I'm finally driving my hotrod a little and found right away I need taller gears, the 3.78s just don't do it. It turns about 2600 rpms at 55 mph. I would like to try some 3.25s but I can't find anything at all for the open driveshaft. Am I overlooking something?
Open driveshaft has nothing to do with your gearing. https://www.speedwaymotors.com/shop/1940-ford-motive-gear~49-1940-54-10245
To aid advice we need to know your rear tire height. There is an online rpm calculator also. 55mph X 3.54 gears X 366 ( a constant) divided by 31 ( tire height) will put you at 2,454 rpm. The numbers you gave indicate your tires may be 29”.
The ones at Speedway are for closed driveshafts unless you buy the adapter. I have a factory open driveshaft rearend from a pickup. My tires are 6.50-16s which are 29" tall and with the 3.78 gears I am at about 2600 rpms which is what the rpm calculator shows. I like that calculator by the way. Using the calculator I need 38" tires to get the rpms where I want them, not happening. So am I stuck buying speedway gears and going with an adapter?
I don't get the same results for that combination. Here is what I get: 3.25 w/ 29" tall tires = 2071 rpm @ 55 mph 3.54 w/ 29" tall tires = 2256 rpm @ 55 mph 3.78 w/ 29" tall tires = 2409 rpm @ 55 mph 4.11 w/ 29" tall tires = 2619 rpm @ 55 mph Example: 55 x 4.11 = 226.05 226.05 X 336 = 75952.8 75952 / 29 = 2619 I think you presently have a 4.11 in there. Now, my question is: Are the open drive pickup pinions 10 spline? If so, what were the available ratios? I just checked and I have a 4.11 R&P 10 spline. I'd bet that was the most common ratio.
What engine? What car? My model a roadster with 3.78s and 31 inch tires, runs 80mph all day at 3200 rpm with a warmed over flathead. I also have a open drive truck rear in the shop and it has 4.11s. That's the most common gear. As far as I know 3.78 open truck rear isn't very common. Sent from my SM-G955U using The H.A.M.B. mobile app
I have 3.78 gears. Actually I have another set also. My yokes and pinions are a smooth taper and a keyway, no splines and 9-34 gears. You need to use 366 instead of 336 in the equation.
The 1942-47 Ford 1/2 ton open drive rear end pinions are not splined. They are tapered and keyed like the axles. They came with all the regular ratios. I've seen several 3.54's in those rears. All the after market gear sets are splined. If you go after market you will need the conversion. If you need the open drive conversion go with Hot Rod Works. They fit right.
I have an 8BA in a 1930 Model A coupe. I guess I never asked what an acceptable rpm is for the flatheads. I would like it closer to 2000 instead of 3000 when cruising down the highway. What rpms do you guys like to run at?
My solution would be to swap the guts from a '49-'51 Mercury overdrive transmission into your toploader box, shorten the driveshaft and be done.
Well that explains it. Thank you. I used that formula and probably just typed it incorrectly to an Excel spreadsheet. Now my numbers agree with yours
I've driven mine for 3 plus hours at 3,000 plus rpm and had absolutely zero issues. My engine is a 59ab with merc crank a small reground cam and 2 strombergs. The 3.25 gear will kill you around town. If you change at all go 3.54 but I would keep the 3.78 and get some 7.50x16 or a 31" rear tire. Sent from my SM-G955U using The H.A.M.B. mobile app
Oh and the speedometer in that picture is GPS and is deadnuts on. Sent from my SM-G955U using The H.A.M.B. mobile app
Dumb question I know but what are you using for an engine and how is it built? Perhaps 2600 is a good cruising RPM for the engine in question? Bear in mind that I am looking at this from a mechanics point of view but it is not good to lug your engine. Also you may be limited as to the gears you can find for your rear.
He said it was an 8ba in a 30 coupe. I really dont see 2600 plus rpm hurting it. I personally would rather have the extra rpm at higher speeds to keep the good pulling rpm around town. Sent from my SM-G955U using The H.A.M.B. mobile app
I had a quick change in my A coupe and usually ran 3.13 gear, my Shoebox had a 3.00 gear. While you can spin them higher mine both seemed to have a cruising sweet spot at about 2200.
Ok I think I will start with taller tires and see what I think of that. It’s the same price as gears but easier to swap out and it will drop me to around 2400 rpm. I’ve never had a flathead V8 so I’m not sure what to expect from them other than expensive dollars to hp ratio. Still cheaper horsepower than my Model T engines by a long shot.
One thing to remember is that the flathead is not the same as the engine on an '05 Silverado. It really wants to spin more then a modern engine.
DYSLEXICS UNTIE! The open drivelines are pickup units as I understand old Ford tech so they will have clod-hopper gears not jack rabbit ones. Both the rears I got to make my dual differential Mysterion rear had 4.11s.
I have a stock 8ba with dual carbs c4 trans 272 gears, seams to like 2500 52 mph 29 1/2 in tire, 31 roadster Sent from my iPhone using The H.A.M.B. mobile app
I don't think you will find any gears below 3.78 for the pickup open drive rear end. I've had the 3.78 & 4.11, and know that a 4.44 existed & maybe some lower They may exist, but I think they would be very rare
Good looking car! The 7.50s will help a ton and I'm sure you'll be fine at speed. Sent from my SM-G955U using The H.A.M.B. mobile app
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