By Tom on Monday, March 12, 2001 - 11:28 pm: Edit

I am thinking about 4.70 gears for my Disco. I have 32" tires, but will go to 35 someday and figure whay buy gears twice. Any negative comments about doing this?

Also, anyone now what the OD ratio is for the tranny? I hear it is .73. With this setup, I would be at around 2500 RPM's at 70 based on the following formula:

rpm = (mph(70) x trans ratio ( .73 ) x rear ratio (4.70) x 336) / divided by tire diam. (32) = 2521

Thanks for any thoughts...devil's advocates appreciated.

Tom

By Clint on Tuesday, March 13, 2001 - 08:14 am: Edit

Check out ashcroft's page under ratios
http://www.autoconv.com/
You can keep your axle gears and just change the high range in the transfer case also. Your low will not be as low this way however.

Your method is correct, the there is another way to figure RPMS.
Take the ratio of circumferences or (diameters)of the old tire and the new tire. That will be the difference in speed or RPM.

example
35/32 is 1.09 times the original speed at a given rpm
but the real question is
35/29=1.2 tire ratio
4.7/3.54=1.32 gear ratio
after tire and a gear change your ratio would be
1.2/1.32=.9 of what it was

Sorry for the rambling, I'm hurrying.

By Ron on Tuesday, March 13, 2001 - 01:46 pm: Edit

4.71 are basically free as they are stock in all series rovers. 35s on a disco. Man that would look cool. D-90 guys have gone to 4.7 no problem and I have seen them on a RR as well. With 35s though you may (have to?) need/want to do the HD shafts, lockers gears and in one shot. If you do ARBs you can wait on the shafts as you just can leave them off except for the monthly keep the seals lubed cycle. I think the aforementioned math is correct.

Ron

Also if you do gears do lockers too as to install one you are installing the other.

By Troy on Tuesday, March 13, 2001 - 02:20 pm: Edit

You will be wayyyy over-revving at highway speed with 4.7 or 4.75s & 32" tires. Heck, even with 35s you'll be slightly over-revving. IMO, if you're planning on using the truck around town and it's not solely a trail truck, go with 4.11s and you'll be decent with either size tire on and off-road.

Of course, if you have your heart set, Doug Marbourg is selling a set of complete 3rd members with 4.7s and ARBs on LRX.com. All you'd have to do is bolt them up and run airlines to the compressor.
Troy