RE: Choice of tranny effect mileage???

[chanclas90063]

I recently purchased a 1970 Chevy C-10 longbed. It is decent condition but needs some attention to some of the smaller issues. The truck runs fine, but very poor mileage. I was told it has a 350 from a 70s Camaro and seems to run ok. It feels to me like I need another gear when I am on the highway and it gets very poor gas mileage. The current tranny slips when cold and as it get warm it runs ok, someone suggested upgrading to a 700R. Any opinions or suggestions on improving the mileage?? I do a lot of highway driving

[cdowns]

yes going to an overdrive tranny like the 200 or 700 will help somewhat burt it is really tryin to beat a dead horse to get good milage out of these trucks without spending in the many thousands of dollars

[Stocker]

These trucks are not known for good fuel mileage. If you're running high RPM at road speeds, an OD trans or a rear diff gear change can help. With your current trans slipping, it may be time to replace with an OD unit. First, though, check its fluid level - if it's low, that could explain the slipping. You also should check the rear gear ratio so you'll know what you are dealing with before changing anything.

[Boog]

Just what mpg is the truck getting? 10-12 is somewhat low. 15-16 is pretty good for these trucks without OD and fuel injection.

[chanclas90063]

Originally Posted by Boog

Just what mpg is the truck getting? 10-12 is somewhat low. 15-16 is pretty good for these trucks without OD and fuel injection.

Yes, it is the 10-12 range. I was hoping to get somewhere around 18-20???? It's not so much as the gas mileage, but the sound of the engine straining at 70mph that is unnerving...

[Boog]

Yeah that's what I figured. How much are you willing to spend to get a few more mile per gallon with this truck, with this engine? Overdrive trans or gear changes aren't cheap and may only net you 3 mpg increase with that engine.
Install a tach to see just how fast the engine is turning at speed.
Small blocks can run all day at 2800 to 3000 rpm and not hurt them. You can go to a taller rear tire to slow the engine speed a bit but it will throw the speedometer off some.
You may find it better to just slow down on the highway until such time as you can make changes to it.

[jocko]

If you change the rear gear to reduce highway rpm, keep in mind you'll lose low end gear advantage (decreased acceleration). Best of both worlds is a true o/d trans. That will give you an increased acceleration off the line (whether you want it or not) because the o/d trannies have a deeper (higher numerical ratio) 1st gear in addition to the overdriven 4th gear. If you post tire outside diameter, we can give you some examples of final drive ration in 1st and 4th so you can see how much you'll be increasing the rpm range you start from a dead stop with as well as final rpm in 4th at various highway speeds. Also need to know your current rear end ratio, if you know it. Generally speaking, a 3.42 or 3.73 is the ideal compromise rear gear ratio for these o/d transmissions, but others are workable - but they can cause issues at the low or top end if you have say a 3.08 (rpm too low at high speed) or a 4.56 rear (rpm will be very high at 1st gear accel, so it will shift to 2nd almost immediately. Not a big deal, it's an automatic!)

[RickDeVon]

My 250I 6cly runs great with the TH350 and 307 rear but like you I feel it needs another gear...

[Fitz]

Originally Posted by chanclas90063

I recently purchased a 1970 Chevy C-10 longbed. It is decent condition but needs some attention to some of the smaller issues. The truck runs fine, but very poor mileage. I was told it has a 350 from a 70s Camaro and seems to run ok. It feels to me like I need another gear when I am on the highway and it gets very poor gas mileage. The current tranny slips when cold and as it get warm it runs ok, someone suggested upgrading to a 700R. Any opinions or suggestions on improving the mileage?? I do a lot of highway driving

A 700R would make a big difference in mileage. The rear axle ratio might be part of the cause. If RPM is above 3000 at 70MPH then you should consider both an overdrive transmission and a higher (numerically lower) gear ratio.

[Mike C]

You won't need to change the rear if you add the overdrive. A TH200 4R has a .68 overdrive. If you are turning 3200 rpm on the road, that drops to 2176 which should be a sweet spot for a carburated motor. A TH 700R4 has a .70 overdrive which drops the 3200 down to 2240.

An OD trans with 3.07 gears or less is almost not useable because it lowers the revs so much. Especially true with a smaller motor and/or one that's not tuned well.

One of the primary reasons newer cars are more fuel efficient and driveable is the more transmission gears not the motor and FI. They help, but it's all about the transmission and final drive selection.

[Fitz]

Originally Posted by Mike C

You won't need to change the rear if you add the overdrive. A TH200 4R has a .68 overdrive. If you are turning 3200 rpm on the road, that drops to 2176 which should be a sweet spot for a carburated motor. A TH 700R4 has a .70 overdrive which drops the 3200 down to 2240.

An OD trans with 3.07 gears or less is almost not useable because it lowers the revs so much. Especially true with a smaller motor and/or one that's not tuned well.

One of the primary reasons newer cars are more fuel efficient and driveable is the more transmission gears not the motor and FI. They help, but it's all about the transmission and final drive selection.

Mike, Mike, Mike. You should have read my post. A turbo 350 with 265/70-15 tires and a 3.73 rear end is EXACTLY 3000 RPM at 68 MPH. So, rpm's above that number would indicate a rear axle ratio lower than 3.73 and there are none between 4.10 and 3.73! So, If RPM's are above 3000 the diff likely has a 4.10 gearset and a change to a 700R yields 2200 at 70mph but a drop to a 3.73 gets 2000 at 70! Better choice in my book if the goal is improved mileage.

[Mike C]

I did read your post Fitz and as usual you have good info. But I responded because I don't think going to any gear less than 3.73 would be a good idea with an OD auto. And you suggested he do OD and a lower numeric gear but didn't specify what that would be.

