Diff Ratio

[fcwrangler]

I’m putting the front and rear ends together for the FC and just wanted suggestions on what diff centre to fit. The car will be running a warm grey with triple strommies, headers, mild cam and 2” exhaust with standard gearbox and EJ front and rear ends. It will be rolling on 15x7 mags with either 205 or 215 tyres. I have the standard diff as well as a 3.55 and a 3.36. Will be used mostly for cruising suburbs and highways.
Thanks in advance,
Jim

[Errol62]

Knowing the aspect ratio of the tyres would help but with 17” rims 3.55:1 would be safe. 


Sent from my iPhone using Tapatalk

[mcl1959]

I agree, 3.55 would be the best in my opinion

Ken

[ardiesse]

Jim,

I'm with Ken.  3.55 would be the starting point.  Mild cam and triples suggest your car would need a bit of a prod to get away cleanly from a standing start.  Hmm.  Compare the diameter of the rear tyres you're thinking of fitting against an original 13" crossply.  If your new tyres stand taller, then you could go 3.89, and change up a ratio if the car feels too low-geared.

Or if you're doing a lot of freeway driving and not much stop-start, then 3.36.

Weight is the enemy.  Resist the temptation to load the car up with accessories.  It might be instructive to compare the weight of your new wheel/tyre combination against standard.  I know, it sounds odd.  Recently I changed tyres on my commuter pushie, and it just felt . . . slow.  My new tyres were about 80 g heavier (each) than the old ones.  But that's how it works with wheels and tyres.  You are storing energy in rotation as well as plain old linear motion, and the effective inertia is twice the tyre's mass.  Imagine a runner lining up for a race in steel-cap work boots . . .

Rob

[fcwrangler]

Thanks guy’s I’m cleaning up the 3.55 diff as well as putting new bearings and seals on the axels. Also in the process of overhauling the front cross member with new bushes, seals and rubbers.
Jim