high speed rear end question

eagle

eagle

Well-known member
I've been looking at the high speed differential gears that Mac's (and others) sells. It says they increase top end by +5 mph. That would be really nice, but how much does it affect 1 st gear? 1st gear is plenty fast stock already, so is reverse. Does anyone have it in theirs? How big of a job is it to install? Your also supposed to change out the speedometer gear. My speedometer is about 3-4 mph fast anyway so not sure if the speedometer gear would be necessary... I'm interested in what those of you who have it think. I can't afford an overdrive.
 
3.54 gears may increase your cruising speed by about 5 mph but you will loose a bit of power on hills. Your 4-DR sedan is one of the heavier body styles.If you live in flat country it may be OK. Generaly,the 3.78 ratio is best for the heavier body styles.If you had a roadster or OCPU the 3.54 or even 3.27 gears would be fine as they are so light.To change the gears is a big job as the rear end assy has to be removed.IMHO,it,s hardly worth the effort for the slight gain in speed coupled with the slight decrease in power.In my S/W sedan I run a Columbia O-D with 4.11 gears & 2.94 in O-D,so I have the best of both worlds-power& speed.
 
I don't seem to have any problem with power. My engine was rebuilt a few years ago but has stock head etc. Although maybe it would be too much for it with the higher gear ratio???? I guess thats why I'm asking you guys. It cruises very nice at 50 although it would be nice if the RPMs were a bit lower. Thanks.
 
What is your current ratio? Mark the tire and see how many revolutions of the engine turn the tire once. Maybe you already have the 3.54 gears.

Charlie Stephens
 
I have two cars, a coupe and a roadster, running with 3.54 gears. I lost top end speed on both when I installed the new gears and recovered it only when I installed 5.5 cylinder heads. Remember, it takes more horsepower to go faster. When you reduce the RPM, you are asking the engine to produce more horsepower at a lower RPM which it simply may not be able to do. Add a higher compression head and the horsepower is available.

Your current error in indicated speeds is most likely an error in the speedometer or the
circumference of the tires. The 3.54 ratio generally requires a 18 tooth driven gear while the 3.78 uses a 19 tooth. That is easy to check.

Gar Williams
 
My roadster has 3:54 rear end gears. I had those put in when the old rear end bearings failed. The roadster doesn't climb hills in high as well as with the 3:78s. My coupe has 3:78s and it has a Mitchell overdrive and a 5.5 Snyder head, that is an ideal combination. I think your best approach would be the 3:54 gears and a 5.5 head for pulling in high gear. The higher ratio doesn't make much difference in low or second as the transmission gears are pretty low anyway.
 
I am running 3:54's and a Mitchell, I have a good strong motor and have no problems on hills. I have even driven away from a dead stop in overdrive without realizing it.
I think the key is a strong motor with high compression head. Mine is a 5.5 head.

Steve
 
We have one member in our local MARC Region that installed a 3.27 in a '30 Murray Fordor TS. Even though he is running a Kelly engine he lost a lot on the high end and realized his mistake after a trip to the east coast from the Chicago area. He is switching (or already may have) to a 3.54, which in my opinion, is still a mistake for the heavy Fordor. I believe the heavier cars should stay with a 3.78 or a 3.70 as many of the early 60 series Fordors had from the factory. My '29 Murray TS has and will stay with a 3.78. Gar Williams
 
thank you for the info . i have a coupe & was afraid of gearing to high . i guess ill put 3:54's in . ........ steve
 
Most of the touring I do is with our model a club and they stay at 45 mph . I have a SW with "B" engine 60 over and a 4spd od and 16 in tires . This is a lot more than I need to run 45 mph. My suggestion ,keep what you have .
 
Eagle. I have 31 tudor , orig engine put in 3.27 gears 2001, all ok live in louisiana and flat ground . Also able to pull from stop sign in 2nd gear, no problem cost was 350.00 for gears but only gives you 7 mph not 10 like people advertise. monthly miles average 1500. 2003 entered in great race added 5.9 brum,mitchell car runs great, out of this world. July 2003 added the web carb, 2009 had inserted and real good cam dyno puts me at 65 hp 2900 rpm. I was told back in 2001 that the A couldn't pull the 3.27 but I knew how my tudor ran and I was good with this, all was ok BUT REALLY for the cost of gear now 980.00 for 7 mph not a good idea . better bang for that would be spend that money on a mitchell. have fun modelA tony Lafayette,la
 
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Thanks for all the info. Maybe I'll keep the std gearing. I have plenty of power, and usually run about 45mph, the rpms are ok at 45. At 50 it seems to be revving a bit more than I like, so thats what I was looking to remedy. Maybe someday I can afford an overdrive. It looks like you pretty much need to plan on close to $3000. The Mitchell synchro transmission is available with 15% higher 1st and 2nd but not 3rd.
 
The easiest way to equate the change is to do some simple math.
At 37.8 MPH with 3.54 gears the engine will be turning the same speed as it would at 35.4 MPH with 3.78 gears. Now just multiply this by 1 1/2 to get your normal driving road speed.

Or, if you want to compare using 3.27 gears then:
At 37.8 MPH with 3.27 gears the engine will be turning the same RPM as it would be turning while driving 32.7 MPH with 3.78 gears. Again multiply this by 1 1/2 to get a good driving speed.

Now, you can see why I'd go with 3.27 gears if I was going to the bother to change the rear end gears. They'll make a noticable difference. I'd also install the 5.5 Snyder's head.
 
tom , thanks . i have a good motor (maybe 80hp) & would like not to run it at high rpm . but i also have a s-10 5 speed & afraid the 3.27 would be to high . any thoughts ? ............
 
Model A's were geared down & designed for climbing out of gravel, creek bottom rural roads, & for climbing steep hills that ocurred prior to States using heavy earth moving equipment to level highways by knocking down hills & filling valleys.

In many cases, for some drivers, the 3.27 ring & pinion is a viable option.

If I remember correctly, the MPH equation is:

MPH = [Engine RPM] X [Tire Diameter] divided by [Differential Gear Ratio, (e.g., 3.54,or 3.78,or 3.27 etc.)] x 336; hence, at 2,000 RPM with a 29.8" tire diameter, divided by a 3.78 x 336 = 59,600/1,270.08 = 46.9 MPH.

However, with a 3.27 ring & pinion, at the same 2,000 RPM engine speed, same tires, = 54.2 MPH; or about 7 MPH gain -- on an 8 hour drive, one could theoretically travel an additional 56 miles in 8 hours; hence, save an hour or so in driving time in 8 hours.

Hope this helps to choose, given different vehicle tire diameters & different experiences with comfortable engine RPM's .
 
HLC , good info ! im curious how much extra hp you need to carry the 3.27 with a trans final of .72 ? .......... thank you .. steve
 
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