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.

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/

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.

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

Re: Rear end ratio help?
 

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

Re: Rear end ratio help?
 

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

Re: Rear end ratio help?
 

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

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.

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.

Re: Rear end ratio help?
 

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

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

Re: Rear end ratio help?
 

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.

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.

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....

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

Re: Rear end ratio help?
 

Thanks for your help mate appreciate it

Re: Rear end ratio help?
 

Tyre size is 285 35 22

Re: Rear end ratio help?
 

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

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.