They invented a neat way to calculate that a long time ago, F=ma.

I've been thinking awhile back of mounting a simple pull spring weigh and measure how much pull it takes to get the car rolling, and to keep it rolling also.
Forwards and backwards, and with and without the car idling in Neutral, to as closely mimic the transmission's state with oil pressure present.

I have pushed against the rear bumper of my pickup with a common bathroom scale.

It is hard to find perfectly flat pavement so you should repeat such a test in the opposite direction.

My prefered test is to coast down an approximately 4% grade hill and measure the steady terminal velocity due to gravity.
Do this at three or four different vehicle weights and you can estimate the three unknown aero Cd, tire rolling resistance and drivetrain friction loss.

long story here...
my first car was a '79 Grand LeMans when i was 14 in 1991. Nice enough car, factory buckets, and console. 350 chevy from a '73 monte carlo( according to the numbers)
being i had a few years before i could drive it, and being young and dumb, not fully understanding drivetrain " combinations' , i decided to start modifying. i did a comp cams " 268 high energy" cam, dual plane intake/650 holley, and cheap headers, and a custom dual exhaust that my dad paid for as a gift. ( the car was flatbeded to the muffler shop for this, as i still has a few months before i got my license.)
now comes the first drive... should move pretty good with the things i did right? NO.
the thing was a stone. i remember getting passed by a full size dodge conversion van, who looking back now, probably didn't even know i was floored... and it got horrible mileage. BUT it did loaf along at something like 2,000 rpm at 70...
after 1 to many 1 wheel burnouts, the rear exploded. THAT'S when i started to understand it's not just about the engine.... pulling the cover and counting ( what was left) of the teeth, i had to count multiple times to believe what i was seeing.. a 2.29 final drive ratio. being a broke kid going to high school working something like 20 hours a week, for minimum wage, i borrowed money from my father to buy a complete rear from a monte carlo SS. that had 3.73's.
Holy Moly! did it wake that car up! it would burn the tires going into 2nd gear. not only did the performance improve, so did the mileage! i could not go almost 2 weeks before fill up, where as before, it was at least once a week! but hey, premium was only about $1 then!

Those super tall axle ratios were used before the overdrive transmissions became common. I recall Trans Ams with 2.41 and 2.56 diffs. Ma Mopar had 2.21 ratios in their Fifth Avenue cars.

I have got in trouble on forums for writing this, but if you take an engineering class on automotive design, in the sections on transmissions and gearing you will be taught that engineering convention years ago (1930s) developed the term “overdrive ratio” and it did not originally mean a gear or separate unit like a Gearvendors.

Engineers originally had the straightforward thought that:
“Top gear should be set to let the maximum horsepower of the engine get the vehicle up to its top speed on level highways”.

If this was set correctly, the “overdrive ratio” was 1.00
regardless whether the driveshaft was turing at a lower rpm than the engine.

If the the vehicle did not reach its theoretical top speed and its engine was BELOW the rpm of maximum horsepower, then the “overdrive ratio” was less than 1.00 and maybe 0.90.

If the vehicle did not reach its top speed and the engine rpm was ABOVE its rpm of maximum horsepower, the the “overdrive ratio” was a number above 1.00, say 1.1

Books like the Bosch Automotive Handbook have a German engineering slant and still teach it this way in their section on transmissions.

Yes, it is more “word definitions” but one ought to respect the past traditions.
When Watt defined a “horsepower” for his steam engine it became a tradition even if that amount of power would quickly kill even a Cyldesdale sized horse.

Just checked my van rpm to speed, 4.10 2930-50 @ 60 mph for 15 miles on a hand held diagnostic. 5.9 Magnum 2001. Couple of questions, is this a good rpm for that engines "sweet" spot? Also It's got a lock up converter but the rpm, set at 60 mph on cruise, fluctuates from 1888 to 1955 up and down slight grades but the hand held never goes off 60, is this normal for a new lockup?
Take out of overdrive it goes to 2770 rpm @ 60
Thanks

my 2000 dodge van with a 5.9 and 3.92s is close to that fluctuation and it definately locks solid in lock up. It will not hold the speed as precise as my 2011 1500 ram did on the data stream on my scanner or in real life on the freeway running side by side.

I have a set of 3.21s to install, I am positive they will give me better highway MPG.

It has more rpm in overdrive?

"Take out of overdrive"

Interesting, my scanner would stay at 60 then going down a hill would creep to 61 but rpm would go down to 1888. All over the place. I'm thinking the converter is not locking up to well (all new parts, electrical and trans). Will the lockup fluctuate 100 rpm?
I have a set of 3.54 for the Dana that I would like to try. Talked to the Dodge service man, he said no problem reflashing pcm for the ratio change.

I don't know what your combo is, but I have a 2002 Ram 1500 van 5.9 with 3.55's. 255/70/15 tire. The van gets 11mpg. Mine is a high top conversion so I think the limiting factor is the giant sail effect.

It has the Hughes baby cam and EQ heads, magnaflow exhaust and cat. After the rebuild and add ons it still only gets .5 mile per gallon better than the old factory stock all oem van with 155k.

Even switched to electric fan, still no difference. I am about to rebuild the rear end and thinking of switching to 3.92.

Dont waste money taking it to the dodge dealer for a "flash" after a ratio change, the vehicle speed is measured after the gears and will not affect anything computer related changing gears.

I have worked with so many of these I am 100% confident it will improve my MPG.

2001 3500 5.9 4.10 gears 245/75R 16 tires, I get around 13-14 mpg now. 186,000 miles, new chain, fixed the gasket on the bottom of intake.

What about the speed sensor on the trans (where the old speedo cable went)?

ok, i'm still not understanding. " 4.10 2930-50@60 mph."
then you say " take out of overdrive" it goes to 2770@60"
if you take it out of overdrive, the rpm should increase.
or am i reading this wrong?

Sorry I didn't preview the post very well, rpm should of read 1930 in overdrive.

Looking at the manifold pressure on the scanner, or simply hooking up a vacuum gauge, will help you find the “island of best fuel economy” in the sea of engine performance.

Notice that in nearly all BSFC graphs the best spot is at low vacuum but at an engine speed where the pistons are moving less than 1500 feet per minute.

Another way of saying this is:

Gasoline engines get best fuel economy when
the engine is making high torque
without scraping the piston rings against bore walls too fast.

High torque,
Relative 'slow' RPM/speed,
Low vacuum (so, lots of throttle opening).