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| Changing rear differential gears | Rate Topic |
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| Posted: 03-06-2018 01:31 am |
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1st Post |
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Pricelessjunk Member 
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I currently have a 3:73 (4 speed), rear axle in my car with a 5 speed Getrag gearbox and I have a 3:45 rear axle from a 5 speed car. Question: Can I swap the ring and pinion gears between the axle housings - using the 3:45 gears in the 3:73 differential housing or do I need to swap the whole rear axle assembly? which would be easier? The 5 speed axle doesn't have brakes and I would need to swap backing plates, wheel cylinders, brake lines, etc so was thinking just changing the ring and pinion may be easier? Thanks in advance for your thoughts. Tim (pricelessjunk)
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| Posted: 03-06-2018 03:43 am |
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2nd Post |
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redracer Member
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Much, much easier to swap the whole pumpkin; the only thing you may have a problem is if the original 4 speed diff. has the smaller flange/U-joints that appeared on the earliest J-Hs. The later 5 spd. had the larger flanges so check before swapping to be sure the driveshaft flanges match.
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| Posted: 03-06-2018 09:13 pm |
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3rd Post |
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answerman Member
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A question for those who have experimented: how unusable is a combination of the stock Rootes 4-speed and the 3.45:1 differential? I could see it help with cruising speed, but first gear is already kind of tall with the 3.73 in my opinion and I could see where it would make starting from a stop "not so nice". Anyone?
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| Posted: 03-07-2018 12:04 am |
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4th Post |
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Esprit2 Member
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J-H First Gear Overall Ratios (1st gear ratio times final drive ratio = overall ratio) First ––– Top Gear 10.76 –– 3.45 = J-H stock 4-spd & JH 5-spd 3.45:1 final drive ... (Ufdah 1st) 11.33 –– 2.69 = Toyota W58 5-spd & JH 5-spd 3.45:1 final drive. (Ufdah 5th) 11.63 –– 3.45 = J-H 5-spd car, stock 3.45:1 final drive 11.64 –– 3.73 = J-H 4-spd car, stock 3.73:1 final drive 12.25 –– 2.91 = Toyota W58 5-spd & JH 4-spd 3.73:1 final drive. I've not driven a W58 with the JH5's 3.45:1 final drive ratio, but the 2.69:1 overall 5th gear would be a really tall cruising gear. I have a hard time imagining that being practical for the stock 2.0 liter 907. 'Maybe' in the flat lands, but not in hill country. Or maybe with a 2.2 912, or a 2.5 hotrod. Something with some torque. Regards, Tim Engel Last edited on 03-07-2018 12:12 am by Esprit2 |
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| Posted: 03-07-2018 07:01 pm |
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5th Post |
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answerman Member
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Going to cherry pick your stats for my answer, Tim: Esprit2 wrote: J-H First Gear Overall Ratios (1st gear ratio times final drive ratio = overall ratio) If I'm reading your math right, there is very little difference in first gear between 4-speed/3.73 (11.64) and 5-speed/3.45 (11.63). What would the ratio be for first gear for a combo of 4-speed/3.45? That was where I was going. Would it be similar to second gear in a 5-speed/3.45? Not that I'm planning on doing this, I was just wondering. EDIT: I read your reply again and see that you did reference the combination I asked about: I know enough Minnesotans to know what "ufdah" means. So, it's working pretty hard to start out in first with that combination. By the way: planning on bringing the Jensen this year to the Intermarque Spring Kickoff: hope to see you there. Last edited on 03-08-2018 08:32 pm by answerman |
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| Posted: 03-08-2018 09:13 am |
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6th Post |
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NigelK Member
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I have a W58 with the stock 3.45:1 cwp axle in my GT, mated to a 912HC spec engine and it's a really nice combination. Relatively quiet cruising in 5th on the motorway and I can pull off in second gear (if I forget that I no longer have the Getrag box installed 😂)...
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| Posted: 03-08-2018 08:22 pm |
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7th Post |
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Jensen Healey Super Moderator 
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My ‘73 has a warmed over 2.0 with a 4 speed and 3.45 differential. It is slow off the line but has long gears. I haven’t driven it in several years due to back problems. Kurt
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| Posted: 03-08-2018 08:28 pm |
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8th Post |
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answerman Member
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Thanks Kurt: that's the real world answer I was looking for. Tall first gear but not undriveable.
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| Posted: 03-09-2018 10:23 pm |
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9th Post |
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Pricelessjunk Member
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Thanks to all who posted. This is very enlightning... with this information my choice may not be to change the rear diff ratio but to keep the 3:73 rear and swap the Getrag for a W58. My intent was to improve highway driveability (reduce the screaming RPM's) I feel bad for the 907 which is all stock (except Dellorto's) driving 75 mph for hours at a time. So with my 23.9" tire my choices are: (@ 75 MPH) 3:73 rear w/ Getrag 5 speed ----3933 RPM 3:45 rear w/ Getrag 5 speed ----3638 RPM 3:73 rear w/ W58 5 speed ------3068 RPM Alright, I've made my choice. If anyone needs a good 3:45 rear axle let me know...
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| Posted: 03-10-2018 04:03 am |
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10th Post |
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answerman Member
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Pricelessjunk wrote: Alright, I've made my choice. If anyone needs a good 3:45 rear axle let me know... Ok, I have to ask: where are you located? I might be interested if the price and shipping is right, I like to stockpile stuff like this for the future in case I ever need it.
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