axle ratio choice

[72 ScoutII]

We are rebulding a 72 Scout 258 cid 6 cylinder with d44 axles. The stock ratio is 4.27. We are planning 33"-35" tires for mostly local off road use but also some driving on pavement around the area in a 50 mile radius on an infrequent basis. Any suggestions as to 4.88 v.s. 5.13 ratio for the new gears? Does anyone know what the stock ratio engine rpm would be at 55 mph, or 65 mph, if it will even go that fast?

 

[RobertC]

Do not know the "max rpm" for a amc 258...

IH sv v8s have a "max recommended rpm" of 3600 - 3900 rpm (depending on ci).

Here is a formula from a previous post:

from a strictly "math viewpoint" - no downhill / no uphill / no head wind / no drag...

3900 rpm -- max recommended for a 304

and a "generic tire dia" of 29"

3.31 = 102 mph
3.73 = 90 mph
4.27 = 78 mph

formula:

speed = ( ( rpm * 3.14 * tire dia * 60 ) divided by rear end ratio ) divided by 63360

for nitpickers 2 * rolling radius is more accurate than tire dia.

 

[72 ScoutII]

Re: axle ratio choice and RPM

For what it is worth, I beleive the 258 max hp is at 4600 rpm and the max torque is at 2300 rpm.

 

[Chris Pucci]

If you plan to drive it on the street 4.88 would be pretty good. If you never plan to drive it around go as low as you can.

But remember you are giving up a lot of strength in the pinion as you go lower in a d44. The 5.13 will have a super tiny and weak pinion and 'May' become your weak link with 35's...