Technical Overdrives, engine size and rear gear ratio- thinking out loud

Hello all, I had some questions in regards to overdrives and rear end gear ratio. I have a 56 F-100 with an inline 6 and a factory Borg Warner overdrive. The truck has factory 4.09 rear end ratio and a pretty small tire (I can't remember the size off hand). The OD works but even with it at 65 the truck "feels" tapped out. I am doing a lot of upgrades this winter, lowering it and such. I am going to swap in an early F100 9", and have a 3.50 gear set and a 3.89 gear set. I am not sure what tire size I will choose, because I am not sure if I will 15" or 16" wheels, but I feel it will probably be an 8.00 15 or like a 7.00 16. I may leave the 6 or eventually upgrade to a mild 292 V8. I know there are a lot of variables in my math equation, but based on this loose info, I am guessing the 3.89 gear set would probably be the wiser choice? Truck won't haul much, but I would like to do 60-70 with ease. I don't want to gear it too high, it is a heavy truck. I have a 53 sunliner with 3.90 gears and non overdrive and the flathead pulls that car 65-70 pretty easy. But, do any of you think the 3.50's may be a better fit?

 

I'd go with the 3.89s. With the .7 OD, that gives you a 2.72 final ratio, still pretty tall. The 3.5 gears would put you at 2.45, I don't think that six would pull that anywhere except on level ground.

 

There are lots of gear calculators on the internet, including many that allow tire size in the calculations. I don't have a handy link right now, but they're easy enough to find.
You probably don't want that six revving much over 2500 at highway speeds.

 

RPM x Tyre diameter [inches] then divide by MPH then divide by 336[constant] = overall ratio
Divide overall ratio by the overdrive ratio = rear end ratio

example
1: 2400 rpm x 26 inches = 62400
2: 62400 ÷ 65 mph = 960
3: 960 ÷ 336 = 2.857 overall ratio
4: 2.857 ÷ 0.7 [overdrive] = 4.08 rear end ratio

 

My thinking as well

Thanks, I am not good at math, but that calculator is great. I tried it on a 30" tall tire, 70 MPH and 2500 RPM...Came out at 4.40. Pretty easy to figure out once you have the formula. Thanks you guys. 3.89 it is.

 

I put an overdrive in a Ford Econline van with 4:11 gears, 200 six, 14 inch tires. Was a wonderful combo, of power, ease of crusing and economy. It’s eay to gear too high with an overdrive. My 200 was a little more comfortable than your 223 spinning some rpm. With you choice of gears I would definitely go with the 3:89 instead of the 3:50 with your six. Just my .02.



Bones

 

Food for thought:

It probably takes 50 to 70 horsepower to push your truck along at 70 mph, on flat level ground, depending on tire type and pressure, drive-train resistance, etc. If my memory serves, a 1956 Ford 223 was rated at 137 PEAK Horsepower. Like Boneyard51 alluded to, it is easy to overgear that truck and get into an RPM range that can not produce the necessary power 50 to 70 Hp to maintain that speed, especially when a head wind or hills are added into the mix.

 

I think 50-70 hp for a level road is a little high if the engine is stock I'd stick to the rpm levels from the factory. It's all about the combo I love those I have a 283 big cam and a tunnel ram with 2x 4s what ratio do I need to run 1800 rpm at 70 mph.

 

http://www.wallaceracing.com/Calculators.htm should know what your engine likes rpm wise . best rpm for power.

 

That safety bead isn't that big of a deal. Lots of '50s cars didn't have those and ran tubeless tires as OEM. The DOT didn't require the beads until the mid '60s, long after tubeless tires became standard on virtually every car that didn't have wire wheels.

 

I had a 1960 F100 with the 223 engine and it ran 3.89:1 diff ratio from the factory.
They could also be optioned in those days with a factory overdrive unit.
I am unsure of what the O/D improvement ratio was but it does go to show that this combination is about right for what you have now.
My F100 referred to above ran non factory 235/75/15" rims.
With no O/D it struggled to hit 65mph and if run all day at that speed all it was doing is wearing the engine out.
Always nice to know at what rpm your engine makes the best torque and try and fit that knowledge into what you are trying to achieve.
I found towing was what I was going to be doing a lot of so I was limited to be able to tow trailers and not bog down.
The other thing I found for my situation with towing was with only a 3 speed tranny, I was always left wanting for that extra gear between 2nd and top.
It's called reverse engineering...

 

370 would be ideal for that
Mine had 390 gears and I thought it could use a little less gearing

 

The 3.89 was a common ratio for the OD cars back when Ford still sold them, but it was usually reserved for the V8 cars. The six cylinder cars all got 4+ ratios except on special order.

