I have a 77' F-100, Short bed, 351-M, C-6 auto, 9" rear with 2.73 gears.
I have access to a good 9" with 3.00 gear set. Would 3.00 gears make a noticable difference on take off? Or should I keep looking for a 9" with 3:50 gears?

 

what are your goals in performing the swap????????????????

 

Senior Chief,
I am really just looking to get a little more get up and go. I don't want to do the work to find that I can't tell any difference. I know a 3.50 gear set would be nice, but how much do you think it would effect my gas mileage?
V/R
ET1 Will

 

Wouldn't the taller gears give *less* get up and go or do I have this backwards?

I'm looking for bigger ratio for higher speed and expect worse take off... maybe I'm barking up the wrong tree altogether.

 

numerically lower numbers (ie 2.75:1) provide a very low mechanical advantage for acceleration. A numerically higher number (ie 4.10:1) will have a much better mechanical advantage with regards to acceleration. The trade-off is highway speed rpm. Since the first number is the amount of driveshaft rotations, meaning that in the first 2.75:1, the driveshaft spins 2 3/4 revolutions to 1 revolution of the axle. Engine rpm's will be very low. The other 4.10:1, means that the driveshaft rotates 4.10 revolutions to 1 revolution of the axle, so given equal tire diameters, the engine will be spinning at a much higher rpm at the same speed.
The 4.10:1 (numerically higher, considered "lower gears) will accelerate and haul loads with less difficulty than the 2.75:1.
The misconeption is that the higher the gear, (numerically lower 2.75:1) the better the mileage. Well this is not always true. Since the engine has to struggle to accelerate, it usually consumes more fuel just to get going. Unless this is a lightweight vehicle with small tires and driven on the highway all of the time, one may never see a didtinct advantage. This negates the whole theory of higher gears getting better mileage. This does not mean that the truck can go any faster either. That misconception that a higher gear can be installed to pick up top end is also a fallacy. This is directly related to hp and torque. Limited hp has limited speed. Trucks with different gear ratios like the above examples will have the top end speed. The only difference is that the 4.10: truck will get to speed quickly, and it will be turning at a higher rpm. The rpm will be the limiting factor here, and in the 2.75: truck, it will be limited by a lack of torque vs resistance, but the top speed will be rather close.
Gear selection is a balancing act, with variables such as weight, additional loads like trailers, tire size, and engine output.

As far as noticing a difference between a 2.75:1, and 3.00:1, you may notice very litttle with regards to acceleration. If you are looking for a "seat of the pants" improvement, I would wait for a lower gear (numerically higher) like the 3.50, or 3.73.
Good luck.

 

I have 3.55 and it's the perfect match between take off/towing and mileage.
I don't think you will see much difference in take off....Just my 2 cents.
Where in VA are you? Norfolk?

 

75F350: Thank you for the info. I misunderstood the gear ratios for sure. Searched for more info and found some good basics at http://www.ringpinion.com/ if anyone other than me is looking.

 

Thanks for all the help I am going to keep an eye out for a rear end with 3:50's. I am in Yorktown. USCG!

 

I did notice when I changed my tires from 235/75R15 to 245/75R16, but the difference was very small, mostly noticeable on one specific stretch of my daily commute.

 

the main thing about rear ratios is what you are doing with the truck and what size tire you are running

 

I am running 275/60r-15... It is a daily driver "stop and go", trash hauler, and what ever else I need to do. Why a 2:73 in a truck? The gearing in this thing is really beginning to irritate me.

 

can't remember all ratios for 9in but something like a 3.20 to 3.35 would be ideal
the 2.73 was a stock gear in the 70's towncar and got best milage at 70 to 75 mph the 3.0 was the towing gear for the towncar
3.50 is not a bad gear for daily driving but with 351M a tad low