What's a good tranny?

[eventhorizon66]

My factory gearing was 2.73 with a 700R4.  My opinion was that this actually hurt gas mileage.  The reason being the engine was bogged down on all but perfectly level highway and if you encountered any type of steep grade a downshift to third (sometimes even second) was needed to maintain speed (forget accelerating).  Somewhere along the road, I got this hair-brained idea that 3.08s would help with this while still maintaining low rpms for economy.  Wrong, that was a colossal waste of money.  Now with 3.73s I can accelerate with ease in fourth at highway speeds.  My mpg has improved (over the 3.08s) at speeds of 55-65 and remained about the same at 65+.  I'm sure there is a trade off point at about 85 mph where the 3.08s would be better for mpg, but I don't cruise that fast, and frankly if you are worried about mpg you should be cruising no more than 60 anyways.

In conclusion, 2.73s are a great economy gear for TH350s, but not for 700R4s.  I would run no less than 3.42s behind a 700R4.

Top speed should be around 300mph.

On the other hand, just think of how much time could be saved traveling around at 300 mph.

[Lt.Del]

so true.  you have to take advantage of the preferred rpm the engine wants to go.  With a high gear 2.73, the engine rpms are just too low for any power or fuel efficiency (if you have to push the accelerator all the time to maintain speed).

[Skunksmash]

So yall would recommend that i change the gear ratio to 3:42's? What RPM should that put me at, if i am traveling at 70mph? It is nice to be able to use the fast lane on the highway. And i do some highway traveling sometimes. Seems strange that your 3:08's didn't work out the way you wanted. And to be honest, the truck burns out too easy as it is lol. (spins tires on takeoff)

[eventhorizon66]

(3.42 x 70 x .70 x 336) / 28 = 2011 rpm

[SUX2BU99]

^ Is that formula assuming 28" tall tires?

Skunk, I'm in the same boat as you right now. I'm trying to determine if I should swap my perfectly fine TH350 for a 700R4. It seems it's entirely dependent on the trans builder and installer as to whether or not the 700R4 will hold up. I haven't determined yet if the person that I found selling a 'built' 700R4 I can trust or not as building a strong trans. I should get some references from him. Anyway, I have a 3.40 12 bolt in my truck and 27" tall tires. At 70 MPH, I'm turning 3000 RPM and I'm not entirely comfortable with that for longevity reasons. Using Event's formula and 27" tall tires, I should get 2073 RPM. My TH350 doesn't have a lock up converter though and that adds 2-300 or so RPM too.

Ideally, I'd prefer to have a 700R4 I don't have to worry about and a 3.73 rear end and a functioning limited slip. That's another $1000 away though.

[Lt.Del]

And to be honest, the truck burns out too easy as it is lol. (spins tires on takeoff)

if your truck burns out easily with 2.73 rear, man, with a lower gear, you better have a few sets of tires on standby.

So yall would recommend that i change the gear ratio to 3:42's? What RPM should that put me at, if i am traveling at 70mph? 

depends if you have overdrive or not.  She'll be tacking quite a bit more but you'll be able to pass others easily just by practically thinking about it.  That's a better rear for towing--you'll have power to actually go over hills without flooring the accelerator.  If you do a lot of city driving, 2.72 gears just are too tall, you have to keep your accelerator down just to get the truck going.  I think you mpg will increase with something like a 3.42 or 3.73

[eventhorizon66]

^ Is that formula assuming 28" tall tires?

Yes.

[rear gear] x [speed in mph] x [trans gear] x [constant] / [tire diameter in inches] = [RPM]

*Assumes manual trans or locked up converter.

[DnStClr]

Just to add a lil more to talk about- what about the addition of an aftermarket overdrive unit, like GearVendors or similar?  What're the pros and cons for Skunkmash ( other than costing about $1300).

[Skunksmash]

Whoa, i did not even know those existed. They are like $2950 lol. I guess they sort of add another gear, in a way?

[eventhorizon66]

Pros: Gear splitting for drag strip performance, OD gear, durability, keep your current trans.

Cons: $$$, no TC lockup, keep your current trans.

Unless you feel you could benefit from gear splitting, I'd just build a 700R4 or 200R4.

[Stewart G Griffin]

More on the mpg thing:

My personal belief is---and we ARE going to split hairs here but so be it, is i think a higher diff ratio with direct drive trans (1:1) will get better mpg than an overdrive transmission with a lower diff gear.  i feel this way because it takes more energy to drive and OD trans vs. direct drive.

i run a TH-350C and 2.56 gears and get 15.8 for a low and 17.20 for a high, so let's just say average 15.8mpg just to rate it conservatively.  This is with scientific testing.  And this is with 350 cubes.

You've got 2.73's, so you're looking good in that department.


2) The 700 is simply a more versatile transmission;  i can't argue against that.  If you want more responsiveness/acceleration/better performance, this is your trans.

3)The energy consumption difference between the two may be negligible, but i still feel there is a difference in economy.  Possibly a small one, but a difference nevertheless.

[eventhorizon66]

On the highway, all other things equal, a TH350C with 2.56's will get about the same mpg as a 700R4 with 3.73's.  The major differences come in when you throw around town mileage into the mix.  A 700R4 has 11.41:1 total gearing in 1st and 6.08 in 2nd with 3.73's, while a TH350 has 6.45:1 in 1st and 3.89:1 in 2nd with 2.56's.  When setup with these respective rear gear ratios, in normal driving, the 700R4 will require much less throttle to accelerate than the TH350, given the same acceleration rate.  Under aggressive driving situations, the 700R4 will accelerate much faster.

[VileZambonie]

Stewart 3rd gear in most transmissions is direct drive.

[Skunksmash]

Maybe i missed something... but is the 4rth gear in a 700R4 direct drive?

[Lt.Del]

overdrive