Mathemeticians.....rear ratio calcultaions help please?

[andrewh]

Ok Guys and Gals. I am looking into 5 speed options, and have looked at the 5th gear ratio on the Jaguar Classic (JLR) 5 speed and it has a ratio of .63, vs a few other offerings in the region of .75. My rear diff is a 3.07. I am considering a 5th gear ratio as a future proofing of my car as the Moss Box is really not drivable by the average younger generation. I also have had enough of the Moss experience. I know I can put a 4 speed synchro box into the car and that is an option. But for the purpose of this request, I am purely asking am I correct in thinking , all other factors remaining even, that a .63 5th gear will reduce my revs at a fixed speed by 37%? JLR don't offer different 5th gear ratios sadly. Thank you in advance. Andrew

[mgcjag]

Hi Andrew...try this calculator https://spicerparts.com/calculators/tra ... calculator

[paydase]

0.63 with a 3.07 looks to me much too low revs, especially for the 3.8 engine.
That would be ok with a 3.54 rear diff

[andrewh]

thank you Steve. I had tried a couple of these calculators but they didnt seem to work, that one does. At 3300 rpm the car would be doing 135 mph, which I may hazard sounds a bit high. WIth a 3.31 it would be doing 125 which seems a lot more sensible. Nothing is ever easy is it ?! I cannot see me stripping my rear IRS down to replace a new 3.07 with the 3.31 after just 4000 miles tbh

[andrewh]

Thanks Serge, our posts crossed. I agree

[paydase]

Both my FHC and OTS have a 3.31 with the Moss box.

When I purchased my first E-Type, fearing too high revs at speed, I thought I would replace that 3.31 by a 3.07 or possibly replacing the Moss box by a 5-speed.

I became however used to the Moss box, especially after having replaced the oil with Redline MTL.
And the replacement options looked to me quite expensive (installing a 5-speed or buying/restoring a 3.07)...

Years later, I observe that I only drive my E-Types on small roads and that I rarely explore speeds beyond 130 km/h on highways.
In these conditions, the 3.31 is perfect, and the Moss box is part of the adventure

[mgcjag]

Future proofing....Will the average younger person even want an E type

[andrewh]

If any car is going to remain desirable its sure to be an E type. But in essence, if I want to leave the car to one of my children they will need to drive it and the Moss is not for the inexperience at Double de clutching.

[MarekH]

Perhaps the sensible real world analysis is to ask what revs do I want to be running when cruising long distance on a motorway, a big A road or along the empty French roads and purchase accordingly? Everything else is lower gear "fun" or gear changing stop-start built up area driving.

kind regards
Marek

[angelw]

andrewh Wrote:

I am purely asking am I correct in thinking , all other factors remaining even, that a .63 5th gear will reduce my revs at a fixed speed by 37%?

Hello Andrew,
The reduction in input RPM (Motor Revs) will be 0.37 per rev when compared to a 1:1 top gear ratio. Accordingly, simply multiply 0.37 by engine RPM.

Regards,

Bill

[andrewh]

Thank you for your replies Guys. I need to drive my car down the motorway when the weather improves and assess its happy place. At least I now know the calculations for any chosen set up. Thank you

[Simonpfhc]

Can’t remember where I found this info, but it has worked perfectly for me:

I have found that the 3.8 and 4.2 Jag engines like a combined OD and final drive to work out around 2.5.
example: 3.54 x .73OD = 2.5842
3.08 x .83OD = 2.5564
4.09 x .63OD = 2.5767 ( used in XK 150 type kits)

I have a 0.8 5th gear, so my 3.31 x 0.8 = 2.648, perfect!
3.31 x 0.63 = 2.085 way too high!

[andrewh]

Thanks Simon. That’s a great rule of thumb. I have a 3.08 so with a 0.63 it’s anyone s guess . Must feel very tall indeed. Why JLR Classic built the gearbox with that ratio is beyond me. Maybe appealing to US 3.54 owners ?

[paydase]

Yes, typically for the 3.54 US owners liking to cruise on the highway