gear ratio and engine speed and mph.

rer_239

rer_239

Senior Member
tried the search but couldn't fine the answer. i have a 4.11 gear set and plan on using 16" wheels (tire dia 28.36). that figures out to 2440 rpm. is that too high for a stock engine/trans?
 
You need to measure the road surface to the center of the axle to get the radius, then multiply it in the formulae. This is because the tire has a shorter radius where it contacts the road.
 
rer_239 said:
tried the search but couldn't fine the answer. i have a 4.11 gear set and plan on using 16" wheels (tire dia 28.36). that figures out to 2440 rpm. is that too high for a stock engine/trans?
Click to expand...

2440 RPM at what speed?

I have run my Model A engine at that RPM at times. It is very well balanced, so quite smooth.

PM me for details.
 
There really isn't enough info on your car. I can tell you that for a street/ touring car you will soon tire of the 2440 rpm. You want to be down around 2000 rpm for a pleasant enjoyable car. The A motor has a lot of torque and you can be happy with the lower rpms. Henry had it about right!
 
Here are the formulas, MPH= RPM x Tire diameter, divided by Gear Ratio x 336. RPM= MPH x Gear Ratio x 336 divided by Tire Diameter. Gear Ratio = RPM x Tire Diameter, divided by MPH x 336. Tire Diameter = MPH x Gear Ratio x 336 dived by RPM.
 
With a 3.78 rear end and 14.4 inches from road surface to center of wheel, the rpm / 44 equals the mph. So on the 3.78 rear end 2440 rpm divided by 44 = about 55.5 mph. to correct this for the 4.11 rear end take the 3.78/4.11 = .92 55.5 times .92 = about 51 mph at 2440 rpm. I do not think that 2440 rpm is too high. I like 2200 rpm better, but I think you are OK.
 
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