Gas: Transmissions and rear gears.

[teepall]

I have a 350/350 in my '70 Cst.
I can't possibly think of whole new drive train at this time. How much difference would changing from a turbo 350 and 3:73 rear gears to a 700 4R and different rear gears make on my mileage.

Any help or suggestion would be appreciated.

Teepall.

[Boog]

There is no guaranteed mileage improvement by doing the swap. There are a lot of variables such as engine setup (stock, cam'd, intake, heads, etc), rear tire diameter, 2wd or 4wd, towing or not, mostly in town, highway driving and so on. I believe you have a good trans and gear selection for lite towing and general around town driving but it won't get great mileage while trying to run interstate speeds. Just installing an OD tranny will help to some degree with the gears you have on the highway. It may not be the only change needed. Many guys use a T350 with 3.08 gears and report around 15 mpg and some even tow with that setup. You can use one of several online gear/tire calculators and swap ratios and tire diameters to see what effects can be made. Here's one: www.richmondgear/calc.com

[69halfton]

IMO, a higher (I.E 3.73) would get BETTER milage in the city then a 3.08. with 3.08s the motor might "lug" a little more then with the 3.73s.

i know that when i have the trailer on my mothers 02 ford exploder, it gets better gas milage out of overdrive. this is because it lugs less.

just my .02

[teepall]

I plan on pulling about 5000 lbs. I want to be able to tow my Spartan Camper, but at the same time improve the mileage somewhat, when I am not towing. The truck does have a bit of a lope, so I am assuming that the cam grind is a bit more aggressive than stock, possibly an RV setup. It towed a camper before. Boy, you should have seen the gas receipts from Mountainview, Ca to Louisiana . Thanks for the info and all the help,
Teepall.

[my67chevytruck]

i would keep the 3:73's and install a 200r or 700r4

[Longhorn Man]

yep, either swap out to a 3.07 gear or keep the gear you have, and put in an overdrive.
With the gear swap, you'll be able to tow on the freeway at the lower ROM... with the overdrive, it would be a bad idea to haul that much weight in overdrive... so you'd be spinning the same RPMs you are now
A gear swap would be much cheaper than a tranny swap.
Personally, if you aren't planning on doing any drag races in it, the 3.07 will be fine for towing, hauling, cruising, and even taking off from a light with all of the above at once. Too many ppl think it's like taking off in second gear or something. It'll still pull no problem, even with a stock small block, it wouldn't be the 'dog' everyone claims it to be.
BTW, a 3.73 and 700R4 or 2004R would be about the same cruise RPM as a non overdrive and 3.07 gears.

[mnunn454]

Have you considered upgrading to a TH350C with lockup converter?

Compared to your current setup, it should save you about 3-4% when not towing and I'd guess maybe 5% or more when you are. Other than wiring up the converter switch there's no other changes required for the swap.

[Longhorn Man]

a TH350C would only drop the revs down about 200 RPMs... not worth the effort or expense IMO.

[teepall]

Could I keep the rear 3.73 gears, install the 700, tow in 3rd on the highway, and in overdrive when driving without the trailer? I 'd like to drive out of town occasionally with out the trailer.

Teepall

By the way thanks for all the suggestions!

[barraza]

You will probably see an increase in mileage with the 700r4 on the highway, but may not see any increase at all in town. What is sometimes forgotten when talking about mileage, is that the load on the engine is what uses fuel, not just RPM. The only rpm variable is that the frictional loses go up as rpm increases, but they are relatively small compared to the aero and weight being moved. For example, if your truck requires 20 hp to sustain 55 mph, it will require those same 20hp whether the engine is at 1500 rpm or 2500rpm. The only difference is that the frictional loses to spin the engine at 2500rpm are more than 1500rpm. A good part of the increase in mileage in newer vehicles using OD transmissions is due to the accurate fuel metering that is attainable with fuel injection at low rpms, something that is not easy with a carb.

[mrein3]

Originally Posted by teepall

Could I keep the rear 3.73 gears, install the 700, tow in 3rd on the highway, and in overdrive when driving without the trailer? I 'd like to drive out of town occasionally with out the trailer.

Teepall

By the way thanks for all the suggestions!

Yes. Tow in drive (3rd) and save OD for non-towing applications. 3rd in a 700r4 is a 1:1 so you aren't going through any gearing. Just engine R's in and same R's out the back when the torque converter is locked up.

3.73 and a 700r4 is a pretty decent combination. I ginned up an Excel spreadsheet to type in rear end gears and transmissions to give you a speed verses rpm chart.

My chart lists my th350 going 3100 r's at 70mph. However my tach says 3400-3500 r's while actually doing it. The difference is the slip in the torque converter. Using the same rear end gears and a locked up 700r4, those r's drop to 2200. That is more that I was expecting.

To make a long post short, I'd go with a 700r4 and leave your rear end gears alone. Tow in drive and leave overdrive for when you aren't pulling anything.

[crazy longhorn]

You wont find any mileage (intown) with the OD trans.....out on the open road , running empty in OD you will see it. Also, you wont find a mileage increase in town on a 3.07 gear....it lugs the motor too hard. For a best all around gear, I like a mid 3's gear. With no OD, it dont lug as hard in town, but still has plenty of git -go. It also turns the motor a tad slower on the open highway, & has enough power for towing. Now you can add the lock up trans to that, or opt for the 700 R4. The700 R4 would probably be the sweetest deal.....loaded , run 3rd gear....empty, find the "sweet spot" on cruise for mileage in OD(my guess around 72- 78 mph). I like the best of both worlds, still enough grunt to tow/haul in 3rd(but not rev the motor too much).....but still be able to "drop the hammer " a bit empty, & RUN crazyL

[67ChevyRedneck]

I don't see much of an improvement "in town" with my 67. Still gets 12-13 mpg (with the crappy winter gas I've been getting 11-11.5 the past few months) with a light foot and limited OD use (the few times I get above 50 mph). On the highway I can pull down 16 mpg going 65 mph.

My ride is a 350/700R4/3.73 combo.

[OrangeCrush1970]

Teepall, not trying to rob the thread, but Im going through the same delma. The only differences are (1) Im running a recently rebuilt TH350 w/a shift kit, so I dont want to spend the $$ on another tranny, and (2) I have a 4.11 rear end. Im turning 3k at 65mph.

Bottom line, guys, is how much would I benefit in the mpg's department from dropping in a set of 3.55's?

Also, can you convert any th350 to a th350c?

Thanks so much-

[crazy longhorn]

You can drop to a 3.42, and it will help on the hyway mileage......but you will not find a 3.55 in a 12 bolt truck rear. crazyL

[El Jay]

teepall, have you considered a Gear Vendors Under/Overdrive?
It's an 80% OD unit, & stout as hell.
Not only do you have the OD capabilities in "D", you can split your lower gears when towing, especially helpful if you're running up & down hills.
They're probably a bit more expensive now, but the last time I looked, they priced out about $2500, plus the cost of a drive shaft mod. (had to have their U joint installed & maybe shortened)

That's just my .02.