You want to be careful about 'overgearing'. A motorcycle forum I'm on (XS11 Yamaha) has had guys experiment with taller final drive ratios and the end result was it reduced top speed as the motor wouldn't pull that ratio in top gear. The bike would go faster in 4th compared to 5th. There's also a danger of spinning the motor too slow at cruise speeds, sometimes leading to bearing damage.

 

I substitute teach at a local school, and if I'm in a math class, I throw out this as an extra-credit problem that usually gets answered by the smartest in the class:
I have a hotrod truck with an AOD transmission that has a .67 overdrive gear, tires with a diameter of 31 inches and a rear axle ratio of 5.14/1. I don't want it to exceed 2,600 rpm on the highway. What's my highest speed to stay within that limit?
Answer: about 70 mph - the state's speed limit.

 

That's like saying 4+4 is approximately 8 to a math professor

 

Steve, I actually had that exact thing happen to me back in about 1976. 750 Honda, with stock gearing would top out at 115, but seemed to running too high rpm crusing. Time for new chain and sprockets, so I geared it up some. Crused a lot better, but in fifth/top gear 105 was tops. 115 in fourth gear. Stayed that way till I sold it.



Bones

 

These are what I use to choose tire size, gearing and what to expect. Mine is 30" tire, 4.11, 3spd o/d and a rpm of 2000 @ 68 ish mph (261 chevy 6)

https://tiresize.com/comparison/
http://www.accuautoparts.com/calculator.php

 

The rpm is the variable.......doesn’t ‘want’ to exceed 2600 rpm.....so ‘about’ is fair

 

I upgrade that F100 to 3.54:1 gears with a SBC engine and T700R4 O/D.
That is a great combination as a street driver, but I would have much preferred in hindsight to have gone with 3.70:1 gears for more versatility when towing.

 

When increasing speed, the horsepower needed to overcome aerodynamic drag is "squared" to the increase in speed. [meaning: If you double your speed you need 4 x the power]

If a truck requires 50HP to drive at a constant 55 MPH and you need to increase this to 75 MPH
that is an increase of 36% from 55 MPH [or 1.36 for calculations]
1.36 squared is 1.8496 [1.36 x 1.36 = 1.8496]

50 HP x 1.8496 = 92.48 HP which would be "asking a lot" from lower RPM's with overdrive.

I see this a lot when people try to increase top speed by simply changing the ratios.

 

More food for thought: The stock 50s Fords with 0.7 OD had 4.11 rears and the stock Mercs has 4.27 rears, but the Merc was heavier with taller tires. Each had about 100 horsepower.

 

The flathead in my 51 mercury with od weighs probably what your truck weighs if no more the stock rear end is 4:27 with the od in it runs at 2200 rpm at 65 the flatty seems to like it there. The 292 will be a better choice and you might want to go with a 3:73 gear with the od.

 

I have a 235/6 , 3 speed OD in my 55 Chevy 210. I know what you mean by tapped out, it feels like running out of gear. I changed to a 3.55 gear ratio and it is great for highway driving. I rarely used 1st gear with the 4.11 rearend, with the 3.55 it pulls great in all three and I still have the overdrive when I want to go fast.

 

Kerry,
That is a very good explanation of how aerodynamic drag affects the feasibility of various gear ratios. Most folks don't understand that the total vehicle package and the operating circumstances will determine the range of gear ratios to be considered. In today's world of computer modeling, the automobile manufacturers can dial in a combination close to optimum for a given vehicle.
My experience shows that Hotrodders fall prey to the following thinking:

IALIGMIB - If A Little Is Good, More Is Better
or
"My buddies car is just like mine, so I'll use what he does"

Really? Same engine, size, cam, induction, exhaust? Same weight? Same transmission, tire size, frontal area?

I developed a procedure for optimizing gear ratios for my Quickchange customers. It is rather lengthy, so I'll not post it here unless there is interest. That is precisely what peaked my interest in the QC in the first place. In a Champ size QC, there are 39 common change gear sets, giving 77 different final drive ratios for a given R&P. In most cases the steps are less than .05. Range with a 4.12 R&P is 2.19 - 7.76.

My mentor, Joy Fair, told me that in oval track racing his experience was that the driver can't differentiate changes less than 1/4 ratio.

Mike51Merc,
That is a very astute observation. I think you identified the reasons for the .16 difference between the Ford and Mercury. So, back in the day (before computer modeling) somebody was thinking about this.

One last thought: The overdrive transmissions from the HAMB era were mainly for fuel economy on flat ground steady state cruising. In the Ford/Mercury example above the final drive ratios were 2.88:1 and 2.99:1, not the low to mid 2's we see in late model, 6 speed transmission cars.

