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EFV-8 Club Forum / General Ford Discussion / Rear end gear ratio

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Posted By Discussion Topic: Rear end gear ratio     ford38v8 05-03-2012 @ 10:50 PM reply profile send p.m. Senior Posts: 2758 Joined: Oct 2009 I'm not a mathematician nor am I a machinist, but I will attempt to explain a couple of things related to gear sets and differential ratios: We identify differential ratios by an approximate number, never the complete number, because the decimal number is infinite. This is intentional, in order to allow the drive gear teeth to roam the driven gear teeth many, many times before eventually nesting with the same tooth again. The result of this "promiscuity" is a polishing of teeth, rather than a wearing down of teeth as would happen with the pairing of the same teeth on a more frequent basis. An example of a ratio that would self destruct quickly would be 4 to 1, attainable with a gear set of 9/36. The gear set of 9/37, on the other hand, has a ratio of 4.11111111111 ad infititum. This is the preferred set, for the reason stated. Gear sets are matched, meaning that they are machined in sets, by the same tooling. The sets are identified with a matching serial number engraved on each of the two gears. This is further insurance of a long lasting gear set, as no two tools can be assured of delivering the same tolerances, and even the same tool at a different time will deliver a different tolerance. For these reasons, then, you will never open up a differential to find worn gears unless it has been set up incorrectly, or that it was damaged by incorrect lubrication, or by bearing failure. The gears themselves will not otherwise wear out in normal use in your lifetime nor your sons, nor his son's sons. I hope I haven't created more questions than answers, but if I have, I'm sure there are those here that are more able than I to answer, and can probably teach me a few things in the process! Alan oldford2 05-03-2012 @ 1:52 PM reply profile send p.m. Member Posts: 275 Joined: Oct 2009 Now that is interesting. Original 3/54 gearsets were 11/39 and repro's are 9/32? Maybe I better try and find old stock (impossible?) or go to a Mitch*ll O/D. John supereal 05-03-2012 @ 1:19 PM reply profile send p.m. Senior Posts: 6819 Joined: Oct 2009 When our industrial base collapsed after the recession hit, almost all the machines necessary to produce gear sets were either shipped overseas or scr*pped for iron. There are very few left here, and the skill necessary to operate the equipment has almost disappeared, as those with it retired or died. The consequence is that many of the "off shore" gears are of questionable quality. Much is also true of machine shops. Years ago, nearly every town had one where equipment could be made or repaired. They have become almost extinct. We operate a large shop mostly equipped with fine machines bought when others closed. We may also eventually fold too, as finding anyone with the ability and interest to learn the trade have virtually disappeared. College diplomas are nice, but they don't do much to keep our machines going. Stroker 05-03-2012 @ 12:02 PM reply profile send p.m. Senior Posts: 1460 Joined: Oct 2009 Gene: Don Rogers and Supereal's numbers are valuable when shopping for a 3.54:1 ring and pinion. Some of our members have purchased after market 3.54's that had fewer teeth on both the ring gear and pinion than the original 11-39 count such as 9-32, etc. I would consider any gearset that does not have the correct number of teeth to be "spurious" as Henry would have said. Gear hobbing is a slow, expensive process so reducing the number of teeth results in cost savings, and a reduction in quality due to higher loads being imposed on the individual pinion teeth. supereal 05-03-2012 @ 10:20 AM reply profile send p.m. Senior Posts: 6819 Joined: Oct 2009 Thanks, Gene. If you are building a driver, I'd suggest that you look into an overdrive unit and keep the 3-78 gears. I have one in my '47, and it makes hiway driving possible with a 3 to one reduction, yet keeps the original ratio for city and hilly use. That way, when you are going 65, the engine is doing about 45, close to the original power band. Gene N. 05-02-2012 @ 4:28 PM reply profile send p.m. Member Posts: 44 Joined: Mar 2010 Thanks Guys Great numbers supereal oldford2 05-02-2012 @ 4:22 PM reply profile send p.m. Member Posts: 275 Joined: Oct 2009 Well, as soon as I get up courage to change our 46 from 3/78 to 3/54, I will restamp the ratio numbers. I plan on buying the gears and then I will be committed. John supereal 05-02-2012 @ 9:44 AM reply profile send p.m. Senior Posts: 6819 Joined: Oct 2009 The ratio codes: 37-9= 4.11, 34-9= 3.78, 39-11= 3.54, 41-9= 4.55, 40-9= 4.44. The numbers stamped on the web of the "banjo" don't always correspond to the gears inside, as many were swapped over the years. Don Rogers 05-02-2012 @ 9:29 AM reply profile send p.m. Senior Posts: 504 Joined: Oct 2009 11-39 should be whats stamped on the rear end. A pinion with 11 teeth and a ring with 39 teeth will give you a rear with a 3.54 to 1 ratio Gene N. 05-02-2012 @ 8:32 AM reply profile send p.m. Member Posts: 44 Joined: Mar 2010 If a rear was stamped with a number 9 34 which when dividing the 9 into the 34 equates to a 3:77 ratio What would be stamped for a 3:53 ratio

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