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

 

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

 

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.