Rear end gears for my 1967 3 speed on the column

[67NC]

I need help, I am getting about 5 miles to the gallon and it is killing me! I have a straight 6, 300 with a one barrel carb. Only thing I can say is when at 55mph the engine is screaming for another gear. Since I only have 3 and I "assume" the trans should be able to handle these speeds? It can only be the rear end gears are to low. Can anyone suggest what gears to go with? or anything else I should be looking for?

Thanks for any help,
Joel
Elizabeth City, NC

[jor]

Third gear, or high gear, is 1:1 so you're right, the differential ratio is the culprit.I like 3.50 for a good compromise. Maybe a little higher than that even, 3.25 or so. Do you have a tach on your truck? If not, install one and you'll have a better idea of what is happening now.
jor

[binder56jd]

3.25 might be a little high for the 3 speed- some thing in the 3.50's would be better imo-jd

[steveheer]

My truck has a 300 with 3.50 gears. It works well. Now I have a 3 speed with the R11 overdrive and my mileage really didn't go up. I will say that I get a minimum of 12ish in town and on the road maybe up to 17. Before the OD I could pull those ##'s too.

I would start by finding out what gears you have now. Jack it up and have a friend help you count driveshaft revolutions. Remember both sides need to turn the same amount to get an accurate driveline count.

[67NC]

Not sure what you mean? Count how many times drive shaft goes around with the wheels? So if I do understand the drive shaft will go around one time per one wheel rotation for a 1 to 1 ratio? and a 3.50 ratio would be 3 turns on drive shaft to 1/2 turn on the wheels?? Sorry I am lost on this. I will go online and do more homework. Please help if you can, thanks.

[spartman]

You could crawl under your truck and see if the rear end still has the ID tag on it.

If so post it here and we can tell you what it is.

Or look at this page.

http://www.fordification.com/rearends-ford.htm

Also if you know that it is the original rear you could look at the code on the drivers side door and tell us what is has.

[steveheer]

You're close. on a 3.5 gear the driveline would turn 3.5 times per 1 turn of the wheels. On a 4.56 gear the wheels would turn 1 turn per 4.56 turns of the driveline. Again, both wheels need to turn exactly the same amount.

[CJ,s 67 F100]

If you have a tach you can use this formula to find out what it is.
High Gear ratio at 1-1 x mph x 336 Divide by tire height
Example 3.25 x 55 x 336 Div. by 28.5 = rpm,s
3.00 x 55 x 336 div. by 28.5 = 1,945
3.25 x 55 x 336 div. by 28.5 = 2,107
3.50 x 55 x 336 div. by 28.5 = 2,269
4.10 x 55 x 336 div. by 28.5 = 2,658
4.56 x 55 x 336 div. by 28.5 = 2,956
Hope this helps, Charles

[rjewkes]

My 72 f100 360 c6 3.00 dual exhausts(turbo style raptors) gets 12-13ish in town and 16-17 or 18 highway. As good as the harley7 f150 5.8L.

[flyboy2610]

My 72 f100 360 c6 3.00 dual exhausts(turbo style raptors) gets 12-13ish in town and 16-17 or 18 highway. As good as the harley7 f150 5.8L.

Those numbers are about what can be expected from these trucks. They weren't designed for maximum MPG.

[fordman]

newer v6 anyone? efi

[mrung0wa]

If you got a 9" I go with a 3.55, that's a good compromise.
Check the tag on the Pumpkin that'll tell you!



Lawrence

[binder56jd]

I don't believe that "fuel effienciency" was a major concern during the era these,and remember they were designed to work not haul mama,kids and groceries, "TRUCKS" were manufactured- so if you're getting 12/13 town and 16/17 hwy that's good imo- jd