According to my calculations, with 215/75R14 tires, a 2-speed Fordomatic and 3.10 final drive, my Galaxie should be doing 2340 rpm at 60 and 2730 rpm at 70.  I assume the number is actually higher than that, given torque converter slip.  Am I correct in thinking that the rolling tire diameter is smaller than the theoretically calculated one I used to get those numbers as well?

Regardless, I don't feel good about cruising at 2730 rpm (or more).  It just bothers my conscience somehow; I can't help but think of the engine wear and fuel consumption incurred by running at such a high speed.  The engine is smooth and quiet even at 70, and doesn't exactly feel wound out, but even so I know it could move the car along more happily around 2000 rpm.

In order to do anything to change it, I see two options: get a 3-speed and change the rear axle ratio to something more like 2.7, which would bring rpms to 2040 at 60 and 2380 at 70, or put in a 4-speed overdrive auto such as the aod, which would bring rpms down to 1570 at 60 and 1830 at 70 with the stock rear gears (assuming 0.67 overdrive, as per Wikipedia).  Of these two, the overdrive trans is more appealing, because it's probably less work.  Though it may be more expensive.

Option number three is the easiest and the cheapest: don't change anything, and don't worry about it.  I prefer this option, but as I mentioned before, I have a hard time not worrying about it.

What do you guys think?

I've got an idea the original final ratio (including the tire size) was a compromise based on highway "gradability", headwind tolerence, etc. - so the tranny wasn't constantly in kickdown mode. Your test might help decide whether a small change is OK. 

http://www.1010tires.com/TireSizeCalculator

BTW I am not sure when FoMoCo made the Ford O a 2 speed but I thought it was 1960.

Crenwelge is right, nobody looked at the feeble torque that's left with a 2.xx gear.  You did the rpm math, now do the rear wheel torque math.

Your engine efficiency isn't based on low rpm, in fact the Y-Block starts feeling its torque curve oats (and higher efficiency) at 2,500 rpm, then it gets even better with higher rpm.

3,000 rpm is a very comfortable speed for the Y.

I never seriously considered lowering the final drive ratio without upgrading to a three speed because I assumed that there would not be enough torque in the 2-speed first gear to cope adequately with much less than a 3.10.  This seems especialy likely if the Y is weak on low end torque.

Does anyone have an idea of how long these engines can last on modern oil?  I have always had the impression that "old" engines in general last about 100k, but have never heard or read a report on how that number changes with modern oil.  I expect at least 200k out of a well-maintained modern engine.

AZ28, I think you're right.  There were probably a lot of compromises involved with having just two forward ratios to cover everything from steep grades to highway cruising.  Seems like the effect of changing tires size on roadability would be comperable to an equivalent rear gear change--just a lot easier to execute.

Doug T, I'm not sure what the range of production was for the 2 speed; I would have guessed it to predate the Cruise-o-matic of '58 but I'm no expert.  Anyway, the data plate on my car calls for a 2-speed and the factory manual covers it, so they were definitely in production for the '59 model year.

simplyconnected, I didn't think about calculating torque produced vs. torque needed.  How do I do that?  I knew there was a tradeoff between rpms and load, but specifics are vague in my mind.  I have noticed that it doesn't feel like I need much throttle to maintain 70, except perhaps up a long grade.

"3,000 rpm is a very comfortable speed for the Y."  This is what I needed to hear!  I wouldn't have guessed it.  I didn't know the Y liked to spin so much!

Well, I suppose I'll feel a lot better about driving my car on the highway now.  Mission accomplished!  I don't get the impression that it's really worth my while to change anything.  Bigger tires (say, 215/75R15, 1" more in diameter) would only save about 100 rpm.

Thank you everyone!

My 2 cents worth.

Torque produced must always overcome torque needed, otherwise your engine will lose rpm's at full throttle, and eventually die.  High ratio gear sets shine at low speeds.  If you are happy with your engine's pulling power at a light, or if you pull a trailer, fine.  If you need more torque at low speed, gearing is the answer. 

My Service Manual says a '55 292 produces 280 ft/lbs of torque.  Let's just use this figure.  Std transmissions output 1:1 in high gear.  Now comes the fun part:

There is a direct ratio between gears and torque output.  At 3.0:1 gear reduction (your gear), rear wheel torque is trippled to 840 ft/lbs, which is great!  Want more torque?  A 4:30:1 gear produces 1,204 ft/lbs of stump-pulling torque (280 x 4.30 = 1,204).  That's nearly one and a half times more pulling power (in high gear) using the same engine and transmission.  The trade-off is engine speed.  Basically, if your car goes 70 mph at 3,000 rpm, with a 4.30:1 gear, your new engine speed would be ~4,300 rpm.

Now for First Gear:  A V8 3-speed tranny's First Gear has a ratio of 2.57:1.  Using the above figures:
*  Your 3:1 TIMES 2.57:1 TIMES 280 ft/lbs of torque= 7.71x280 = 2,159 ft/lbs.  (This is your maximum torque at the light to your tires).
*  4.3 X 2.57 X 280 =  3,940 ft/lbs of torque!  This gear nearly DOUBLED the engine torque for a great launch at the light.

Tire diameters can change ratios just like gears.  Your tire diameter calculations start where the gearing leaves off.  Racers use very large tires with rear end gears of 4:30 (commonly).

Another side advantage to using these high ratio gears, the rear ends tend to stay together.  If you had 600 ft/lbs of torque (or more) you can see where a 3:1 gear has a harder job transferring torque than a 4.3:1 gear.  The 4.3:1 gear spreads out the same torque over an additional 1-1/3 driveshaft turns than the 3:1 gear set.

Sorry to be so long winded, but I hope this helps you understand the difference in gear sets.

  - Dave

HT32BSX115 (6/20/2009)

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46yblock (6/17/2009)

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I agree with most or all above. Try some larger diameter tires. As for a high rear end, I have one, 3.00. 70 mph cruise is about 2500 or less. 3.00 is too high unless all you do is freeway driving. With non freeway cruising of 55 the rpm is only 2000-2100, too low and not efficient. At 3000 rpm I would probably be over 80 mph in danger of the cab blowing off . Someday maybeitwill bechanged to 3.25 limited slip. That would allow me to increase the tire diameter at1-2 inches to fill up the wheel wells and give better ride,which cant be done now.

Hi Mike! Now you have me worried.......I'm going to make sure that securely bolt my cab on!!!


I have 40" tires, 5.83:1 axle and E4OD transmission (0.70 OD). I expect about 2100 rpm at 60mph.!!

Rick, are you planning on some Interstate speeding with the F-600 !