I would like to get some opinion on rear end gearing. I have a 1953 Mercury Flathead with a 1952 Mercury 3 speed transmission what is the best gearing I should go with and having the correct combination. Just a regular street car 1929 Track T project just want to have it reliable on the freeway cruising. I appreciate everyone input Thanks Frenchy
Selecting Proper Rearend Gear Ratio A number of factors must be considered when selecting a rearend gear ratio (ring & pinion set). Tire diameter, transmission choice (with or without overdrive) and desired highway cruising speed. Tire diameter is the first consideration because styling and available space within the wheel opening dictate the size of tire / wheel combination that will be chosen for the car. The choice of a direct-drive (1to1) or overdrive (usually 30%) transmission will determine driveshaft RPM at cruising speed. A highway cruising speed of 60—65 MPH is considered a good target speed. Engine RPM at cruising speed should be maintained between 1800 to 2100 RPM. Lower RPM at cruising speed will produce a vibration that is often mistaken for driveshaft imbalance. This vibration is actually caused in a typical V8 engine by acceleration of the piston on a downward power stroke that is only cancelled out by other cylinders doing the same in rapid succession. This only occurs when the engine is revving in excess of 1800 RPM. This vibration isn’t noticeable when the engine is idling or accelerating through the gears but is often misdiagnosed as a driveshaft out of balance of out of phase. The proper rearend gear ratio can be selected using the following simple formulas whether using a direct-drive (1to1) or overdrive (usually 30%) transmission. FIGURING TIRE DIAMETER (Height) FORMULA: Tire Size ÷ 25.4 x Aspect Ratio x 2 + Wheel Diameter = Tire Diameter in Inches (Height) EXAMPLE (285x70R15): 285 ÷ 25.4 = 11.22 x .70 = 7.8540 x 2 = 15.70 + 15 = 30.70 FIGURING TIRE CIRCUMFERENCE FORMULA: Pi (3.14159) x Tire Diameter = Circumference in Inches EXAMPLE (285x70R15): 3.14159 x 30.70 = 96.446813 CONVERTING TIRE CIRCUMFERENCE TO REVOLUTIONS PER MILE FORMULA: Tire Circumference in Inches ÷ 12 = Tire Circumference in Feet FORMULA: Number of Feet in a Mile ÷ Tire Circumference in Feet = Tire Revolutions Per Mile EXAMPLE (285x70R15): 96.446813 ÷ 12 = 8.0372344 EXAMPLE (285x70R15): 5280 Feet ÷ 8.0372344 = 656.94239 Below is a simpler formula used to figure tire revolutions per mile with an acceptable variation from the more exacting formula using tire circumference to figure tire revolutions per mile. FIGURING TIRE REVOLUTIONS PER MILE FORMULA: 20168 ÷ Tire Diameter = Tire Revolutions Per Mile EXAMPLE (285x70R15): 20168 ÷ 30.70 = 656.93811 FIGURING REAREND GEAR RATIO (Ring & Pinion) FORMULA: Engine RPM @ Cruise ÷ Tire Revolutions Per Mile = Rearend Gear Ratio EXAMPLE Without Overdrive: 1800 RPM ÷ 656.93811 = 2.73:1 EXAMPLE Without Overdrive: 2000 RPM ÷ 656.93811 = 3.04:1 EXAMPLE With Overdrive (30%): 1800 RPM ÷ 656.93811 = 2.73:1 x 130 = 3.54:1 EXAMPLE With Overdrive (30%): 2000 RPM ÷ 656.93811 = 3.04:1 x 130 = 3.95:1
Frenchy, assuming you wish to run something like a 7.00 x16 on the back, anything in the 3.54 - 3.73 range is gonna be happy at our somewhat ellevated freeway speeds around here at the expense of a quick takeoff. With something light weight though, acceleration will still be pretty strong.
I don't know....I just took a 1000 mile trip in my 57 chevy one ton truck, with 31" tall tires, 3.73 gears, and manual (non-overdrive) 4 speed trans. It was happy at around 68 mph. I wish I had some 3.42s to put in the rearend.
For what v-8 engine are you calculating your 1800-2100 rpm cruising speed? Or from what book did you copy your information. I know that you are not a builder or you would not have made a blanket statement like that. Some engines are lugging at that RPM which also causes a vibration. Engine cruising speed is based on engine build, can be effected by operating range of the cam, shaft as well as compression, heafds used, flywheel/crank shaft weight, type of fuel, intake design, ignition or any combination of the above. He could also spend hours with a calculater ot slide rule. or he could wimp out and use one of these to calculate engine RPM at given speed. http://tri-5.chevyrides.com/rpm.php
Here's a calculator that will do the math: http://www.roadkillcustoms.com/.../Differential-Gear-Ratio-Calculator.asp
squirrel, if memory serves correct, that Merc trans has a pretty tall first gear in it. If it was mine I would difinitely err to the 3.54 side of the equasion myself. Maybe a touch lower as you suggest.
Just keep in mind that there is a tradeoff there...gearing it for lower rpm on the highway, means you lose acceleration, and get to do more downshifting.
You mean a touch higher? or maybe lower... I don't know.... http://www.classicoldsmobile.com/tech/69.shtml
I have a 378 in my rear end which is out of a 1947 Ford truck. I have the 1932 Ford with the V12 engine and it is running well. And it has the 39 Zephyr trans. Thanks for the great feedback. Frenchy
Here is something else that I missed in your analysis of gear calcualtion. Drievshaft speed is at cruising or whatever speed is not determined by finl drive gear not by transmission final output. Crankshaft speed or engine RPM asr altered by the output gearing of your transmission but your driveshaft will be turning the same speed for a given highway speed regardless. I am not picking on you, this is very well written. It is just that it is not good information. Someone somewhere down the road is going to read it and say gee this is correct and run with it.
Ok nowing nothing about the engine other than it is a Merc I am going to agree with R frederick. A 3.5 or a 3.55 in a light car producing 100 hp is a pretty good tradoff. With a 30" tire it should be turning about 2600 RPM not allowing for parisitical loss. A stock flatty should be real comfortable at that speed.
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