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Old 03-18-2012, 02:42 PM
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Ratio Chart is this correct?

motivegear dot com has a gear ratio chart that shows at 55MPH the engine RPM range from 2050-2400RPMs for cars geared for cruising instead of stoplights. With a 29in time that gives a 3.20 to 3.90 ratio.
Why question is this assuming no Overdrive? (I think not)

If I want a 5,000Lbs car to cruise the interstate, not smoke tires it looks I need 3.42 to 3.73 gears.

But I have a LS1 -4L60e (.75 overdrive) So:
3.90 = 2.93 = 1875RPM @ 55
3.73 = 2.78 = 1745RPM @ 55
3.55 = 2.66 = 1715RPM @ 55
3.50 = 2.63 = 1700RPM @ 55

I have heard it is better to run a engine 100 RPMs too high than 100 RPMs too low- The engine noise and lower MPG are nothing compared to an engine that luggs at all times.

I had planned on 9in Ford with 3.50 or an Explorer with 3.55-- Should I look for 3.73 or 3.90?

I have a 62 Chrysler with 2.92 gears and love it (4,200Lbs) cruises great and barks going into 2nd with stock 413 engine...

Thanks for thoughts!
Charles

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Old 03-18-2012, 02:54 PM
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Last explorer diff I saw installed had a 4.10 ratio, multiply X .75 and you come up at like 3.08. That would be perfect. IMHO
i think 2200 at 60 mph would be better, not 1850 or whatever. That is too low to be in the optimum power making rpm range.JMO
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Old 03-18-2012, 03:59 PM
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3.73 and 4.10 are both common expo ratios.
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Old 03-18-2012, 06:06 PM
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3.73 is a very common exploder ratio
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Old 03-19-2012, 06:43 PM
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3.55 3.73 4.10

So am I reading this chart right?

http://www.motivegear.com/pages/charts/CHART.pdf

I am just shocked I would be putting a 3.73 or a 4.10 in a cruiser that I wanted to get good gas mileage.... If so that is cool, just doesnt sound right...
-Charles
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Old 03-19-2012, 07:00 PM
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3.73 -4.10 is what the charts shows for a 29 inch tire to be in the optimum rpm range for power and economy.
The overdrive gives you the stoplight to stoplight ratios in the first 3 gears and gives the economy in overdrive.
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