Got a 48 59A-B flathead with a c4 transmission with 35inch tall tires. What’s a gear ratio I should be looking for in an axle. For a 30 model A Tudor Show car. Can only drive it 6 months out of the year. (Live in the upper Midwest) don’t see myself driving this thing on the interstate much. 60-65 cruising speed. Thanks in advance. Sent from my iPhone using The H.A.M.B. mobile app
A 35” tire turns 593 revolutions per mile. 60 miles per hour is 1 mile per minute. So if you take 593 and multiply it by your gear ratio, you get your RPM at 60 if you have a 1:1 final transmission ratio. For example, if you have a 3.50 rear gear, 3.50 x 593 = 2076 rpm at 60mph.
Car don’t make any difference, he gave the engine, transmission and tire height! The only thing he didn’t give.... and it’s important!! What is your optimum speed? With out that it cannot be figured! Bones
Ah yes...the ol..."lack of details" post..! Is this - 1. A show car that won't get driven much ? 2. Is this a weekend cruiser that will be driven around town ? 3. Is this a car that will see many freeway miles ? 4. Is this a car that will be making long freeway trips ? 5. Going drag racing ? 6. Making trips down the Bonneville Salt Flats ? Etc., etc. Mike
Engine modifications, or more specifically HP ratings, are a key consideration. Too high a ratio (low numerically) could result in necessity to downshift on a slight grade, or worse yet, lower top speed in high gear than the next lower gear. Also, 35" tire diameter seems awfully large. Typically, stock Fords with V8s used a 3.78 ratio, with 28" diameter tires. Multiplied by the wheel diameter ratio gets you to a ratio of about 4.7 for reasonable drivability. Most Model As were 3.78, or optional 4.11. A very few had 4.56 ratio; good luck finding one!
Here is a simpler tire/gear/speed = RPM in ------- calculator that works pretty good for transmissions with 1 to 1 output ratios but may not be that accurate on OD trans https://spicerparts.com/calculators/engine-rpm-calculator
I understand the engine makes a difference, but he already stated he had a Flathead with a C-4 Auto. Chances are he has or is going to have a modern rear end. The most important thing he left out was the optimum driving speed. With the little info he provided, we already know this is probably not a real high performance engine, or he would have stated that fact. We know he has slippage with the C4 and tall tires. You are going to have to turn that Flathead a little higher that a larger V-8 in order to get enough horsepower to pull the car and C-4. So the rpm we should shoot for at his speed should be around 2800 to 3200+ at high cruising speed. Which he didn’t tell us. Definitely not enough info to answer! Bones
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