Jump to content

Tyre Sizes + Gear Ratio's

Recommended Posts


after fitting a discovery transfer box to my standard 110 defender, and after covering 4000 miles across spain and france, i have come to the conclusion that i need to claw back aroud half a gear ! at present i have 235/85/16 all terrain tyres fitted, i am hoping that if i simply fit 235/70/16 tyres this may achieve my goal ?

as i need a recon gearbox and recon transfer box anyway, would i be better keeping the taller tyres and altering the transfer box ratios instead ?

my present setup is fantastic on frances long flat roads, and its fine on the spainish roads, but any hills and mountainous regions see me changing gear alot,( not towing ) and i am now planning a trip to italy, but this time towing a caravan, and i know that the present setup, will have my slipping the clutch just to set off etc, and thats before i hit the alps !

i really enjoy the reduced engine noise and the taller gearing, and i dont want to fit bigger intercoolers, mess with the pump timing etc, just end up with something between standard defender gearing and the discovery ?

maybe having a gearbox built with standard 1st,2nd,3rd gears and then taller top gears would do the trick ?

its a minefield and some would probaly say, leave well alone, i dont go serious offroading, but i would like to keep my low box standard, just in case..oh and no overdrives please :-)


1991 3dr 200tdi defender 110 (called poppy)

1974 s3 airportable (with no name as yet)

Link to comment
Share on other sites

Am I right in thinking the disco 'box is the 1.2? I'd be thinking 1.4 [+ overdrive in the long term, although at a price] would suit you.

what's the gen on putting a 1.4 in a n/a d as an improvement from the 1.667 on normal 7.50 tyres? I could do with a marginally higher top speed myself.

Link to comment
Share on other sites


yes your right that the disco transfer box is 1.2 as opposed to the 1.4 in the defender, i didnt really want to go faster,( my standard defender transfer box propelled me over the speeds limits !)

most of the countries i have/am visiting have speed limits similair to the uk, and i have seen the results of some of these clowns in big japanese 4x4's hulterling down roads with caravans/trailers, going far to fast, not good !

as for fitting one to a n/a 2.5 diesel, i cant help really, my original tyres were 7.50x16, but they needed replacing anyway, so i bought the 235/85s which lifted the gearing very very slightly.

i have spoken to several people reguarding overdrives..but at a £1000 i can have a gearbox built !


Link to comment
Share on other sites

235/70 are standard Disco 200/300 tyres, gonna look a bit silly on a 110, lots of air beneath the arches :blink:

You can't fit a higher top gear to the main box and there's nothing in between a disco 1.2 and a defender 1.4 transfer.

Better off keeping the 235/85s, 1.2 transfer and getting the injection pump tweaked to give you a bit more oomph and torque on the hills. Details were on the old forum for a diy tweak. Anyone got a link to the US web page it came from originally??? Take it we are talking about a tdi 110? :unsure:

Link to comment
Share on other sites

Hi dasterdly - I had exactly the same issue with my 90. Although it's a V8 I fitted 235/85 x16 tyres which raised the gearing just a little too much. The original transfer box ratio was 1.22 - very close to the disc gearing. I had a 1.411 transfer box fitted from Ashcrofts which has totally transformed my truck! :o

Although it only raised the revs by a few hundred when cruising it's responsive and doesn't feel out of sorts any more.

I would strongly recommend you take this route too.


Link to comment
Share on other sites

i guess this is a follow on thread from the lre forum regarding the fitting.

when i first had my td5 i went through the same thought process but decided against the 1.22 box due to the area i live in being quite hilly and i tow alot, and thats with 125 or so bhp so 15bhp over a tdi in std tune. i am running 235/85s.

in the end i bit the bullet and fitted a GKN overdrive, thus retaining the std gears but with that longer gear for motorway/main road crusing. it will pull o/d from 50mph. abd drops rpm by 800 at 70mph. it was fitted at 20k miles and i have done 70k miles now with no problems attributed to the o/d and mpg increased 10% or so as well.

Western recently picked one up for considerably less than the new price so they are around. try ringing someone whos selling a defender with one fitted and ask if you could have the o/d separately. in my opinion its well worth the investment. plus you can keep the 235/85.

alternatively rebuild the t box (again) back to the std 1.4 gears, as prev mentioned theres noting between the 1.4 and 1,22 box and keep the 235/852

or another option change to 4.11 diffs, thats a mod fitted that will take a 1.2 high range box almost back to std 1.4 box, it may be slightly higher than a 1.4 box in the end although again prob cheaper to just go back to 1.4 box.

try ringing Dave at Ashcrofts if he doesnt shopw here.

Link to comment
Share on other sites

just dug this out again, my findings from the GKN overdrive, it raised some questions on the old forum:

Having being asked by several forumers for my thoughts on the GKN overdrive I thought I would compile the points as below and post for all interested parties. Primarily i thought these points may be of interest to those considering fitting the unit off set against cost/economy gain. I fitted mine on 22 April 2000 at 22,700 miles. The vehicle now has 67,900 miles on it.

Prior to the purchase I looked at engine speed per 1000rpm for the different gears and worked out what reduction the overdrive (O/D) would make assuming a 25% reduction in engine speed. Results are shown:

mph 50 60 70 80


Gear Mph/1000rpm Std O/D Std O/D Std O/D Std O/D

3 12.57 4.0 3.0 4.8 3.6 5.6 4.2 6.4 4.8

4 17.56 2.8 2.1 3.4 2.6 4.0 3.0 4.6 3.4

5 22.81 2.2 1.6 2.6 2.0 3.1 2.3 3.5 2.6


a. All engine speeds x 1000

b. The figures in italics show theoretical engine rpm.

c. Shaded boxes show likely main usage area.

