Rear end ratio help? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Sparky dave · 06-11-2019, 06:44 PM

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.

**Sheepdip** · 06-11-2019, 07:38 PM

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/

Mike C · 06-11-2019, 08:37 PM

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.

RustyBucket · 06-11-2019, 08:53 PM

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

Sparky dave · 06-12-2019, 04:10 AM

Quote:

Originally Posted by Sheepdip

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/

Thank you I’ll go work it out now. I did a diff oil change recently and never checked what rear end I had and don’t fancy draining the oil again so this helps me a lot

Sparky dave · 06-12-2019, 04:12 AM

Quote:

Originally Posted by RustyBucket

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

Thanks for clearing that up. Ah so it’s my converter lock up that I’m not using then. Will my rpm drop even more if I start using it

Sparky dave · 06-12-2019, 04:14 AM

Quote:

Originally Posted by Mike C

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.

Ok mike I’ll get the tape measure out. Thanks

rpmerf · 06-12-2019, 09:31 AM

If you just give us the size, we can calculate the diameter. Don't need to get the tape out.

rpmerf · 06-12-2019, 09:41 AM

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.

**tdangle** · 06-12-2019, 09:54 AM

Quote:

Originally Posted by rpmerf

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.

that doesn't sound correct. with 3.73 and those tires with a th350 at 70 I would think you are closer to 3000 rpm. I have slightly taller tires 4.10 and .7 overdrive with lockup and at 70 I'm right at 2000

rpmerf · 06-12-2019, 11:30 AM

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

**tdangle** · 06-12-2019, 01:18 PM

Quote:

Originally Posted by rpmerf

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

Sounds about right. didn't realize you were comparing to a different vehicle than in your signature. And yes I have 265-75-16 with a dia of about 31.6" so just over 2K is correct. Way nicer than when I had the TH350 trans.

Overdriven · 06-13-2019, 01:03 AM

Quote:

Originally Posted by Sparky dave

Thanks for clearing that up. Ah so it’s my converter lock up that I’m not using then. Will my rpm drop even more if I start using it

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.

*randy500* · 06-13-2019, 01:07 AM

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....

Sparky dave · 06-15-2019, 06:37 PM

Quote:

Originally Posted by randy500

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....

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

Sparky dave · 06-15-2019, 06:38 PM

Quote:

Originally Posted by Overdriven

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.

Thanks for your help mate appreciate it

Sparky dave · 06-15-2019, 06:47 PM

Tyre size is 285 35 22

Sparky dave · 06-15-2019, 06:52 PM

Quote:

Originally Posted by rpmerf

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.

It comes out at 29.9 in tyre diameter so I’m thinking it might be 3.73 s like yours. Thanks for the info

RustyBucket · 06-16-2019, 12:17 AM

Quote:

Originally Posted by Sparky dave

I was thinking gearing feels a bit long and was wondering if it would be a bit more fun with a different rear end.

Dave, regarding your initial question. NO, it would be a bit more fun with more POWER. Looks like you've got plenty of gear.

06-11-2019, 06:44 PM	#1
Sparky dave Registered User Join Date: Nov 2017 Location: Leeds west yorkshire Posts: 307	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.

06-11-2019, 07:38 PM	#2
Sheepdip Senior Member Join Date: Nov 2018 Location: Waterford California Posts: 2,365	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/

06-11-2019, 08:37 PM	#3
Mike C Registered User Join Date: Dec 1999 Location: Austin, TX, USA Posts: 7,709	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. __________________ 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

06-11-2019, 08:53 PM	#4
RustyBucket Registered User Join Date: Jul 2009 Location: Space Coast, Fl. Posts: 1,050	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 __________________ 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!!

06-12-2019, 09:31 AM	#8
rpmerf Registered User Join Date: Feb 2011 Location: Parkville, MD Posts: 1,024	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. __________________ 1970 C20 Custom Camper - 350, TH350 1997 GMC Suburban 1994 Acura Integra GSR 1987 Dodge Daytona Shelby Z

06-12-2019, 09:41 AM	#9
rpmerf Registered User Join Date: Feb 2011 Location: Parkville, MD Posts: 1,024	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. __________________ 1970 C20 Custom Camper - 350, TH350 1997 GMC Suburban 1994 Acura Integra GSR 1987 Dodge Daytona Shelby Z

06-12-2019, 11:30 AM	#11
rpmerf Registered User Join Date: Feb 2011 Location: Parkville, MD Posts: 1,024	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 __________________ 1970 C20 Custom Camper - 350, TH350 1997 GMC Suburban 1994 Acura Integra GSR 1987 Dodge Daytona Shelby Z

06-13-2019, 01:07 AM	#14
randy500 Account Suspended Join Date: Jul 2016 Location: Tacoma, WA Posts: 6,873	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....

06-15-2019, 06:47 PM	#17
Sparky dave Registered User Join Date: Nov 2017 Location: Leeds west yorkshire Posts: 307	Re: Rear end ratio help? Tyre size is 285 35 22



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