Ring and Pinion size needed for bigger Tires

[moparchub]

Hey Guys, I'm putting 20" wheels with P285/40/20 tires on my 68 Charger
I know this is going to have a big effect on the power out of the hole.
Its not a race car but it does have a stock 440 HP2 with good acceleration and want to keep the power it has always had. The new tires are 29' replacing 26.6" Im not sure where to start so hopefully some one out there has been through this, any information would be appreciated.:icon_shaking:

 

[Average Joe]

What do you have for gearing now?

 

[5.7 hemi]

4.10's will do just nicely!

 

[Kern Dog]

A little math is in order. Figure the % difference between the old diameter and the new. Add that percentage to your existing gear ratio and pick the closest one available.
Another issue to consider is the weight difference between the two sizes. The 20s might weigh more, resulting is longer stopping distances and slower acceleration, though the difference may be slight.

 

[NCC1968]

I use this one:

http://www.justdifferentials.com/Articles.asp?ID=251

If you plan to drive it on the streets you'll want to determine just how high you want to spin on the freeway....then limit it at the bottom. If you want to mostly race you will need to get into it more and calculate the actual power to the ground and figure out shift points and finish line RPM.

Luckily since I have a 511 cid engine I can go with 4.10 and melt the 29inch tires, but still turn a very good 1425RPM at 60MPH......due to double OD 6speed.

It will be worth the time for you to study and get it right on the first try.

 

[moparchub]

Im running 2:76 gears with a pretty much stock 440 HP2.
Its pretty much a cruiser but have wanted to give up top end for better launch.
As far as braking, The whole thing started with a 4 wheel disc brake conversion so stopping should be good.
I'll try to do the math and get some idea, the 6 speed would be nice... some day

 

[Kern Dog]

As simple as it may seem, a 3.23 was the factory standard ratio for a 440 powered R/T with an automatic. This is a really decent gear as far as balance between low speed response and top end.

 

[Average Joe]

I agree with Kern,3.23 or 3.55 would be a good combination for what your doing. I run 4.10 with a five speed and its at about 2400 rpm at 70 mph in overdrive.