View Full Version : Rear End Gearing

06-05-2012, 09:02 AM
Does anyone know what gearing came with the Cabriolet stock? I haven't had a chance to get under mine recently and I'm trying to calculate gearing for a Cummins swap :clap

06-05-2012, 10:44 AM
there is no easy way to know and it depends on what year your truck is. check the doorjam for the axle code and you are likely to be able to find the information that you're looking for.

06-05-2012, 11:01 AM
Will do. Thanks for the reply

06-05-2012, 11:01 AM
I agree there is no set gear ratio they used.
Jack up one wheel.
Spin it a full revolution.
Count exactly how many times the drive shaft turns.

You will end up with 4.1 turns or 3.75, meaning 4.10 ratio or 3.73 ratio.
If it is a limited slip rear end I think it is doubled? So you will count 8.2 turns of the drive shaft for a 4.10 ratio.

06-05-2012, 02:22 PM
hi, im interested to know what problems you think the diff ratio might course you after fitting a cummins into your cabriolet as ive just fitted one into my cabriolet complete with an allison 4 speed AT545 box, i havn't road tested it yet as ive got a problem with the power steering box. as soon as iv'e replaced it i'll try it out. have you actually started the engine swap?

06-05-2012, 02:49 PM
No I haven't. I'm planning on cutting out some of the flooring and doghouse to fit a NV5600 in it. The issue I was concerned with was the IDI's (And presumably) 460's can run more RPM's than the 6BT's can. I didn't want to have the motor completely strung out at 65MPH where as the IDI I have now, along with a C6 is getting close at about 70MPH.

06-05-2012, 04:03 PM
right good point, not sure on that how mine will pan out until i road test it. if i find its an issue with the top end i may have to consider some work with the final drive, but for now i just need to get this truck on the road to give it a shake down incase their are any issues with the engine and box or indeed any of the fabrication work i've done. i hope not because im desperate to get the truck back on the road were it belongs instead of in my driveway in bits. let me know how you get on with the swap. :bigok:

06-06-2012, 10:38 AM
I wrote a little Excel macro that I use to determine axle ratio or RPMS. It's pretty basic math but it works.

Basically there are five factors involved.

Tranny ratio (any gear)
Axle ratio
Tire Diameter

Knowing any four of the five my macro will calculate the missing factor. With GPS accurate speed is easy to get and most OD trannies are 1:1 in third gear and if not you can easily get the final ratio off the internet for pretty much any transmission.

Like I said I did it in excel but if anybody wants it just shoot me a message with your email address and I'll send it along.


06-06-2012, 03:48 PM
Theres also:


I'm not sure what tire setup you have on yours but going off of a 40" tire and a 4:10 rear end you'll be at ~2200 RPM in fourth at 65MPH vs. 1600 RPM with a G56 in sixth gear.

06-17-2012, 05:48 PM
If your Cabriolet is the same as mine it should be a 4:10 Dana 70HD

02-27-2015, 09:08 PM
well you can call ford and ask them , probably 4.10, just like mine

07-27-2015, 11:35 PM
most are 4.10