Ok so I am trying to pick out what gear wood be best for me. I have the stock 48 rear end. I am turning about 2800 to 3100 55mph 60 mph. Wood 3.54 bring it down a lot or not much. Or should I go with 3.25. Tire size is 205/35/15 I think. It's a 1948 merc with flathead and 3spd. I like to able to go 60 on freeway with 2200rpm 2400rpm. I don't go up much hills.
***uming your stock gear is 3.78, and your are turning 3000 rpm at 60 mph with a 205-75 tire, then to turn 2400 rpm with the same tire you would need : 2400 divided by 3000 times 3.78 = 3.02 gear ratio or 3.00 to 1 I don't know where you are going to find gears like that for your rear end.
I'm going to take out Saturday and write down what rpm I am hitting at 60 and 65mph I think may have it wrong.
Make sure the tire size. And what gear ratio you have for sure and come back with that hard core info. Sent from my Z981 using The H.A.M.B. mobile app
The normal rear axle ratio for 39-48 Mercury car is 3.54. Before getting excited about changing things, verify what you already have. In my experience a well balanced flathead likes to spin around 2600 rpm at highway speed. Much lower than that bogs the engine down. With factory diameter tires the 3.54 ratio is perfect for those rpms and 60-65 miles per hour. Also, are you really running 205 / 35 / 15 tires? Those would be about 20 inches tall. The factory tires are 28 inches tall, a huge difference.
OLD rule of thum from my roundy-pounder days : For each POINT of change in gear ratio (.0) will affect about 100 RPM in engine speed - with all other factors remaing the same. In the OP's case, going from a 3.54 down to a 3.25 will REDUCE engine speed by 300rpm, at the same road speed. Going from a 3.25 to a 3.54 will INCREASE RPM's by 300. WHAT is the stock ratio? WHAT is the 1st-gear ratio? Is your area HILLY, and you'll need to smoke the clutch to take off on a hill? Gear ratio is a COMPROMISE between take-off grunt and easier freeway crusing. Going too tall out back might make the car a PITA around town, at lower speeds. Kinda why most guys ditch the stock driveline for 4 or 5-speeds with a broader varity of gear ratios in the 'box and more ratios to choose from for a later diff. Easier to locate parts too! Good Luck, Tim
Flip the rearend housing upside down, there will be numbers stamped into the 1/2" or so wide web that runs from the torque tube collar to the now top of the center section... the stamped numbers are the number of teeth on each gear, divide the I think its a 9, into the other #, that will give you the ratio the car had when it left the factory... rock on !
Ok the stock speedo says 60mph at 2600rpm on 205/35/15 tires. Any thing p*** 60mph is 2800rpm to 3000rpm
I didn't believe such an animal existed, but it does! This is what the diameter is according to the tire site: 20.65 inches (524.5) The second thing I can't believe is that you would run such a tiny diameter tire on a car like that! Any wonder it is spinning high revs. For god's sake man, put a proper tire on it, like a 70 or 75 and life will return to normal.
Thomas marmont. Get all the correct info from your car man. Go and write down what your tire size is. Maybe take a pictures. Also go look what your ratio is You got these guys answering things and your mistaken your own stuff. I've said this yesterday. Sent from my Z981 using The H.A.M.B. mobile app
lowest Ford went was 3:54's and I ***ume they had a reason for that. A buddy of mine had a '47 with a ford 9" with 3:00 behind a flathead and he'd run out of poop trying to pull any hill.
One possibility is consider switching to an original Mercury overdrive transmission. Occasionally they are posted for sale on here and the Ford Barn complete with controls etc. That would allow the engine run strong with the original rear end ratio. The flathead can still make rpm for torque for the hills running in 3rd gear and then be able to run with the OD where it is easy cruising. That may avoid the risk of under powering it with a 3.25 rear end when it comes to hills that would force a downshift anyways with higher rpm or hold up traffic. The engine is compatible with the factory OD transmission so it is a low risk upgrade. Use the advance search on the HAMB & Ford Barn to check if there are any for sale.
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