Optimal Setup

[OlSmokey]

Hi there guys, I'm pretty new to the concept of gearing and stuff like that. I have a 1972 C10 with a 350 and a TH350. I'm not sure what my differential is geared at. Ultimately my goal is to have the truck be a daily driver and I'm trying to plan out my build. My question is, is the 3 speed transmission good enough for both in town driving, as well as long road trips (which I plan to do) and if so, what would be an optimal gearing setup for the diff? Also another piece of info is that I will not be towing anything heavy, and if so, I have a very small brown trailer I have access to that I may tow every now and again. But not for long distances.

I'm also not opposed in this stage to have something to aim for in terms of transmission upgrades. Maybe something along the lines of a 5 speed or 6 speed auto transmission if that's even a possibility.

Thank you!

[Mike C]

Usually 350/350 trucks came with 3.07 gears. With 31" tall rear tires it would probably be decent for longer trips.

My 3.07 geared truck with 29" tall tires is pretty comfortable at 65 mph, but not 75 IMO.

The single thing that makes late model vehicles more driveable is the overdrive transmission. I would be looking at going to 3.42 or 3.73 gears with TH700R4 for maximum value. 3.73 is a common ratio in 6 cylinder trucks so you may find a complete used rearend fairly cheap. And budget TH700R4 can be done for $1250 I think if you shop around.

Swapping a 427 into my truck and will be running 3.73 and TH200 4R because I like the gear ratios better. But it's not going to be $1250....

[Alex V.]

Firstly, these trucks were designed for *all* purposes when they were new. Some combo's aren't as ideal as others, but they didn't get to be the icon they are by being mediocre in stock form.

For a truck that isn't going to pull a whole lot, sitting on a close to stock size tire for a C10, 3.07's are good. On my Suburban with 30.5" tires that ratio is on the fast side if you want any oompf down low or much towing prowess, but with 3.07's and a 29" tire your 60 MPH RPM is only about 2,200. That puts 75 MPH up to about 2,700 which is right in a stock SBC's power curve. Many people think that speed sounds like the engine's about to come apart but it's the nature of the beast and is where these engines were designed to work best. My '82 Mercedes 5 cyl. turbodiesel turns about 2,700 @ 60 and those engines last half a million miles - most people have just gotten used to engines of more recent times that have the tech to develop a lot of low-end power so they can push vehicles along at freeway speeds at <2,000 RPM while still being able to pull hard past 4,000 RPM when the driver asks for it.

I'm used to my 1/2 ton Suburban (3.73's on 30.5 tires, 4-speed manual) and 1-ton GMC (TH400 with 4.10's and 32's), both of which turn right around 2,600-2,700 at 60, so your truck would probably feel pretty sluggish to me, but then again I don't usually sustain more than 65 MPH in mine while yours would feel a lot better up over 70.

[RodnRudy]

My 68 SWB C10 with fairly stock 350 engine, mild cam, Eddy 1406 and mid rise manifold, 350 Turbo trans and 3.08 gear gets average of 14 MPG. Rear tire is 29" tall and speedo is off about 3 MPH. I run her down the interstate 70 with no problem.

[OlSmokey]

Oh okay very cool. Thank you guys for the responses!

[jocko]

I'd tinker around and figure out what you have first. There are threads on here as to how to take a wag at it and guess (that are pretty accurate) that don't require removing the rear cover and counting teeth.

Since you mention you aren't opposed to a future trans upgrade, then I'd simply figure out what you have now and then save up for an o/d trans upgrade to a 2004R or 700R4. Many many folks on this forum have done that, and it's worth the effort. It is important to figure out your rear ratio beforehand - if you have a 3.07 now and add an o/d trans, you may find that your rpm is too low at mild highway speeds. 3.07 is a great highway gear, and if you're not aching to do burnouts, you might be just as well served to keep exactly what you have now and in good mechanical condition (I'd recommend doing that anyway). An o/d trans is really best suited to a tighter range between suitable rear end gearing options - my 2 cents would be between 3.42 and 4.10. Go lower numerically and you start getting lousy o/d trans performance/too low rpm in the 55-60 mph range - go higher numerically and you'll be chirping the tires at every stop light even if you don't really want to (because, in addition to the 4th gear overdrive, o/d transmissions generally have a deeper first gear than their non o/d counterparts). 3.73 is the absolute sweet spot for an o/d trans (for "most" tire diameters, different story if you have tiny tires or big mudders) and this is why GM used 3.73 in so many car/truck configs that came with o/d transmissions.

If you don't trust the counting driveshaft rotations vs tire rotations method to figure out your current ratio - if you tell us your tire diameter (measured, use a level across the top of the tire), and actual speed (on a gps, not your speedometer), and your rpm at that speed (if you have a tach) we can tell you your gear ratio. Without that info, you'll have to go old school and either count teeth or rotate the d/s and count revs under the truck.

EDIT: Btw, as Mike mentioned above, your truck would have come standard with a 3.07 ratio (assuming it was built with a 350 originally). If it was born with something optional for the rear ratio, it would be annotated on the SPID. Also note that if the 350 is not on the SPID (and your VIN has an E code rather than an S code), then it was born with a 307 engine and would have had a 3.73 rear end. If you have confidence in your truck's lineage/originality (and most of us don't, ha), then you could assume 3.07 of nothing else is on the SPID. Also, if it seems like it isn't exactly a drag racer and it isn't rev'ing real high at higher highway speeds, it's probably a 3.07.