 

That is your perception and it is real. I have a 3.90 and B-W .7 overdrive, 235 6 cylinder in my '49 Chevy 1/2 ton. Maybe similar weight to your truck. I live in the Piedmont region of Virginia with rolling hills and a local mountain pass that I go over maybe once a month. My wheels are artillery type 17" and are 30.9".

The smartest addition I ever made was adding the overdrive, I thought. Approximately 30k miles ago I acquired a Motor Minder Vacuum gauge and it changed my highway driving 100%. I now seek out the 'Green Zone' of optimum vacuum and I know I have easily saved the cost of that NOS period piece.

Conclusion: Perception is real, until a reliable gauge teaches you that you were driving incorrectly given the demands of terrain and engine.

 

My later model ( 1970 ) 250 likes 2400 rpm at highway speeds, 65-70. Fuel mileage is 21 on level ground but head winds will knock it back 2 or 3 mpg. This is in my '37 Chevrolet truck, 3300 lbs, slight down in the front stance. Wind is a killer! 3.08 gear, 1:1 final drive automatic trans. This is a 100 hp at wheels engine.

 

Also to add; While increased fuel economy &/or speed was the main selling point, HAMB-era OD's were also used to extend the life of the mill (usually seriously dead at ~100k, or at least needing a rebuild by then), & also to quiet things down. NVH was lessened by more than the ~30% reduction in engine & trans rpm & associated noises.

GQCE; if you don't mind all that much posting the gear-choice optimization formula, I'd be real happy, n thankful. Or PM it to me? Either way, if you can, whenever, TIA.

I can delete the following Q's n start another thread on these subjects, if they don't fit in here. But:
While I'm asking & we're on the subject of OD n QC... What I'd like to know is...
Since I've read somewhere(Must be true... ) that the QC back gears took ~20hp loss to run. What accurate info do you have on that? & how much can the gears be thinned vs strength vs power consumption? Straight cut n helical?
Also on the R&P; is there a listing of the drag = required hp consumed to run certain "styles"? Say, the ford banjo w/the pinion centered consuming the least (xx) to the ford 9" type offset consuming the most (xx)?
& last, on the HAMB-era OD's, power consumption in OD(since 1:1 requires the least)? & what is the formula for figuring out if the OD is strong enough to live behind some whatever-mill? Like say behind a supercharged hemi, or Stude, or whatever. It seems they are rated in 10k or 20k lbs, (for towing? IDK, wtf?), but I've never been able to find anything to convert back into HP, or preferably into lb/ft output from some mill. Like say, torque input into OD limits, & torque output from OD limits?

TIA.
Marcus...

 

How does the 223 compare to a 235 Chevy engine? Reason I ask, I put a D44 w/3.54's in my '59 1/2 ton. Sure could have used another gear. My 235 was stock. Trucks are similar...couple of brick walls pushing air

 

This is a page from an old catalog I published. The formatting isn't great in the conversion but I think it will show the procedure.

CARE AND FEEDING OF YOUR NEW QUICK CHANGE REAR END
RATIO SELECTION

This is the fun part for a Gearhead. You probably chose your first ratio based on what you thought you would need. Now, you may want to make some adjustment or maybe you just want to smoke the tires till they go flat. If you want to fry the hides just get the lowest gear available put the big one on the top and do it.

If you are fine tuning, here is the procedure:

Drive down the road at a constant speed in high gear. Choose a speed that you would like to be able to accelerate from without shifting. Look at the tachometer (you did put a tach in didn’t you?)


Step down on the throttle and see if it pulls steadily. If you break the tires loose or the reaction is too violent, you need taller gears (lower number). If it feels like you put your foot in a bucket of oatmeal you need to drop the gear (higher number). How much change is needed is determined by repeating the procedure at a different speed, lower speed if you need more gear, higher speed if you need less.


When you get the right feel note the tach reading again. From this you have determined the RPM that the engine likes to cruise at. Now, take the RPM at your desired cruising speed and divide it by the RPM that the engine likes. This will give you a ratio.

Example:


RPM @ desired cruising speed 2550 = 1.159 (ratio change needed)
RPM that engine pulls from 2200


Now take the ratio and divide it into your present overall rear end ratio.



Present gear ratio 4.56_ = 3.93 new gear ratio
Ratio from above equation 1.159


Let’s reverse this just for the heck of it.


RPM @ cruising speed 2200 = .863 (ratio change needed)
RPM engine likes 2550

Present gear ratio 4.56 = 5.28 (now there’s a gear)
.863

 

Thank you. Marcus...