you'll be at 1400. Guess it depends on where you'll be driving. I really want to see what bd, vile and hatzie have to say to tell you the truth

[MrFiveOh]

With the 3.42s and my tires:

1935rpm @ 70mph / 1797rpm @ 65mph

Is that not high enough?

Since my swap ive driven about 800  miles. I have 3.73 gears with a 28inch tire 1600 stall. 1st shifts into 2nd at 15-18MPH around 2k rpm, into 3rd at 30MPH around 2k rmp, into OD at 50-54 around 2k rpm. I cruise anywhere from 65-80 (Texas Highways) at 80mph im around 2200rpm.

[OldGray]

I'd love to hear as many opinions as I can get!

Again, my goal is MPG.  I don't tow a lot, and when I do it might be a motorcycle on a 5x8 open trailer, so no heavy stuff really.  I am not looking for hot-rod performance, but a bit more off the line (than I have) would be great.  Here are the numbers (IN OD) again for comparison:

3.42's - 70mph = 1935rpm / 65mph = 1797rpm

3.55's, - 70mph = 2008rpm / 65mph = 1865rpm

3.73's  -70mph = 2110rpm / 65mph = 1960rpm

[Irish_Alley]

if your goal is mpgs 65 mph isnt a good speed to judge that on.

[Tazman]

I will have to look at my IPO codes to see what gearing I have. I do have the 700R that came with the truck and the stock 350.I don't drive it hard and have always had good luck with the mileage.