And it's probably not relevant to the OP but a stall converter can skew the RPM at speed as well. My brother's Camaro with 3.73 turns more R's on the highway than mine with 4.10's and that's because mines a manual and his has a 2800 rpm converter. And on top of that, the OP says the tranny slips at speed. And he has no tach.

Since he didn't say what gear he had we all made some assumptions. If he had 3.07 and added an overdrive that would be too low an RPM for the most part. And either 3.73 or 4.10 would work fine on the highway with an OD to your point. The only disagreement I had was doing the lower numeric gear (since I was assuming he has 3.73 which is highly likely) AND going to a lower numeric gear.

[Mike C]

Originally Posted by Mike C

The only disagreement I had was doing the lower numeric gear (since I was assuming he has 3.73 which is highly likely) AND going to a lower numeric gear.

I meant going to a lower numeric gear ala 3.07 AND the overdrive transmission. My bad.

[CC69Rat]

700R4 would definitely help in overall fuel economy but as others mentioned.. check and make sure the fluid levels are good in your TH350 and that might buy you some time at least.

A good tune up, new spark plugs, wires, maybe upgrade to HEI, check and make sure the exhaust is not old and the inside of the mufflers are rusted and collapsed (?) Is the air cleaner dirty or stopped up?

Anything you can do to get the fuel to the engine, get it burned and more free flowing exhaust will help with the fuel economy. The engine has to breathe to be efficient.

Try the cheapest options first and save a little along the way for 700R4.

What size tires are on it now? Are you planning to put a larger tire on it at any point? That will impact the rear gearing too. . I'd also factor that into play before regearing.

[Stocker]

It's possible that at least part of the problem with the engine sound may be coming from a fixed-blade engine fan. If you're not sure what you have, post a pic. Replacing a fixed-blade with a thermal clutch fan will help relieve a lot of the roar.... and can free up a bit of HP and fuel mileage too.

[1968aj]

Originally Posted by Stocker

It's possible that at least part of the problem with the engine sound may be coming from a fixed-blade engine fan. If you're not sure what you have, post a pic. Replacing a fixed-blade with a thermal clutch fan will help relieve a lot of the roar.... and can free up a bit of HP and fuel mileage too.

my fan clutch was froze up. sounded like it was turning way too many rpm. once I changed that it made a major difference it sounded like it was stuck in second gear .it now sounds totally different

[chanclas90063]

Thank you all for your words of wisdom. The truck goes to the wrench tomorrow and we are going to see what we can do with the tranny and the condition of the suspension system. I look forward to enjoying this truck for a many years. This forum will be my bible

[Z10]

Larry,

I'm running a zz4 350 with FIRST TPI, a 700r4 and a 3:73 rear. Everything is new and now with 7000 miles on this set up, I'm getting about 16.5mpg at 75mph. I'm guessing 20mpg is going to be really tuff to get. I think the main issue now is these trucks have the aero of a brick.

[Mike C]

Originally Posted by Z10

Larry,

I'm running a zz4 350 with FIRST TPI, a 700r4 and a 3:73 rear. Everything is new and now with 7000 miles on this set up, I'm getting about 16.5mpg at 75mph. I'm guessing 20mpg is going to be really tuff to get. I think the main issue now is these trucks have the aero of a brick.

Truth. First thing to do is tune it up. Start with HEI, plugs and new wires. Adjust the carburetor. Narrow tires with high inflation pressure, lower the truck as much as feasible and still driveable, add an air dam, overdrive, clutch fan. All will help with mileage. But the easiest thing to do is slow down. Much easier to aproach 20 mpg at 60 mph than it is at 70. One thing these trucks have going for them is they are light which helps a bunch.

[sleepertruck72]

Congrats on the truck ! I have a 72 C20 with a 350 HO and 700R4 4.10 geared Dana 60. The overdrive is nice, at 80 MPH I get 10-12 MPH Driving 55 MPH may help, but that's boring

[chanclas90063]

Here is a photo of my truck. It is a 1970 Chevy C-10 with a "68 front clip. The engine is strong but the tranny slips when cold. I do travel back and forth to San Diego and would like to keep this as my DD. Changes to the tranny and adding a bed cover may increase my mileage slightly I hope. Thanks for all the help and I will continue to use this board as my guidelines

[RustyBucket]

Nice lookin' truck. .. Guess if you REALLY want to keep/use it for a DD a 4.8 or 5.3LS drivetrain swap would be worthwhile. Lots of LS powered trucks available for salvage for reasonable prices. Both of my trucks have overdrive transmissions and neither are much for mileage. One's BB stick and one's SB auto. I don't check mileage much but it seems like every time I drive one of them I have to buy gas. I imagine if I ran the interstate at a constant 75mph they'd probably give 15mpg. or so but they're a lot more fun to cowboy around in the lower gears. My DD 6.0 crewcab GMC gives about the same mileage as the old trucks .. but it's a lot heavier. Bet a well tuned 4.8LS in your truck would give 20mpg.

[jocko]

Here are some numbers for you. First set of tables (left column)=29" tire, 2nd set of tables (middle column) =30" tire, 3rd set of tables (right column) =31" tire. (EDIT: ignore the "primary option" wording - that has nothing to do with these tables...)

[chanclas90063]

WOW!!! I am overwhelmed with the information coming from this group. I feel empowered with this much knowledge in one group. I was hesitant to take this project on, but my confidence has skyrocketed due to my association with this group. I look forward to many years with this truck. Thanks all!!!!

[randy.powell]

So my experience with my '71 Cheyenne Super long bed is -
350 SB, 400 Turbo, 4.11 rear = 12.5 hwy mpg
Same drivetrain, no changes but installed 700R4 = 18 hwy mpg

Love it! Blew 3rd in the turbo 400 so had to change it anyway...

Randy