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[Snowman]

My Jeep is confusing me; I need help.
It has had a new rear diff; I'd like to identify it.
It seems to be some sort of lunch box locker, and it is marked 'J D'.
Any ideas besides 'Jeep differential' or 'John Deere'.
(Did John Deere make a diff that would fit?)

Secondly.
The ratios have been changed.
It's not lifted, it has standard 19" wheels.
The speedo is 15% over by my best guess.
By my caclulations I'd need 33" tyres to make this s right..
. is that correct.

Alternatively, 31s and a new speed sensor wheel.
But, I'd like to know the ratio.

It has a towbar, is may have been adjusted for towing.
But also has two locking diffs, wheel spacers and a few other off road mods.

Any comments or guesses are welcome.
Thanks

[Mike Pavelin]

Standard 19" wheels?

Count the number of teeth on the crownwheel and divide it by the number of teeth on the pinion. eg. 41/11 gives 3.73:1. Simples.

[Snowman]

Sorry.
That was a typo and a bad description.

It has standard 29" tyres. 

[Mike Pavelin]

If the speedo is reading slightly over and you're on standard tyres, I would say that's normal and the gearing was standard.

[Snowman]

It's not slightly over, it's a full 15% at least.
When I'm doing 70 my speedo reads 85.

I just wondered if there's a 'standard' regear that would do this?

I can't get into the diff to have a closer look - it's just had fresh oil and been done back up and I don't want to take it apart again.
The mechanic was pretty sure it's been regeared too.

As for the new diff.
The logo looks like a J D with a P inside the D.
Although it might not be a J.
I know that's a damned vague description, I'm just hoping for ideas.

[bigjeepzj]

could try jacking it up and counting how many times the prop turns to the wheels

[Jonny Jeep]

As already stated, jack it up and count the prop turns for 1 wheel rotation. And since it seems it has a different ratio than you had don't use 4WD.

[Snowman]

Why wouldn't I use 4x4?
Both diffs have been changed and the gears all match.

The question was just whether anyone knew what ratio change would result in a 15% increase at the speedo.
I just though someone might have been in a similar situation on 29" wheels while waiting for new wheels to arrive.

[JamesH]

My Dad's 2.5 XJ read about 15% over from the factory, I pointed this out and the dealer changed the speedo gear to make it about 5% over. It was less than a yr old at the time, standard 4.10 diff and 28/29" tyres. I think some just come from the factory like this. It's a new speedo gear to correct or do the diff cover off investigations to count R&P teeth and see what's actually going on inside. If it has been re-geared then you could go for the bigger tyres and suspension lift if that's what you want to do anyway.

[Snowman]

Interesting.
Obviously TJ's in the UK  are 3.07 from the factory as standard.

29" wheels have a circumference of 91"
Adding 15% to that means I'd need a 105 circumference to get my speedo back.
That's a 33" wheel.

Unless my maths are wrong.

[Ezz]

That's a 33" wheel.

33" tyre

[Jonny Jeep]

Why wouldn't I use 4x4?
Both diffs have been changed and the gears all match.

You had said "It has had a new rear diff" which is why I figured only one diff had been changed and said not to use 4x4.

[MOCAJ]

every car or truck when new reads 10% or more over the speedo 

Can you post some pics of your truck and diffs that would make identification easier, is it lifted and what makes you think the ratio is altered. It is usually only done to run different tyre sizes associated with a lift for off roading so some history would be good.

You can alter the gears on the speedo gear that fits into the transfer case if its an issue for you

[Snowman]

It's not lifted.
I've had a full service at 4play Jeep, the guy there knows his stuff and he thinks it's been regeared.
From what I've read about how Jeeps normally drive I believe him - but this is my only Jeep so I have no point of reference.

I didn't ask him to check the pinion because I didn't know to ask, and he thought I knew it had been regarded.

So, I am where I am.
It's no big deal - I'll get a new speedo gear or bigger tyres at some point.
I was just wondering if someone could make an educated guess.

As it happens, I'm more curious about the brand name on the diff to find out if it's a decent one or a cheap one.
J D. With a blocky P within the D.
Any ideas?

I will try and post some pics if I can work out how.

[MOCAJ]

I would say the diff has an aftermarket cover, probably like this one with maybe a bit of wear

if your guy is an expert he should be able to tell you all this 

[Snowman]

Nope - it's got the original/standard diff cover.
The only way to know it's been changed is by looking inside.
I believe it's called a lunch box locker according to wiki.

Same for the front and back.

The marking is on the gear not the cover.
And it's not a brand he recognised, and I've been unable to Google the logo.
Just wondering if anyone had any ideas.

[gazjeep]

Standard 19" wheels?

Count the number of teeth on the crownwheel and divide it by the number of teeth on the pinion. eg. 41/11 gives 3.73:1. Simples.

  seems the easy way to know for definate

[Snowman]

Does anyone happen to know the rpm of a TJ when doing 60 mph in 5th on a standard 3.07 diff and 29" wheels?

If you do, then I can use a GPS to find my rpm at 60, and then do some maths.
That'll be a lot easier for me that dismantling the diff.

[Snowman]

To put it another way;
In 5th gear.
My gps says 60 mph.
My rpm says 2500
My speedo says 69 mph.

Does that sound right or wrong to people with experience of such things? 

It's a 4.0 5 speed manual.

[Snowman]

If anyone is still interested....

I found this formula;

(RPM X Dire Dia. in Inches)
------------------------------------------------- = MPH
(Diff. Ratio X Transmission Ratio X 336)
Your trans ratio in 5'th gear is .84

By putting in all my numbers I reckon I have a 4:10 ratio in there.

Definitely time for bigger tyres.

[Ezz]

Don't go to big on the tyres,

I've got 4:10 with 35" tyres and the gearing is pants

[Duncan C]

To me it depends on weather you are happy with the way it drives now - you seem more worried about the speedo reading over than the actual drive. If that is the case just change the speedometer gearwheel in the transfer case

 http://www.4x4xplor.com/speedo.html

It takes about 10 mins and cost about £30 - a lot cheaper than new bigger tyres!

If you go down the other route I had 4.56 ratios with 33" tyres on a 4.0L manual TJ and the gearing was really nice, albeit a lower than standard, so you perhaps wouldn't want as big as 33"

[Snowman]

The gearing isn't good.
It's quite fun having too acceleration, but the lack of a top end does bother me on the motorway.

I think I'll stick with it till the tyres are worn out - then I'll ask around about a good choice of new 30-31" tyres on stock wheels, or maybe some new wheels.

[gordy]

Before I had my 98 TJ re-geared, running 33" tyres on 3.07 the sat nav said 30 the speedo said over 40, more like 42 IIRC and in 5th gear the RPM was like 1500 when doing 50 on the sat nav and the speedo was reading 65.

[Jonny Jeep]

Plugging some numbers into the gear calc at http://www.grimmjeeper.com/gears.html I'd say you have 4.55 gears which are ideal for 33s.

Selecting AX15, NP231, and entering 29 inches for tyre size and 4.55 for axle gear ratio it shows 2500 rpm in 5th giving 60 mph.