MPG difference between th-350 and 700r4?

[1986Silverado]

Looking for the difference in gas mileage between the two. I have a 700r4 in my truck now but it took a dump on me Saturday and was thinking about a th-350. It will be my everyday driver once I get the tranny back in and I only drive 12 miles round trip to work and back and maybe 50 miles on the weekends. Funds are limited right now is the reason but if the mpg are not much of a difference I may go with the th-350.

[gldevall]

1st off, welcome to the board. 2nd I think it may be a toss up on the mileage considering you are not driving very far. I have a 34 mile round trip commute and with my TH400 and 3.73 gears I have been getting 13-14 MPG. Costing about $40 a week for gas. Hope this helps.

[bpmcgee]

1986,

This is sort of a tough question to answer. At the simplest level, the 700R4 has a .7 Overdrive, compared to the direct (1) 3rd gear in the TH350. That means your engine is only turning over 70% as much at a given road speed. So, you might anticipate a 30% fuel mileage increase.

However, this is dodging the bigger question, that no one really has a firm answer to -- is the fuel useage vs RPM curve of your engine really linear? In other words, if you go from 1000 RPM to 2000 RPMs, does your fuel useage double? Most people agree it doesn't, but I've never seen anyone who was able to give you any formula or data to prove this.

So, to summarize, we don't know how much it will save. 30% seems to be the maximum it COULD save, but more likely to be about 10% to 15%, depending on all sorts of factors like driving habits, traffic patterns, engine components, etc.

I hope I made this clear as mud for you!

Brian

[kevinr1970]

Originally Posted by bpmcgee

..... but more likely to be about 10% to 15%, depending .....

I remember reading something recently where people who bought the new elec/gas hybrids (and some diesels) were complaining because the cars were advertised to get around, say for the sake of arguments, 45mpg but were only getting around 40mpg.
It was roughly a 10% difference.
The writer pointed out that if your car was advertised to get in the range of 18mpg (SUV's) but only got 16.2mpg people wouldn't complain.
Same 10% difference.

My contribution to the thread???
How long will it take for your 700r4 to make up the $ difference in mileage savings vs. how much cheaper it will be to rebuild a T350?

[86k10]

Well I think it depends on your engine and rear axle combination. Higher gearing like 3.08 is probably better suited to a 3 speed trans, but get in the neigborhood of 3.73 and 4.10 upwards than definitely a 4 speed. I just recently bought a 74 C 20 that was advertised on here. It originally had a 4 speed manual, and of course that being unacceptable to me, I figured hmm I have a 700 I had rebuilt about 6 years ago sitting in my basement. So after a quick change from a 4x4 unit to a 2 wheel drive, it's sitting behind a mildly built 350. It has 4.10 axle gearing and gets in the neighborhood of 20 MPG, however driving in direct it gets about 15. So in my instance it will pay for it self quite fast, and if you have a good 700 core that needs nothing but a overhaul kit and no major parts, they are not that expensive to fix and will outlast a 350 2 to 1.

[kevinr1970]

Maybe I need to talk to different people then?!? The T350 in my '81 Jimmy is out and I have access to a 700r4 from an '84 K5 with all associated parts but whenever I ask people which I should put money into rebuilding they always say the T350 because its stronger??? And this is like 100% from different people and shops?

[cliffsta]

Originally Posted by kevinr1970

Maybe I need to talk to different people then?!? The T350 in my '81 Jimmy is out and I have access to a 700r4 from an '84 K5 with all associated parts but whenever I ask people which I should put money into rebuilding they always say the T350 because its stronger??? And this is like 100% from different people and shops?

Yeah from what I have read, the early 700R4's were problematic. I have never used one so i wouldn't know, but that's what I've read. Now the TH350 on the other hand is bulletproof if you maintain it and aren't hard on it. Mine has never been serviced as far as I know, and it still works (other than a tad sticky in the cold). Hell, my dad says the only thing holding it together is all the crap in it.

[Grim Reaper]

For the type of driving your doing I doubt you would ever break even on the difference in cost between a TH350 and a 700. You will almost never see OD and locked converter and that's where a 700 makes it's MPG gains.

It is true...engines have a sweet spot. My 454 in my 88 Burb it likes to cruise at 2500 RPM (63mph) and it doesn not care what's behind it it can hold that RPM all day. I just came back from Macon with 5600lb on the hitch and that motor never broke a sweat. I was getting about 10.2 mpg. Normal city driving it gets about 9.2. That's running mixed hwy running 70-75 mph at around 2900rpm. Yet if I ran 2500 RPM I would probably pick up 2 mpg. Best I have ever got out of that truck was 11.5 and that was Cruise control on about 65 for 300 miles non stop truck empty.

You can pick up more MPG with driving habbit changes for your type of drives.

[1986Silverado]

Very good information there. I may go with the th350 after all.

