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Discussion Topic:
1946 Mercury
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Peder D |
05-13-2012 @ 1:40 PM
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Member
Posts: 95
Joined: Oct 2009
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Hey Guys! My car has the 3:54 rear. Would I notice any considerable change in rpms if I installed a 3:25? Thanks! Peder
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TomO |
05-14-2012 @ 6:57 AM
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Senior
Posts: 7250
Joined: Oct 2009
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Peder, I would not recommend a 3:25 ratio. Your engine would be working too hard to move the car. Your overall gas mileage may go down with the higher ratio. A 3:54 is the best all around ratio for your car, unless you are in a mountainous area. I suggest that you look for a Columbia rear end or an overdrive.
Tom
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carcrazy |
05-14-2012 @ 9:19 AM
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Senior
Posts: 1651
Joined: Oct 2009
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If you did install a 3.25 ratio rear axle your engine speed would be 91.8% of the speed with the 3.54.
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shogun1940 |
05-24-2012 @ 5:04 PM
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Member
Posts: 464
Joined: Feb 2010
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for every inch of tire diameter ,it changes ratio one tenth,,3.54 would be 3.44 wth a bigger tire. Maybe you could borrow some tires and rims to try it out
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Peder D |
05-25-2012 @ 12:13 PM
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Member
Posts: 95
Joined: Oct 2009
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Should hasten to say that I bought and had mounted P205/75 R15 radials back in 2004. Was the only tire available at the time...so this increases the engine rpm since this tire is smaller than the original 650x15? What is the difference in numbers? Thanks again, Peder
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carcrazy |
05-25-2012 @ 1:16 PM
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Senior
Posts: 1651
Joined: Oct 2009
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Your original 6.50x15 tires have an aspect ratio of 90% so their overall diameter is 26.7 inches. Your new P205/75R15 tires have an aspect ratio of 75%. Converting the metric 205mm into inches yields 205/25.4=8.07 inches. Your new tires have an overall diameter of 27.1 inches. Your original tires turn 755 revolutions per mile while your new radial tires turn only 744 revolutions per mile. Therefore your new tires offer a very slight overdrive of 1.5% slower engine speed for a given vehicle speed.
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TomO |
05-26-2012 @ 7:41 AM
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Senior
Posts: 7250
Joined: Oct 2009
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Hi Peder, The original standard size for your Mercury was 6.70x15 and the optional tire was 7.10x15. The closest radial tire diameter to the original 6.70x15 is the 225/75 R15. You can check out tire diameters at http://www.cokertire.com/ without having to do the math. Your radials are now 8 years old and are near the end of their safe use life. If you can find some 7.10x 15 tires in your country, you could use them, otherwise I would go with the 225/75 R15 radials.
Tom
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Peder D |
10-28-2012 @ 11:14 AM
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Member
Posts: 95
Joined: Oct 2009
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Hi again! The suggestion was to look for a Columbia or an overdrive...I see Mitch*ll Manufacturing is advertising in V8-Times a model 1000 overdrive...any experience with this unit...a good buy? Thanks....Peder
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supereal |
10-28-2012 @ 8:25 PM
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Senior
Posts: 6819
Joined: Oct 2009
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Have an overdrive in my '47. It isn't a Mitch*ll, but operates the same. I chose a 33 percent reduction with my 3-78 rear gears. If i did it again, I'd go for a 30 percent, as it lugs down below 40 mph in OD. With a 3-54 gearing, I wouldn't go for anything more than 25 percent. Overdrive is great for highway cruising, but a bit pricey if you don't need it.
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Peder D |
10-30-2012 @ 12:27 PM
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Member
Posts: 95
Joined: Oct 2009
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Thanks for the input. Mitch*ll saying a 26% reduction with the unit they offer. Interesting you mention lugging. My car does that even with the 3:54 at lower speeds. Mechanic says "it's an old car". Thinking carb. or ignition? Don't like this situation and hoping still there is a solution to lugging. Everything under the hood upgraded. Any suggestions? Thanks again..Peder
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