I have a 49 ford original with OD with a 3.78 gear. Question, will a 3.54 gear give better gas mileage. Thanks for any info. Slim.
If your '49 motor is stock it will have a tough time pulling a 3:54 gear in overdrive. You will be shifting a lot, or find roads that are all downhill. A 3:78 is a good gear for this car.
Mileage could be the same, better or worse........depends on whether or not the engine works 'harder' i.e. wider throttle opening, lower manifold vacuum, results in poorer fuel economy. IIRC, factory equipped O/D shoebox Fords had a 4.10 ratio. In which case the O/D may produce the desired improvement. With your stated 3.78......maybe not so much. IMO, the minimal potential gain, and possible loss, makes the effort , and associated expense, not worth it. Overdrive has the best potential for fuel economy improvement when the engine, at normal highway speeds, has 'surplus' torque available to propel the vehicle at reduced rpm. Ray
Ray is right on the money. I'm in the same process right now, 56,000 mile '50 Shoebox. Was originally a 3 speed car but I found an OD and had it rebuilt. I'm running the 3 speed rear gears as well, hoping to improve mileage and cruising speed. My 8BA is warmed up a little with heads, cam, aluminum flywheel, and dual 97's, but just a clean up bore and no stroke. Blueprinted and balanced, line bored and decked, Johnson adjustable lifters, full flow oil system etc. Hoping to get 140+ horsepower and 210 lb. ft of torque. Highway speeds around here are 75 and some areas are 80. I'm hoping I'll have the torque to comfortably run 75 and pull hills without dropping out of OD. Wish me luck. Should have the car on the road in a few weeks. Sent from my DROID device using the TJJ mobile app
You can go to an online RPM calculator and see what your final RPM is. Try here: http://www.csgnetwork.com/multirpmcalc.html If you were running a 3.90 gear and dropped down to 3.54 with OD, I'd say it'd be worth it. The split from 3.78 to 3.54, with OD, I don't think is worth the expense... probably only a couple hundred RPM at freeway speeds. And as someone else said, you might have a hard time pulling that going up hills, in OD. -Brad
Vacume gage is your best friend! when you keep it green your doing the best mileage you can get. any set up is a crap shoot till you get on the road. that right foot is always the problem. most car can get 200 mpg while coasting down hill throttle closed. the deeper the pedal the lower the MPG. I had a friend with an old Studebaker flat six, OD and a vacume gage in the dash. could pull consistant 35 mpg till it hit the hills.
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