Ken, With out doing the math how does changing the rear gear ratio change the spreading of the transmission ratios as they never change?
Only way I know of changing the trany spread is to change the tranys gears.
You can have wide & close ratio tranys with in the same model trany.


Also the 3.54 & 3.15 gears we where talking about are in a AMC car that we swapped out the stock AMC T10 for ford's WC T5 with a 2.95 first gear.
Dave ----

 

I see FuzzFace has answered this, or addressed it. I think I know what you mean, Ken, so let me give it a try with my take. I'm no Ernest Hemmingway, bear that in mind lol!

If the rear is lower (higher numerically), then the speeds in each gear are less, serving to effectively shorten the gears in the transmission.

I know with the 2.46 that's in it now, I can wind those out quite a bit - well, as much as the low revving 300 six will let me. My speedo reads low, but I see 30+ in 1st, 2nd to 50, and 3rd pretty much higher than I can do OD 4th because it has the power to pull 3rd, but not 4th. A lower rear should change all that and allow the gears to be shorter, and IMO a better/more usable spread.

I don't care that the 1st gets down to what the NP435 is, but don't mind it being shorter. I mainly brought up that example from my '85 in saying that I use all the gears even tho most folks drive a granny low 4 speed like a 3 sp unless they are loaded or towing. I effectively do that now in the '81 Srod but in the other direction (no OD use usually).

 

This site says the torque peaks at 1,600 RPM. I'd set the gears up to run at 1,600 at whatever speed I'm going to normally drive.

 

Ok I got it now, it does not make the trany a close ratio as I said.
What it does do is moved the shift at speed lower, not the rpm, with a lower (higher number) gear set.


A close ratio trany has the shift at speed closer between gears (less rpm drop) where the wide has the shift at speed wider (larger rpm drop). Again this is done with the trany only not the rear gear change. This is to keep the motor in the power band.
Dave ----

 

I've heard that before to set speed at peak torque. 300s don't rev much and most of the power is down low.

 

When I bought a Honda CB175 new in 1972 the manual showed the torque curve and fuel consumption on the same chart - and the fuel consumption was minimum at max torque. Later I bought a college text book on internal combustion engines, and it stated that if all else was equal max torque and minimum fuel consumption coincided. (Actually, what it said was "brake specific fuel consumption", or BSFC, is minimum at max torque. That means that if you require as specific amount of power out of the engine you'll get the least fuel consumption at max torque.)

 

Exactly right......the transmission ratios don't change, but they effectively close up (or open out), driving & shifting.
Graphed, the effect is clear.

Gary, I didn't know the 300 max. torque figure was so low......so 3.54 is probably too low (short) an axle ratio for the application.

 

I have 235/75R15 tires, and an online source puts them @ 28.9" tall.

There are numerous calculators online; here's one:

RPM/MPH From Gear Ratio Calculator by Wallace Racing

This allows up to 3 final ratios. I put in 2.46, 3.00, and 3.25

On the 2.46 ratio, 1600 works out to be ~71 mph. I can say that that is the minimum where there is any power, and really it's still gutless for the most part.

Interesting though, I'll have to take this into account. I guess there are trade offs depending on whether it's setup for max mpg, or better power and drive ability. Remember too that the national speed limit was 55 when this truck was made, but it still makes me wonder what group of engineers pushed the 2.46 with a .79 OD to the folks that had the final decision on what to offer to the public.

Calculator or not, I still want more gear! This is a good example of how the book says this is perfect, but real world experience proves otherwise.

 

We need to get David/1986F150six involved in this discussion. His truck has 3.08 gears and an OD tranny and I think he said it runs about 1,700 RPM at 62 MPH where he cruises. It seems to have good power, and he's commented that it is fun to drive where his son's truck, geared much like Festus, is not.

Further, he's getting ~20 MPG - but he drives like there's an egg between his foot and the accelerator. I think he said his son's truck will do better than that, but his description of driving it is exactly how you've described driving Festus - if you can see a hill you should have already downshifted.

 

I have also use Wallace Racing when setting up my drag car.
When all the math and information that is out there you just have to pick a few and throw a dart to pick one as a starting point.


I have also wondered if they ever drove what the engineers pushed as some of them back then just did not work!