[86k10]

Well since I have a similar truck I can tell you my experience. I did a mileage check during 2 trips to Florida and back. And that was back during the 65 MPH speed limits. I ran an entire tank out more than once running in 4th and did the same running in 3rd and my difference was 16 in 4th to 15 in 3rd, not much difference. And that was when I was still running the original 305 and 3.08 gearing. As for what it gets now with the big Olds and 4.10's is about 14 in 4th and 10 in 3rd. So I think you would not be that much better with the 4 speed auto. But you can always save up and put one back in if you desire. Just my 2 cents.

[grayw0lf]

86k10,
How did she cruise w/ the original 305, 3.08's & 700R4? Was it realy strong enough to push the 700R4 at cruising speeds... say around 80-85mph?

[86 Red_Bowtie]

Before i changed my rearend gears from 3.08 to 3.73 i ran 1700 rpm going down the highway at 70 mph and was only getting 12 mpg. After the change it runs at 2600 rpm
going 70 mph and i am getting 15 mpg and i doesn't kick down on every hill i go up like it did running 3.08. So all in all if your going to stay with 2.73 rear end gears i would go with the TH 350.

[bluex]

Originally Posted by 86 Red_Bowtie

Before i changed my rearend gears from 3.08 to 3.73 i ran 1700 rpm going down the highway at 70 mph and was only getting 12 mpg. After the change it runs at 2600 rpm
going 70 mph and i am getting 15 mpg and i doesn't kick down on every hill i go up like it did running 3.08. So all in all if your going to stay with 2.73 rear end gears i would go with the TH 350.

Mine was the same, I got right under 13 running mine right after the rebuild with the 3.08's. It was running about 1300 locked up at 60. It would downshift sometimes to 2nd for even a tiny hill. I've just got it back somewhat reliable so I dont have a number for switching to the 3.73, it does run right at 2k @ 60 (I think speedo is waaay off) so hopefully that takes care of all the downshifting and will result in better milage. This is with a 275/60/15 rear tire.


86--if you swap to the T350 make sure to get a 9" tailshaft version and you wont have to mod the DS. You will have to move the crossmember forward (towards the engine) about 2" or so. If your lucky the holes will already be in the frame.

[86 Red_Bowtie]

Originally Posted by bluex

Mine was the same, I got right under 13 running mine right after the rebuild with the 3.08's. It was running about 1300 locked up at 60. It would downshift sometimes to 2nd for even a tiny hill. I've just got it back somewhat reliable so I dont have a number for switching to the 3.73, it does run right at 2k @ 60 (I think speedo is waaay off) so hopefully that takes care of all the downshifting and will result in better milage. This is with a 275/60/15 rear tire.


86--if you swap to the T350 make sure to get a 9" tailshaft version and you wont have to mod the DS. You will have to move the crossmember forward (towards the engine) about 2" or so. If your lucky the holes will already be in the frame.

I'm running 295/50/15 for rear tires on on my truck.

[Palf70Step]

As other have stated, it depends on your motor alot and the type of driving. I do not know how scientific this is, but I was told you'll get your best MPG when you run at your engine's peak torque RPM. So if you have a turbo 350 and say your engine's peak rpm is around 1800 ( I think that's about where a stock SB is) and that's the rpm you run at 65, then you switch to a 700r and the same driving drops your RPM down to say 1500, it is possible you'll maybe see a drop in your MPG.

It seems to have always worked for me when I had engines built and set up a car that if I got my highway RPM's in that peak torque range, I got the best mileage out of the combo. If I mess with tranny or rear gears that would change where that peak torque was, then I would lose MPG.

Again I don't remember what it was based on, my Grandpa showed me that when I was around 13/14 (long long time ago in a land far far to the north )and it always has seemed to work out.

[86k10]

Hey Graywolf to answer your question, no it didn't in fact after I had changed to my 455, I was really astounded when it would just hold those high speeds and would not gain any

[crossy]

i've owned lots of these trucks and i've also driven through, 'wide open' Mississippi , but i pretty much agree with everyone else's comments. your truck most likely has 2.73's in it now. and unless your driving the stretch of interstate that i did and running 85+ plus like everyone else was then it will only make a negative 1-2 MPG at MOST. you will loose a little umph out of the hole though as the 700 has a real low first gear that helps with those salt flat gears. DAVE

[apstguy]

Early 700s had a life of 145,000. Less if you weren't nice to them. In the 350 vs 700, wouldn't the engine rotation vs driveshaft speed would be different because the torque converter would still slip some (unless you got a TH350C) so the actual ratio isn't 1, it would be slightly more. A 700 is definitely .7 because the converter locks. That just came across my mind but I thing the differnce would be too small to notice. I do have to say I now have a Bowtie overdrives TH700R4 stage 2 and it rocks. My cousin got in and said This thing shifts like a @$@! I love it! - Give it some gas and it shifts FAST. Faster than I could ever shift a manual. They use a 88-93 core so you don't have to worry about the earlier's failures. I do have to say my 305 is not enough engine for the tranny - it has the corvette servo in it so to increase speed on the freeway, you have to almost floor it - The 305 bogs if you push halfway down & it doesn't downshift. I guess I'll just have to get a 350...

[86 Red_Bowtie]

Mine is a TH700 not a 700R4 . It has no lockup converter.