New gear ratio - 300 six in a highboy

Hi guys

I've been doing lots of reading and am sending myself into circles trying to make a decision. This is where you come in.

My 1976 F250 highboy has 4.1 gears but I will need to replace at least the front one as it has damaged teeth. While I have the option I am thinking of changing both fronts and rears to 3.73 or 3.55 gears.

The truck has the 4 speed manual and will be running 33's. (possibly 35's in the future). Most of my driving will be at 50 to 60mph (and occasionally 65mph). I will be towing a 5000lb trailer on a regular basis. What gear would you recommend. Below are the expected RPM's at 60mph

4.1's - 2504
3.73 - 2278
3.55 - 2168

This makes the 3.73's as a pretty good intermediate option. Obviously I am trying to strike the balance between power and fuel economy but also looking at the impact of possibly moving to 35's in the future which would result in the following RPM's.

4.1's - 2361
3.73 - 2148
3.55 - 2044

What would you guys recommend?

Cheers
 
Dont you think that 2500 RPM's is too high for sitting on 60mph without getting less than about 10mpg...even unloaded?
 

sixtseventwo4d

Well-known member
The two things I weighed out was the regular towing of 5000 lb trailor. And that you were considering goin to 35's.
It would stand the 3.73's also and do fine. Consideration should be given for the elevation your driving at.
 
Elevation...haha. Im in Western Australia. Very flat part of the earth and only about 100 feet above sea level.
Yeah. The towing is an issue but I still thought it would was generally recommended to keep the RPM's below 2500. As this is my first 300 I will happily defer to those with better judgement than me.

I should also include that the engine will have a 500cfm Summit carby running through an Offy C Series with the EFI manifolds.

Also, Ive just realised that some of my numbers were a bit off. So lets try that again and completely forget 3.55's. They wont suit.

33's
3.73's at 60mph = 2300 rpm
4.1's at 60mph = 2500 rpm

35's
3.73's at 60mph = 2200 rpm
4.1's at 60mph =2400 rpm

Something that I also need to take into account is that we always look at the higher end numbers for revs, but I will also spend a lot of time doing 45mph and at those speeds I really dont want the motor chugging too low in 4th.

33's
3.73's at 45mph =1700 rpm
4.1's at 45mph = 1900 rpm

35's
3.73's at 45mph =1600 rpm
4.1's at 45mph =1800 rpm

So, what would you do? If I new for certain I was staying with 33's I would go to 3.73. Its the option of the 35's that is causing me the concern. Would 35's make the 3.73's a bad choice or just not optimal?
 

BigBlue94

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4.10s...

I run 4.56s with 37" tires, and at 70mph, im around 3000 rpms, with an NP435 trans.With the intake, big carb and efi manifolds, you should have power to about 5000 rpms. I run mine on the highway at 3000 rpms with no problems. Done a couple 12hr, 700 mile trips from 1000' to 13,000' above sea level and back down. Just need to make sure the engine is on good shape with good cooling and oil systems.

I get 12mpg on the highway, in a 6" lifted 85 bronco
 

bubba22349

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Don't have a direct compareision but maybe it will be of some help. I had a 1965 F350 for many years it was built to tow or haul lots of weight and it did that very well. Was a short wheel base with stake bed and dump was just under 6,000 Lb. 300 six, NP 435 4 speed, 4:88 gears, 255 70 16 wheel / tire combo of 30.1 inches diameter. On hyway it topped out or stopped pulling at 63 to 64 MPH so there wasn't any point in pushing it any higher, I didn't have a tach but estimate that was at 2900 to 3000 RPM I usally got 15 MPG if I drove it at 55 to 60 MPH. My 300 six that i built for it as a medium duty low compression engine almost stock except for a Melling cam which gave it great pulling torque from idle RPM on up. But it didn't like the sustained higher RPM's, it needed an overdrive for running at any higher speeds. I am sure your 300 has a higher compression ratio so it will have more top end power, I think if your going up to 35's and are towing often then the 4.10 are good the 3:73 might also work good if you don't tow as much. Best of luck
 

BigBlue94

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Another example...

My 99 e250 has a 4.2L v6 making 200 HP and 250ft/lbs with a 4r70W trans and 3.73 gears turning 31" tires. I recently pulled my 2015 john deere gator on a sturdy 12ft trailer on a 3hr trip. Trip was through SE KS, where there are some good hills to tax the engine. Didnt really struggle too much, but I do wish id had 4.10s. Would have kept the trans from shifting as much.
 
Thanks for all of the replies and sharing of knowledge.

I wont be changing the gearbox to an overdrive.

As this truck is not currently driveable and is being rebuilt I have never had the chance to drive it with its existing gears. Also, as one of my current ring gears is damaged I cant even test out the existing 4.1's without having to get alot of work done. Aaaargh.

I guess what has clouded my decision making is trying to achieve respectable MPG figures (hoping for 16mpg average) while not making the truck a dog with regards to acceleration.

Also, while I will be towing on a regular basis, it is probably only 20% of the time and even then it will be at a max mostly between 50 and 60mph for about 50 miles.. Most of my driving will be unloaded. So with that in mind I reckon that irrespective of using 33's or 35's I should be ok with 3.73's.

With 33's I'd be looking at between 1900 and 2300 RPM's from 50 to 60mph.
With 35's I'd be looking at between 1800 and 2200 RPM's from 50 to 60mph.

Being only a 4 speed I need to get my head around the fact that 4th will primarily be my Overdrive, even though it is 1:1.

Im going to post a question about Lockers in a minute but will open a new page for that.
 

BigBlue94

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I think the 3.73 will be a good 'compromise'. My bronco has the 4.56 and no OD turning 37s. If I did have overdrive, I would want 5.13 gears. Especially if it was an automatic. Ive now had two broncos on 37s. First was an E4OD with 4.88s. The ONLY thing it did better than my current one was cruise at a lower RPM. But it didnt get any better milage, with a healthy 351. In fact it got about 10mpg whereas i get 12 in the current one.
 
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