Rear Gears
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Joined: Aug 2003
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From: South Carolina
Rear Gears
Hi Guys, Haven't posted here in a looong time. I have a 74 F100 2WD 351M C6. The rear end let go today. It came with 3:00 gears in it. I'd like to change the ratio, for better MPG, I do alot of highway driving with it. I see where you can get 2:90's thru 2:70's ratios. Should I stick with original ratio or will changing ratio hurt. I don't pull a trailor or haul anything heavy(<1000lbs) with it. Any input is greatly needed and appreciated.
Thanks
MarkC.
Thanks
MarkC.
Post Fiend
Joined: Jan 2003
Posts: 10,323
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From: Montana Territory
Rear Gears
There are a lot of neglected 2.75:1 9" rear ends out there. It really won't help your gas mileage much but your top end speed will be faster, I forget what the formula for tire size and re3ar end ratio to engine revs is but at highway speeds you should be at least a few revs lower. You aren't gonna hurt anything by going higher (lower numerically).
Logistics Pro
Joined: Jan 2001
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From: Central NYS
Rear Gears
MSC,
FWIW, I swapped out my 2.75's for a set of 3.25's. (2.75 was just too tall a gear for my little 302 in a 4400# truck.)
My fuel mileage on long Interstate trips dropped from between 21 and 22 mpg to between 20 and 21 mpg, BUT my fuel mileage improved on the local two lane blacktops from 17 and 18 mpg to 18 and 19 mpg.
With the 3.25's and 237/70-15 tires it turns about 2000 revs per mile, and about 2200 rpms @ 65 mph. On a trip to South Carolina, where most states allow 70 mph, I thought the 3.25's were a little much. At that speed I think 3.00's would be the gear of choice with my OD transmission.
So, with your extra 50 cubes and non-OD C6, I wouldn't think you'd want to go any taller than 2.75's, and you'd hardly notice the difference between your 3.00's and some 2.75's.
FWIW, I swapped out my 2.75's for a set of 3.25's. (2.75 was just too tall a gear for my little 302 in a 4400# truck.)
My fuel mileage on long Interstate trips dropped from between 21 and 22 mpg to between 20 and 21 mpg, BUT my fuel mileage improved on the local two lane blacktops from 17 and 18 mpg to 18 and 19 mpg.
With the 3.25's and 237/70-15 tires it turns about 2000 revs per mile, and about 2200 rpms @ 65 mph. On a trip to South Carolina, where most states allow 70 mph, I thought the 3.25's were a little much. At that speed I think 3.00's would be the gear of choice with my OD transmission.
So, with your extra 50 cubes and non-OD C6, I wouldn't think you'd want to go any taller than 2.75's, and you'd hardly notice the difference between your 3.00's and some 2.75's.
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From: South Carolina
Rear Gears
dfisher1, pdqford;
Tire size is 235 70 15's. I would like to cruise around 65. I should tell y'all that the tranny is supposed to be out of a Lincolin. So right now at 65 on my speedo it tachs about 2600 to 2800. I'd like to drop that off some. That's why I want to go to a higher gear. I'm sorry I wasn't clear in my first post. So how much do you think my rpm's would drop with 2:75's? My mpg no matter what I do is around 10mpg. The motor is old and I need to make this last untill I can redo the whole truck.
Thanks Guys
Mark C.
Tire size is 235 70 15's. I would like to cruise around 65. I should tell y'all that the tranny is supposed to be out of a Lincolin. So right now at 65 on my speedo it tachs about 2600 to 2800. I'd like to drop that off some. That's why I want to go to a higher gear. I'm sorry I wasn't clear in my first post. So how much do you think my rpm's would drop with 2:75's? My mpg no matter what I do is around 10mpg. The motor is old and I need to make this last untill I can redo the whole truck.
Thanks Guys
Mark C.
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Joined: Aug 2003
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From: South Carolina
Rear Gears
Well, I finally got to my truck yesterday. Found out I only have a broken L/S axle. Over five years ago the bearings went bad. Well when I pressed them off the L/S was gaulded on. So we built up the surface by welding it and turned it on a lathe, which of course hardened the axle(never thought of it then). That's where it snaped. Pretty lucky though, it lasted over five tears. Good friend is getting me one as I write this.
Thaks for all your help guys.
Mark C.
Thaks for all your help guys.
Mark C.
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