I think I would go with the 1700-1800@65 mph (average speed around here) so when you slow down like hit a hill you are in the peak range. I also know what I needed when I was in CT with BIG HILLS to what I have now here in NC are 2 different things and each person has to plan for that. Also what the truck will be used for, mine will be a back up rig to tow my 20' open deck car trailer when my 02 Durango (5.9 w/3.92 gear) is down.


My plan is to keep the T18 my truck has and once on the road and bugs worked out would like to install a Gear Venders OD unit. This way if I want I can split each gear and have OD (0.78) when in 4th.
Dave ----

 

I think that's a great idea, more first hand knowledge can only help.

3.08 means he's got a 8.8; I've got a 9" so choices are 3.00 or 3.25. I have not seen any gears available in between those two for a 9", at least none that I know of, and I still think that 3.00 isn't enough, but admit even that would be a huge improvement over the anaemic 2.46 that are in there now.

 

Sorry I am late to the party...

My son's 1984 F150 with 4.9L and 4 speed manual OD was "birthed" with a 2.47 rear end ratio. It still has the feedback ignition and carburetion system. It was my truck before becoming his so I had the opportunity to make observations for a number of years. With the 2.47 rear end, the truck could approach 40 mph in 1st, but would not idle at less than ~8 MPH. This was rough on the clutch and as has been mentioned previously, one had to anticipate oncoming hills and down shift "before" encountering the hill. One time, an experienced mechanic friend of mine and I took a trip in the truck and he was driving. He went for miles, as we talked, before I pointed out to him that he was driving @ 65 MPH in 3rd gear. There was no engine noise at that speed. Before the advent of gasohol [in that geographic area], this gear combination would return consistent 26+ mpg @ 65 mph. If a vacuum gauge had been installed, it likely would have indicated a very low number since at that speed, the gas pedal might have been depressed 2/3 of the way. This is contrary to what most will say is the "winning combination" for good gas mileage, but what was happening is that the engine was in the sweet spot regarding torque plus with the throttle being opened widely, the engine had very little pumping loss.

The truck was a "dog" when driving in town. The clutch had to be slipped quite a bit when starting with a load. The only redeeming factor was the engine's fat and flat torque curve. I certainly would not have wanted to ask a 302 to work with this combination!

The rear gears were changed to 3.55 and basically, the O.D. gear had the engine spinning at just about the same speed as was 3rd gear BEFORE the rear end change. The truck became much more pleasurable to drive [even sporty-like]. The drop in engine speed between 2nd and 3rd was less noticeable. The engine sounded a little tight to me @ 70 mph and the gas mileage on the highway dropped to ~21 mpg.

My 1986 truck is equipped the same with the exception of 3.08 rear gears. It drives well, with my conservative driving style, and returns very good gas mileage [18+ in normal driving with as high as 25-27 mpg on the highway, with a little aerodynamic help]. Mine has been converted to Duraspark ignition.

Since your truck has the 9" rear end, I would go with 3.25:1 and stock sized tires. Until corrections are made, your speedometer is going to be WAY off!

 

David, I'd rep you if I could. That's excellent, first-hand info from someone who has driven these trucks with three different rear axle ratios. Thanks!

 

Thanks, Gary. If I may digress just a little, prior to these two trucks, I owned a 1978 F100 LWB Custom. It, too, was powered by the 300 engine but had a C6 automatic transmission with 2.70 rear gears. The truck was heavier and was before FORD designed the aerodynamic improvements of our era [1980-1986] trucks. The best I could ever do on the highway was 16-18 MPG and about 13 MPG in town.

 

Excellent write up, thank you! I'll take that and go with it; 3.25 it is.

Now, I need to decide on doing a re-gear myself, or just try and find a decent 3rd member rebuilt or used. I also am not absolutely sure if it's a big bearing and if it's 28 or 31 spline. From some online research, I'm thinking big bearing and 31 for truck use. 28 was mostly for passenger car.

I had a '73 Mach 1 Mustang Q code 351C + 4 sp. That had a limited slip 3.25 gear and I found out later that they used 28 spline until 3.50 ratio, then went to 31. But again, I don't think that applies to trucks. I'll dig into it some more.