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#1 |
Registered User
Join Date: Nov 2017
Location: Leeds west yorkshire
Posts: 307
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Rear end ratio help?
Can anyone give me an idea of my rear end ratio? I have a 69 longbed 350, 700r4 with a 12 bolt POSI rear end. When I’m doing 70mph my rpm is approx 2000rpm in 4 th gear ( lockup not used). oh I have 22 inch wheels on. I was thinking gearing feels a bit long and was wondering if it would be a bit more fun with a different rear end as I have play in the prop shaft anyway.
Last edited by Sparky dave; 06-12-2019 at 04:19 AM. |
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#2 |
Senior Member
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Location: Waterford California
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Re: Rear end ratio help?
You'll need your rear tire actual O.D. not the rim size. Just punch in your rpm 2200, speed 70 and tire O.D. and hit calculate...... will then give you a close ratio.
https://speedmaster79.com/tools/gear-ratio/ |
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#3 |
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Join Date: Dec 1999
Location: Austin, TX, USA
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Re: Rear end ratio help?
22 inch wheels don't help at all. You need to provide the outside diameter of the wheel/tire combo. I would guess you had 3.07 and tires were 30" tall or so.
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44 Willys MB 52 M38A1 64 Corvette Coupe 68 Camaro 'vert LT1 & TH700 69 Z/28 355 12.6's @110 69 Chevy Short Step 4 1/2"/7" drop 72 Jimmy 4WD 4spd 4" & 35's 02 GMC 2500HD 4x4 Duramax |
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#4 |
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Join Date: Jul 2009
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Re: Rear end ratio help?
If you're in 4th gear with your 700r you're IN overdrive. Third gear is a 1 to 1 ratio. I think the O.D. ratio is approx. .70
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Dad always said, "Son, WISH IN ONE HAND, and ......." -------------------------------------- -------------------------------------- Current toy trucks: '71 GMC 1500 SUPER CUSTOM short/step, orig. dk. blue, 350/700r, ps,pb, A/C '72 Blazer 2wd, ochre & white, 454, tremecTKO 500 5spd. ps, pb, A/C, tilt Last 10yrs of hobby vehicles, had a FEW more in the 50yrs. before these: '66 Plymouth Belvedere City of Miami cop car clone. '70 Nova 406"sb, 13.5-1, solid roller, Brodix, p.glide/t.brake, back halved, 9" Ford, spool 4.88, cage,ladder bar/coilover, 10.5 tire....... SOLD!! '67 C10 short/step side mount spare -- SOLD!! '72 Jimmy 2wd, 350/350, ps, pb, fun driver, lots of bondo & a shiny red paint job..... SOLD!! '69 Nova 350, 4spd, A/C, ps, p.b, ...SOLD!! |
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#5 | |
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Re: Rear end ratio help?
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#6 |
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Re: Rear end ratio help?
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#7 |
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Re: Rear end ratio help?
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#8 |
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Join Date: Feb 2011
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Re: Rear end ratio help?
If you just give us the size, we can calculate the diameter. Don't need to get the tape out.
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#9 |
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Join Date: Feb 2011
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Re: Rear end ratio help?
Given that you are at 2000 RPM at 70 MPH, that means you are doing 35 MPH per 1000 RPM. This is the ratio in my suburban with 245/75R16 (30.5" diameter) and 3.73 rear gears. If your tires are smaller, you have a lower ratio rear, if your tires are larger, you have a higher ratio rear.
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1970 C20 Custom Camper - 350, TH350 1997 GMC Suburban 1994 Acura Integra GSR 1987 Dodge Daytona Shelby Z |
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#10 | |
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Re: Rear end ratio help?
Quote:
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Terry 1970 Custom Camper/C20 , GM Crate 350/7004R, Dana 60, factory AC |
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#11 |
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Join Date: Feb 2011
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Re: Rear end ratio help?
My suburban is a 97 - overdrive with lockup. I can plug it into a calculator and see 34.7 MPH per 1000 RPM.
To get ~35 MPH / 1000 RPM with a 0.7:1 gear ratio and a 4.10 rear, you would need to have ~33.5" tires. I'm assuming you have 235/85R16, as that's what came on my 70 C20. They are 31.7". With OD and a 4.10 rear that should give 32.9 MPH / 1000 RPM. You should be closer to 2,125 RPM at 70 MPH. Here is a calculator to get the speed at a given tire size, trans ratio, rear ratio, and RPM. https://spicerparts.com/calculators/...rpm-calculator
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1970 C20 Custom Camper - 350, TH350 1997 GMC Suburban 1994 Acura Integra GSR 1987 Dodge Daytona Shelby Z |
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#12 | |
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Re: Rear end ratio help?
Quote:
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Terry 1970 Custom Camper/C20 , GM Crate 350/7004R, Dana 60, factory AC |
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#13 |
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Join Date: Mar 2016
Location: Long Island NY
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Re: Rear end ratio help?
Yes, rpm will drop more as torque converter slip is eliminated with lockup engaged. Typical rpm drop is around 300rpm but depends on the converter.
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#14 |
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Re: Rear end ratio help?
You probably won’t have to worry about the gear ratio much longer, trans will be blown before you know it without lockup in overdrive....
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#15 |
Registered User
Join Date: Nov 2017
Location: Leeds west yorkshire
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Re: Rear end ratio help?
Is lockup for all gears or just 4th? I’m hardly ever in 4 th long enough to need it but if it’s real important I best wire it in then
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#16 |
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Re: Rear end ratio help?
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#17 |
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Re: Rear end ratio help?
Tyre size is 285 35 22
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#18 | |
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Re: Rear end ratio help?
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#19 |
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Join Date: Jul 2009
Location: Space Coast, Fl.
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Re: Rear end ratio help?
Dave, regarding your initial question. NO, it would be a bit more fun with more POWER. Looks like you've got plenty of gear.
__________________
Dad always said, "Son, WISH IN ONE HAND, and ......." -------------------------------------- -------------------------------------- Current toy trucks: '71 GMC 1500 SUPER CUSTOM short/step, orig. dk. blue, 350/700r, ps,pb, A/C '72 Blazer 2wd, ochre & white, 454, tremecTKO 500 5spd. ps, pb, A/C, tilt Last 10yrs of hobby vehicles, had a FEW more in the 50yrs. before these: '66 Plymouth Belvedere City of Miami cop car clone. '70 Nova 406"sb, 13.5-1, solid roller, Brodix, p.glide/t.brake, back halved, 9" Ford, spool 4.88, cage,ladder bar/coilover, 10.5 tire....... SOLD!! '67 C10 short/step side mount spare -- SOLD!! '72 Jimmy 2wd, 350/350, ps, pb, fun driver, lots of bondo & a shiny red paint job..... SOLD!! '69 Nova 350, 4spd, A/C, ps, p.b, ...SOLD!! |
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