C-6 Tranny with a 2.75 Gear Ratio?
 

I have a 300 six with a C-6 tranny and a 2.75 rear axle ratio. I get distressing Gas Mileage, like about 11 MPG. Is there a different combination of gears that would improve my MPG? I get this mileage with normal driving to and from work, with nothing in the back. I am thinking of puting in a traction control/pos traction rear end. Open for suggustions.
thanks
John

What size tire are you running? I may be wrong here but the 2.75 gears are going to keep your engine RPM down which I would assume would help save gas mileage. If you go to a lower gear (higher ratio) like 3.54 or 3.73 you'll spin a lot higher RPM to go the same speed . . . which would increase your power and acceleration but would also increase the RPM needed to go any given speed. But a lot of the gearing is going to come down to the diameter of the tires.

Ivan you are pretty much right except that if the engine is turning at such a low rpm to be below the ideal cruising range it will use more gas also as it is having to lug all the time, and if it is any stop, and go driving then you are pulling harder with the high gears to get going, and this will kill mileage too. As far as recommendations we need tire size, and typicall speed you are driving and/or what rpms are you turning most of the time, in a truck I would say you would want no higher than about a 3.25 ratio for typical cruising range ie 2200-2600 rpm. An example of this is I drive a Semi best milleage I get with it is at 1450-1475rpm which is around 63-65mph, if I drive in a 55mph state, and run that speed at around 1400 or slightly less I loose .5mpg which translates into almost a 10% loss of fuel economy but if I run 70mph at around 1525rpm I only loose about 5%

Good point Monsterbaby. I just figure with the slippage of the C6 it's going to compensate somewhat but this will still lug the engine a bit. 2.75 does sound way too high a ratio unless you're running some tiny tires.

I agree the engine is way below it's power band and is struggling.

For my two, I'd say a 3.50 would help the little bugger much better. That C6 is whooping up your engine enough. Most of the older truck had 3.75 and lower rears. That's what makes folks thing the I-6 is so powerful.

John :-X06

I don't know about the RPM, but my speeds are 40-65, mostly in the city, some Highway. My tire size is P235/75R15. I did replace the 3 speed stick with a C-6 and kept the orginal rear end. I always thought this was a mistake. But back to the orginal question, what about the posi traction and the correct ratio for a set up like mine.
Thanks John

IVAN, Your statement about good MPG with the 2.75 ratio. I get terrible MPG in the city or on the road, especially when I am pilling my little 14" Aluminum Boat. I can just about watch the gas needle go down.
thats why I am wondering about the rear end ratio with the C-6
Thanks John

Yikes!!! With your tire size you really do need to be running something in the 3.50 range. I read an article a while back that recommended a you calculate your gear ratio at .12Xtire diameter. In your case your tires are right about 29" which would put this ratio at 3.48. This seems to give an optimal RPM for power and fuel efficiency . . . so keep an eye out for a gear set in this range.

Now if someone can answer your posi question you'll be set. :)

Thanks Ivan

if you want a posi, it is a good choice if you do any driving in snow or mud if you only drive on dry pavemant then it isn't a needed thing but doesn't hurt any either other than the checkbook on the first purchase. As far as finding one look for a bronco, as far as I know all of the early broncos had traklocks in them, and I think that continued on through the 79's, and alot of the early 80s also, and alot of them had 3.50 gears so that would solve both of your needs in one shot

Sgt Wonderful, Something is way wrong. I have a 460 and c6 with 3:00 gears and I get 11.5 MPG. You definately need to go up to at least a 3:25, and I don't think a 3:50 would hurt anything. You do have the ford 9", correct?

Have you ever noticed that max horse power is listed with rpms well above 4500? All engines build power with more rpms until they run out of their power band. Your I-6 needs to turn more rpms to make it's power to turn the big C6 (usually used with big V8s) and the large tires that you are running.

It never reaches it's power band with the 2.75 rear gear, and is struggeling.

Posi is nice a few times a year, if ya got the funds and find one, buy it. You can change it out in an hour.

my 2

John :-X06

My Ford Shop manual for 79 light truck, lists the following Gear Ratio:
2.75-1
3.00-1
3.25-1
3.50-1
3.55-1
But it dosent say anything about what shouls be used with what.

John

Docholiday: Yes I think it's a 9"

John

Sgt Wonderful, It's a good deal that you have the 9". You can change the gears in about 1 hour. Do you know how, or do you need to know how? I realy think a gear change would solve a lot of your problems.

