Engine vs Trans vs gearing

[Ultrasummer]

I wanted to pick your brains and see what you suggest. I have a 1968 short bed C10 stepside. I've had it for about 6 months now cleaning it up and fixing the minor stuff. It seems pretty much all stock. It has a 307 engine with a 4 speed on the floor. Runs great, starts right up. First gear is a granny gear and getting up to highway speed, I have no tach but it feels like about 5000 rpm. I don't want to do some crazy LS swap or lower it. I just want a cool looking old truck that is fun to run around in on the weekends. It's not my primary vehicle. It would be nice to be able to go down the highway at 60 mph with lower rpm's. Plus, I could do without the granny gear. Just kicking around ideas for now. Just change out the gears in the rear end? Crate 350? If I did either of these, would I have to address the trans as well? Oh and from what I could tell by the internet, rear end gears are 3.73. thoughts and suggestions are welcomed.

[Ultrasummer]

Pics after getting new tires from Coker

[notsolo]

Engine is not the problem IMO..Rear. axle ratio sounds like it's closer to 456....Nothing on the spid?..change the gear oil..pull the cover and check...I think the 12 bolt 373 rear axles were something like 42-11 ring and pinion.

[Ultrasummer]

Spid

[jimijam00]

Block the front wheels so that they cannot move.
Jack the rear of the truck up so that the rear tires are off the ground.
Put the vehicle in neutral (but not running).
Mark the driveshaft somewhere you can see it from a safe area.
Rotate the rear wheel towards the front of the truck one full rotation while watching how many times the driveshaft turns.


2 ¾ rotations = 2.73 gears
Just past 3 rotations = 3.08 gears
3 ¼ rotations = 3.27 gears
3 ½ rotations = 3.55 gears
3 ¾ rotations = 3.73 gears
Just under 4 rotations = 3.90 gears
Just past 4 rotations = 4.10 gears

[Ultrasummer]

Interesting. I'll give that a try. Thank you

[Ultrasummer]

Ok I just did it. Jacked rear of vehicle up. Put it in neutral. Marked the drive shaft. Turned the rear tires counterclockwise one full revolution and counted how many revolutions the drive shaft turned. The answer is ....2. Not what I was expecting.

[RustyBucket]

http://www.how-to-build-hotrods.com/gear-ratio.html

Here's a pretty good description of how to determine rear gear ratio by turning the wheel on an open rear end.

Good lookin' truck..... I'm partial to short step/sides.

[SWEET7T]

Originally Posted by Ultrasummer

The answer is ....2. Not what I was expecting.

Make sure BOTH tires made that full revolution. Or, jack up just one rear tire and spin it 2 revs and count the drive shaft revolutions.

I have a 3.08 rear. Turn about 2700rpm at 70mph.

[Ultrasummer]

two complete revolutions of the rear tire while up on a jack = 3 3/4 revolutions of the drive shaft. And by Rusty Bucket's link, in a truck without posi, that looks like 3.73. Which is what I read online that my model and year had. So now that we've established that - and going back to the original post ......any thoughts?

[BigBird05]

Your transmission has a final drive ratio of one to one. The only way to lower your RPM's is to replace the rear end gear or put in an over drive transmission.

[Mike C]

Or taller tires. Taller tires can make a dramatic difference. But the reality, is the 3.73 are quite a bit of gear for that 307 to run on the road. My 69 short step with its 307, TH350 and 29" tires on a 3.07 rear gear is happy on the road at 60-65 but 70+ too much.

For now, a set of 3.07 gears would help a bunch with driving on the road. Another 50 lb/ft with a 350 would be nice as well. A cam swap in the 307 might help as well by extending the upper rpm range 500-800 rpm higher.

Just no cheap and easy solutions.

[Ultrasummer]

Something to consider. Thanks for all the helpful info. One last question though - When people talk of the TH350 and TH400 which came from the factory, is that only for automatics? I ask because LMC has a page describing the two and mine doesn't look like either of those.

[HO455]

Yes the T350 and the T400 are automatic transmissions. If you transmission is still stock it most likely is a SM465.
The simple way to get the RPM,s to drop is to put taller tires on the truck. The next way would be to go to a different rear end ratio. Most likely you have 3:70 gears going to a lower number will help.
The factory "Highway" gear ratio was 3:07 but you could go with 3:55's or 3:23's. You may have to change your ring gear carrier inside the axle depending on how low numerically you decide to go.
There are dozens of gear ratio, rpm, tire diameter calculators on the interweb to help with your decision.

I do like your nice green truck!

[jocko]

Since you still have the original 307, I'm gonna go out on a limb and guess your rear gears have never been changed, but you never know till you crack it open and count. Your truck would have come from the factory with a 3.73 rear end, which shouldn't feel like 5000 rpm on the highway unless you're doing 90. So maybe it has been changed out.

Engine has nothing at all to do with highway rpm, only trans final drive ratio, rear gear ratio, and tire diameter - all of which have been mentioned.

All factory transmissions that were available in these trucks, regardless whether automatic or manual, have a 1:1 final drive ratio.

