I have 3.73 gears in right now and thinking about going to the local bone yard to pick up like 2.75 gears.... a ratio of 3 or below.. I think trucks came with 3 and above. Im asking cuz im going to be driving 1300 miles to phoenix. so here come my questions.

1) I dont know my gas mileage, im assuming around 10 in my 390 with 3.73 gears. What difference in mileage would i most likely see from my gears to a ratio between 2.5 and 3.0.

2)What application could i get the 3rd member out of that would bolt up to my f100, that has the gears i want?

3) how much should i pay for these gears at the boneyard? (the guy tried charging me $100 for 3.00 gears... I would really like to get into the 2's if its worth the mileage.

4) will my speedometer be off at all, and if so, how far off will it be going down to those gears.

I just want an extra set of gears for mileage purpose. I will not be towing anything and the only thing i will be hauling are my clothes in the bed of my truck for when i move to az. i figuered if I got a 2 mpg increase and it cost 100 bucks then it wouldnt be worth it for that one trip.

my calculations...
1300 miles divide by 10 mpg is 130 gallons in gas... At 2 bucks a gallon that is $260.

1300 miled divide by 12 mpg is 108 gallons in gas... At 2 bucks a gallon that is $216.

The difference in the two is $44.

 

I doubt the mileage difference will be enough to make it worth the cost of the change. You'll likely save more in gas by simply driving slower. 55mph is boring, but it sure does save alot of gas and it doesn't cost anything either. I have very low gears in my truck (4.56). I had 3.55's in it before. With the lower gears I can get similar mileage as I did with the high ratio gears as long as I keep the speed down. It's when I try to go too fast that it starts sucking up gas.

That said, yes, your speedometer will be off, assuming it's correct now. The truck will be going slower than what the speedometer reads with the new higher ratio gears.

Not sure how much you should pay for a used set of gears. Personally, I would buy the whole third member instead of go through a gear setup just for one trip and then do it all over again later. Just changing the 3rd member out is alot easier. This is assuming your F100 has a Ford 9" axle.

If it is a 9", the third members come in 2 varieties: 28 spline carrier and 31 spline. Trucks usually have 31 and cars usually have 28, but there are still exceptions.

 

Just put a pair of 31 or 32 inch tires on for the trip and drive slow. You will still be time ahead going 55-60 mph vs. the time it would take to swap out the diff.

 

I already have 30 inch tires on, and yes it is a 28 spline. Ive already replaced it, the one i get for the boneyard will be like an extra set. I mean im driving 1300 miles and Phoenix has a lot of freeways. Im not driving 55 mph considering i would get pulled over for going too slow. I have done the calculations with the tires and everything, By the way i found a set of 2.44 gears and carrier and one 3.00 with carrier, thats it, But Mil1ion one of the members and moderators says with 2.73 i should get around 17-19. Would 2.44 be way too drastic, i mean i guess i would have to drive really slow until it got to speed, and besides that i calculated i would pay for the rear end in gas money when i go down there. I'm going to go to the boneyard tomorrow and try and get the carrier and gears for 50 bucks, if its a no go then i will think about it and wait till i get more influence by you guys. I mean its not bad to have an extra set of gears that are ready to pop in, and it doesnt take long. But you guys have me thinking now if it was a 28 or a 31. I just rebuilt the damn thing and i cant remember, im sure i posted it in one of my threads. I'm going to go double check. it was a while back.