You have posted a list of ratios. What it means is the engine turns this many times for the tires one time. Since all non over/under drive trannys turn 1 to 1 with the engine, a 2.75 ratio for example the engine is turning 2 and 3/4s times per the tires one rotation.

As long as the engine can handle the power, the lower the numbers like 2.75 the wheel is turning faster at a lower engine speed. With 3.55 or 4.11 the engine is turning a lot faster to acheive the same tire speed.

The I6 in your truck needs to build more speed to make it's horsepower and the 2.75 ratio is not allowing that to happen. A 3.50 or 3.75 ratio will allow the engine more rpms and it will make it happier, and better gas milage.

hope this helps

John :-X06

jowilker: Thanks for the info. It was never explained to me that way before. But it seams to me that if an engine is turning faster it would use more GAS to make it turn faster, is that correct? If so how can it have better MPG, turning more RPM's

You need to know the torque, not horsepower band for your engine. Usually, engines are most efficent near the torque peak. The 300ci six and the deisels tend to have peak torque at a lower rpm than the gas V-8's. Lets say that your engine torque peaks at 2000 rpm and you like to cruise at 65mph. Find the gearing, with your current tire size, that will give you that rpm/speed combination in high gear.

For example, my old Ranger 2.9 V-6 had the Mazda 5-speed OD, 3.45 gears and 14 inch tires. It all worked out to a 68mph speed at peak torque, and it got the full EPA estimated highway mpg at that speed. My current F-150 4x4/5.0/E4OD/3.55/29in tires gets way less than the EPA estimated 18mpg, more like 15 or less. I have to drive at 75mph to get the engine up into the torque band and at that speed the wind resistance eats up too much gas. At lower speeds, it labors and wont stay in OD, again eating up gas.

Jim

i thought the peak efficiency had to do w/the vacuum. i read somewhere that a vacuum guage can tell where your engine is operating most efficiently. most gassers creat their peak torque at 3-5k rpm's. try cruising at that range and look at the gas bill.

Jessfactor, that's peak horsepower you're thinking of. Most gas V8's make thier peak torque below 3500 rpms. My 351 makes it's peak torque around 2800, which is actually a little under my top freeway cruising rpm, since I have low gears in the truck.

What you have to remember is the amount of gas used is controlled by how much you open the throttle, not how fast the engine is spinning. If your engine is performing in its optimal RPM range it will function with just a little throttle and keep you moving down the road. If you lug it down you're going to open the throttle more to keep your speed up. This doesn't necessarily mean you're going faster, just that you might have your foot too the floor and be dumping a lot of gas in the carb to keep it going. Think of it as driving up a steep hill in 3rd gear vs. driving on flat ground. If you go up the hill and stay in 3rd gear you can put the pedal to the floor but the RPM's will stay fairly low because you're not in your power range. You're still dumping fuel into your carb full speed but not going any faster. Now if you drive that same truck in 3rd on flat ground you can hold the throttle at half and travel twice as fast as you did on the hill because the engine isn't overloaded and can operate in the power range. This is the same thing happening with poor gearing, you can't get to that power range but you can still dump more gas in the carb trying.

Hope that made sense, I've had a bit to drink and I'm a little incoherant. :cool:

Interesting, thanks Ivan.

Modern passenger car engines sometimes have torque peak way up there but most, not all, truck engines are near 2000-2500. A vaccum gauge would make a lot of sense, especially when combined with a tachometer. Most small airplanes have both. You set a fixed rpm with the propeller control (similar to a transmission gear selection) and vary the power with the throttle, as indicated by the manifold pressure gauge (a vacuum gauge, but calibrated differently). Low rpm and high pressure (low vacuum) is bad for the engine.

Jim

Doc, No I dont really know how to make the change. If I got a rear-end (just the gears) from a ford Bronco, could I just take my gears out and replace them with the ones from the Bronco?
John

Basically if you get the entire drop out from another 9in, and the axle spline count is the same then yes you just pull the axles, drop the entire center section out clean up the sealing surface use some silicon on it (I don't use the gasket just blue RTV) install the new center section reinstall the axles, and fill with gear oil. pretty basic simplized description but the general idea. if you are just needing to change the ring, and pinion it's a little more complicated but still not difficult

Thanks for all the info. I am still thinking about it.