Total Bolt In Astro T5 OD Trans (250 I6) All GM Off the Shelf Parts-1965 C10 SBSS

[Old Bleu]

Todd, is there a chance that the tach is set for a 6 cylinder engine while you are now running a V8? Is there a switch on the tach to select between six and eight cylinders? Or when it was calibrated, could it have been for a 6 cylinder.

Sharps and Lugnutz are way beyond me when it comes to knowledge of these trucks, but that is what came to my mind when reading your recent posts.

[Todd W. White]

OK - let's go with the idea that there is no tach, since the one in there now is probably out of calibration and I don't have time just now to check it.

That being said -

* I have P235/75R15 tires, which, if the internet chart I looked at is correct, are 29" in diameter.
* I know the rear end is a 3.08, because I installed it myself.
* I am quite sure the clutch, which is one by Sachs, is good. It's only 1-1/2 years old, and it doesn't slip, as far as I can tell.
* The transmission is an SM420, which was rebuilt before it went in.
* The engine is a 283 bored to 0.060"-over, and runs well.
* My speedometer is within 2 mph, based on my GPS.

Based on these factors, then - what "should" the numbers be with regard to actual RPM's, and, therefore, would the T5 - or anything - be a benefit that is worth the time and money?

Oh - and I talked to Williamson's Instruments, and mentioned that I have a more modern tester that can measure the RPM's using an induction clamp and has a large meter on the front that is calibrated for that, and he recommended measuring the RPM's with that, which I will do and let ya'll know...

- Todd

[Lugnutz65]

Originally Posted by Todd W. White

OK - let's go with the idea that there is no tach, since the one in there now is probably out of calibration and I don't have time just now to check it.

That being said -

* I have P235/75R15 tires, which, if the internet chart I looked at is correct, are 29" in diameter.
* I know the rear end is a 3.08, because I installed it myself.
* I am quite sure the clutch, which is one by Sachs, is good. It's only 1-1/2 years old, and it doesn't slip, as far as I can tell.
* The transmission is an SM420, which was rebuilt before it went in.
* The engine is a 283 bored to 0.060"-over, and runs well.
* My speedometer is within 2 mph, based on my GPS.

Based on these factors, then - what "should" the numbers be with regard to actual RPM's, and, therefore, would the T5 - or anything - be a benefit that is worth the time and money?

Oh - and I talked to Williamson's Instruments, and mentioned that I have a more modern tester that can measure the RPM's using an induction clamp and has a large meter on the front that is calibrated for that, and he recommended measuring the RPM's with that, which I will do and let ya'll know...

- Todd

Todd,
I like how you are very thorough and you know the facts about your truck.

I firmly believe that your engine is doing approximately 2320 plus or minus 50 RPMs at 65 MPH. Since your final gear is 1:1, the drive shaft will turn at the same speed as the engine crankshaft. Everything behind the transmission never changes. The math doesn't change. I realize that you know this, but it's worth saying for the other readers who might be thinking about this for the first time.

If you want to reduce the RPMs with an OD gear so that you save on fuel costs, then your only viable choice is a 0.86 OD.

A 0.86 OD gear will reduce your RPMs a little. You really don't want to reduce them drastically or your engine will lug. I am not familiar with the torque curve for a 283ci, but I bet another forum member named Kieth can tell us.

With the 0.86 OD gear, your RPMs will change as follows.

at 60 MPH current = 2141
with 0.86 will be 1841 RPMs

at 65 MPH current = 2320
with 0.86 will be 1995 RPMs

at 70 MPH current = 2498
with 0.86 will be 2148 RPMs

In my opinion, the 0.86 OD gear will only be useful at 65 MPH and above.

I honestly think that the gain in MPG will be negligible and that the expense cannot be easily justified. But then again, these trucks are for fun. It should be what you want. That's the bottom line.

[Todd W. White]

Thanks Lugnutz. I appreciate the advice.

What if I were to change the rear end to a different gear ratio? What are my options, and what would that do for me?

Incidentally, I filled up today - after in-town driving with low winds except for today, which amounted to around 60 miles of heavy winds, my gas mileage was 17.40, and, from the replies I've been seeing, that's pretty good, I guess. If I changed the rear end, how much could I realistically improve on that, and, in your opinion, would it be worth it?

I'd be interested to hear more about the torque curve on my engine.

- Todd

[Sharps40]

Changing the rear gear so you can change the tranny to an overdrive is tradin delicious red apples for delicious red apples.

Ya seem to have a V8 that pulls satisfactorily with the 308 rear gear (fine choice for V8 cruise/mileage gear too!) and gets great mileage.

Going to say a 352 or 373 rear gear so you can put in a tranny with .76 OD you would note that it will pull even more satisfactorily and maybe get as good on mileage.

But that second bushel of apples is gonna be dayumed spensive for limited performance gain....but perhaps the satisfaction gain is what counts here?