Since you just bought new tires (and they look great, btw) I'm assuming you don't want to change them to something taller. Tire size, unless you go with LARGE diameter tires, as in a lifted 4x4, aren't going to make a "lot" of difference in your highway rpm - they'll only reduce it slightly if you go a little taller (and your speedo would then be reading slower than you were actually going, ticket magnet )

That leaves the trans and the rear. 3.73 is a good all around gear - especially when coupled to an overdrive transmission. So, personally, here's what I'd do:
1) determine actual rear end ratio - crack it open and count, that's the only way to be sure.
2) if you confirm it's a 3.73, and you plan to keep the 307, and you aren't a burnout fanatic, I'd swap in a T5 trans. Search on here for the T5 swap threads, there are several. It will drop your highway rpm significantly and also increase your driveability.

I assume you don't use the granny 1st much, and generally start off in 2nd? If so, you'll like the increase in apparent acceleration in 1st compared to granny 2nd starts. If you do use 1st gear on your current trans, well, you won't miss it and you'll like the T5 better. It is a fairly deep 1st gear also, and coupled with a 3.73, you will be shifting to 2nd by the time you hit mid-intersection.

IF you determine that someone has swapped in a 4.10 gear, I'd still do the same thing.
IF you determine that someone has swapped in a 4.56 gear, well, I'd probably swap the rear end in this case and leave the trans alone (or do both if you can afford it). I'd swap to a 3.73 or lower numerical ratio (higher gearing) if your goal is highway driving. If you are going to swap rear and trans for an o/d trans, I'd always opt for a 3.73 - it is truly the sweet spot for any o/d trans - best of both worlds.

Based on your described goal (lower highway rpm and you could do without the granny) and the likelihood that you still have the original 3.73 in it, a manual overdrive trans is your best bet to achieve your goals. Leave the engine and rear alone. An engine change will not affect highway rpm no matter what.

Recommend compare your speedometer reading against a GPS speed indication. If they are close - that's another indication you have 3.73 gears. Back in the day, if someone swapped a rear gear, they USUALLY didn't go to the trouble of also swapping out the speedo gearing so that the speedo would read correctly. If your speedo is notably different than the GPS reading, then I'd suspect a rear gear swap - in which case you'd definitely want to crack it open and count teeth. Might want to do this anyway, change the fluids and get free peace of mind on what your actual gearing situation is before you plunk down $$ on a transmission.

One more note - if you DO intend to swap the engine out at some point to a higher hp/torque engine, the T5 might not be the best bet, unless you know you will drive it sanely, no hairy burnouts, etc. I had a 283/T5/3.73 combo and it was a lot of fun to drive.

[RustyBucket]

For less than $20 you can buy a cheapy little tachometer. Don't even have to mount it unless you want to.

[Ultrasummer]

These are the opinions and suggestions I was hoping for. The beauty of these old trucks is they are easy to work on and easy to understand. Makes for a great hobby. This site is great to hear different ideas on how to bring these old trucks back to life. I appreciate your wisdom.

[truckster]

Here's a handy tool for tire size comparison, if you want to see what changing the tire size will do:

https://tiresize.com/comparison/

[Ultrasummer]

Ok for all those that keep talking about changing tire size. NOT GOING TO HAPPEN. I just spent $1400 on 5 new tires from Coker's to get the look I wanted.

[truckster]

Originally Posted by Ultrasummer

Ok for all those that keep talking about changing tire size. NOT GOING TO HAPPEN. I just spent $1400 on 5 new tires from Coker's to get the look I wanted.

It was just a suggested possibility. No need to take umbrage.

[typerr]

I'm in the middle of doing an upgrade from my original Saginaw 3-speed (which had already been converted to floor shift) to a freshly rebuilt T5 WC 5-speed that once lived in a Camaro. I'm using an S10 tailhousing & mainshaft to work with my bench seat. I've also got 3.73 rear gears and the .63 overdrive gear will go a long way to reducing revs on the highway. If you aren't afraid to take things apart and put them back together this upgrade can be done for not much cash.

[weim55]

Having done many of these various mix and match vehicle-engine-trans-gear swaps here's what I would do given the facts you put on the table. Keep it simple and have the rear gear swapped to a 3.07 by a shop that specializes in such. I've done the 3.73 to 3.07 change more than a couple times. In a V8 application such as yours that 307 will have plenty of power to cruise that gear on the highway. With the RPMs lower @ 65mph the truck will be MUCH quieter at easier to live with. And your all original truck stays that way, simple and reliable......

Steve weim55 Colorado

[garyd1961]

Originally Posted by Ultrasummer

Ok I just did it. Jacked rear of vehicle up. Put it in neutral. Marked the drive shaft. Turned the rear tires counterclockwise one full revolution and counted how many revolutions the drive shaft turned. The answer is ....2. Not what I was expecting.

Do 2 revolutions of the wheel and then divide the turns on the driveshaft by 2. Gives you a more accurate reading.
Nevermond, lol.

[Mike C]

Leaf spring or coil spring truck? Shouldn't be too hard to find a set of 3.07 gears in a coil rear. Maybe a little harder leaf, but they are out there. I would just keep my eye out for a complete rear axle assembly and swap it in.

[Ultrasummer]

Originally Posted by weim55

Having done many of these various mix and match vehicle-engine-trans-gear swaps here's what I would do given the facts you put on the table. Keep it simple and have the rear gear swapped to a 3.07 by a shop that specializes in such. I've done the 3.73 to 3.07 change more than a couple times. In a V8 application such as yours that 307 will have plenty of power to cruise that gear on the highway. With the RPMs lower @ 65mph the truck will be MUCH quieter at easier to live with. And your all original truck stays that way, simple and reliable......

Steve weim55 Colorado

I'm liking this keep it simple philosophy. Sounds like a good starting point