As far as torgue ... most normal every day use motors seem to run bestest 2000 to 2500ish rpm.....a range you are evidently already smack dab on spot with. Going to OD with the 308 pushes you below 2000 rpm into an area where your engine is making nominal HP and too low on torque curve. You will be lugging around all the time and listening to the engine and truck rattle unless you keep it wound up.

Overall, No I don't think you can "Drastically Improve" on 18 to 21 MPG in a 50 year old truck that weighs in at 3500ish lbs wet and is shaped like a the blunt end of a giant turd. You have already achieved a goal of mpg many of us quest for....specially wif a small displacement 8

[brokenspoke]

Originally Posted by Sharps40

Changing the rear gear so you can change the tranny to an overdrive is tradin delicious red apples for delicious red apples.

Ya seem to have a V8 that pulls satisfactorily with the 308 rear gear (fine choice for V8 cruise/mileage gear too!) and gets great mileage.

Going to say a 352 or 373 rear gear so you can put in a tranny with .76 OD you would note that it will pull even more satisfactorily and maybe get as good on mileage.

But that second bushel of apples is gonna be dayumed spensive for limited performance gain....but perhaps the satisfaction gain is what counts here?

As far as torgue ... most normal every day use motors seem to run bestest 2000 to 2500ish rpm.....a range you are evidently already smack dab on spot with. Going to OD with the 308 pushes you below 2000 rpm into an area where your engine is making nominal HP and too low on torque curve. You will be lugging around all the time and listening to the engine and truck rattle unless you keep it wound up.

Overall, No I don't think you can "Drastically Improve" on 18 to 21 MPG in a 50 year old truck that weighs in at 3500ish lbs wet and is shaped like a the blunt end of a giant turd. You have already achieved a goal of mpg many of us quest for....specially wif a small displacement 8

^^^^I couldnt say it better^^^^^

[Todd W. White]

Thanks, Sharps40 - that was what I was needing to hear from someone who knows.

I'll just learn to be content with what I have, then, though I am wondering if Mr. Langdon's suggestion to get a "true" HEI for the truck, rather than the one from Skip White Performance that I have with the separate MSD coil might be a possible upgrade that could help some on mileage.

[Sharps40]

Lots of complaints here on the skip whites. I have Langdons stock gm hei on my 6. Ranks among the top $100 investments I've made on any vehicle. Set it, forget it. Smoother, more powerful and near instant starts hot or cold....may help with mileage but you are on the high end of potential, I suspect the Tru HEI will be an upgrade that increases reliability more than mileage.

Make durn sure you put in an internally regulated alternator first. The old external regulation alternators produce HEI frying voltage spikes that will run great till yer 100 miles south of East BomFizzle Arkansas on Sunday at 9 PM and no parts stores for 23eleven miles an sleepin in the bed of the truck for lack of cellular signal. $50 for a modern 10SI or 12SI alternator will make yer GM HEI the reliable and stable ignition source its intended to be.

Ya can always set up yer driveline anyway ya want. But before ya dump $2000ish in a tranny/rear to replace a good running set up....drive the truck 6 months and make a plan that you can live with and stick to it. It'll keep ya happier with the truck, the truck will be more reliable and you are less likely to be up on cragis list in three years with an "I intended too but can't so here it is for sale disassembled" kind of ad.

[Todd W. White]

Great advice, Sharps40 -

I haven't had any problems with the one from Skip White yet. Had it for over a year.

I'm hoping I can make a true GM HEI fit my truck - that's the big issue I have now.

As for the alternator, I have a 12SI 3-wire. Works great.

In addition to replacing all of the original wiring with American AutoWire's (almost) Perfect Fit harnesses, I upgraded it using Mark's New System from M.A.D. Electrical (http://madelectrical.com) . . .

[Sharps40]

Sounds like you and Mr. Truck need to go on Honeymoon for several months and discuss/write down all the dreams, the budget, the skill sets and then dovetail together a plan that balances dreams and reality.

Go drive, a lot. It'll tell ya what ya want and need to do!

Luck!

[tkeduk8r]

This has been a fantastic post.... spend my lunch break the past few days reading it all... I, like many of you, have been wanting to do something like this for a long time.... I dont have alot to add to the discussion, but Ill throw this out for the crew looking to gain MPG. I have been involved in building supermileage cars as part of a high school build competition and one of the rules of thumb is that your engine is running at peak efficiency at the lowest RPM that your torque flattens out. So if your camshaft profile & displacment start your torque curve from 1800-3500 RPM you want your cruising RPM at that 1700-2000 RPM range.... you just need to be close as each engine behaves slightly different. Figure in the coefficient of drag of your truck (very bad) and tire width (wider is worse) and that will net you your MPG or GPM .... a 283, built correctly, will be a great dd and tuned right, you should see 16-20 MPG for sure....
--Mike

[Todd W. White]

Thanks, Mike.

I believe you and the others are correct - apart from a completely different power train, I'm doing the best I possibly can.

- Todd

[Sharps40]

This ol thread is getting more of a work out than I thought. Very interesting discussions and salent points, from youngins and ol pharts alike.