The table shows that 3rd + O/D is the same engine speed as std 4th, 4th + O/D is the same engine speed as 5th. (when I same the same within 200rpm)

Thoughts prior to purchase and subsequent Findings

1. 70-75mph with the O/D engaged has the same engine noise as 40mph in 5th, although still have the tyre noise and wind, just makes them more apparent. You don’t need to turn the stereo up!

2. My economy went from:

a. ‘A’ roads old 27/28mpg now average 30/31mpg

b. Motorway old 29/30mpg (65-70) now 31-34 (70-80)

c. Constant 60 motorway old 31mpg now up to 33/34mpg

3. To recover the purchase price of £992 it will take the following miles to break even:

Fuel Cost O/D saving on fuel

20% 15% 10%

0.889 p/ltr 40,200 miles 50,800 miles 73,700

The different % savings are applicable to how you drive the vehicle. I primarily stay at the same speed as I used to drive without the unit fitted and have averaged a predicated 15% saving

4. I use mine primarily as a 6th gear although it can be engaged in nay gear above 30mph, It is suggested not to use O/D in third due to the high level of torque that can be put through the unit.

5. The O/D puts less stress on engine, drops revs I reckon by 800rpm at 70mph which was calculated and is proved by the fitted rev counter. Which now means 70mph is closer to peak torque for the engine at 2100rpm.

6. Easy to use, the button is ideal on gear knob although it looks ………. Different. You soon get used to it

7. The whole unit is transferable to other vehicles on sale.

8. Excellent for towing as smooth full on power shift from 4th to 4th + O/D instead of using 5th which means no loss of momentum. The TD5 will pull 65mph with a trailer and O/D engaged although you will need to change down for hills. See note below on economy though.

9. For economy have found it best to change to 5th at 70 on hills rather than stress the unit in Overdrive.

10. No problems off road with it fitted – its seems quite durable and is well machined finished. It is not used off road but is obviously still fitted. Clearance of the td5 exhaust middle box can prove to be a problem and should be monitored when first off roading to ensure it doesn’t foul the unit. If it does I would suggest a straight through pipe to replace the middle box. Mine is still running the mid box, off roading hasn’t proved to cause any problems in this area.

My Problems experienced.

11. It takes about 1000miles (of O/D engaged) for the O/D to wear in. Up to this point due to the lack of room for oil within the unit, mine used the breather pipe as an escape route for the ATF. This mean’t when the unit had warmed up the fluid moved up through the breather pipe, due to heat expansion, resulting in the engine bay being covered in ATF. (This is where the breather head was routed to). To overcome his I mounted the breather up by the air cleaner/bulkhead, incorporated a ‘U’ bend and cable tied it into position. I then fed it through the top of an old orange squash bottle so that any ATF that came up the breather was contained. You could also see how much oil had come out and top up the unit as needed. It started off emptying about half the oil over a 2hr motorway journey. As it bedded in it threw out less ATF. The total oil used was about 1.5L total over 1000miles. It is my understanding this issue has now been overcome with a different breather set-up. My O/D was one of the first units produced and no doubt like LR do the customer was the trials period!

12. I found no difference in economy for the 1st 500 miles of O/D use. Although this could have been because I was driving faster!

13. I had problems aligning the unit to start with on the interface resulting in a leak, I had to remove unit and refit (twice) used instant gasket in the end having destroyed the original that came with the unit.

14. The filler plug leaked, I overcame this by adding a home made washer from an old inner tube. You can otherwise over tighten the steel filler plug in an ali body and end up trashing the thread.

Fitting the unit.

This was reasonably straight forward, remove PTO cover on back of transfer box, remove transfer box gear and associated bearings, then offer up the O/D interface plate with appropriate gasket, torque up. You will need allen sockets (sockets with part of an allen key in them) to get the right torque (40Nm) and add O/D unit to plate. It extends about 9ins from the rear of the transfer box as it stands now, I had to make up a bracket to drop the exhaust 1†as the mid box just caught the end of the O/D unit, although I believe this is unnecessary on a 110 as the mid box is further back. It took me about 2hrs to do the above in slow time, plus you need someone to help with the O/D unit as its quite heavy to man handle under a vehicle. I did it in a garage with no ramps etc, so therefore had to remove the cubby box etc to allow top access.

Its then only a case of fitting the breather and electric’s. I fitted the control box to the base of the cubby box.

The whole lot is very well explained in the pack that comes with it.

Make sure you have about 3L of ATF prior to fitting to allow 1L to fill and subsequent refills – see problems!

Overall it took me about half a day total to fit the unit. My recommendation would be to fit the unit yourself.

Tools Needed(suggested min)




Allen sockets

Torque wrench

Extra cable ties

About 3L ATF

Nimble hands

Sense of humour

A helper for actual fitting of unit


I told my insurance company about it (NFU) and it made no difference to the premium.

The unit has now been fitted for over 40k miles and since the first 1000 miles has been completely trouble free. I change the ATF in it every 36k (so have done it once) and use it daily.

Out of interest I was going to get a set of disco high range gears fitted. However as I live in a hilly part of the country and do a lot of towing I was advised against this. The general consensus around friends with disco gears fitted is that overall economy around ‘A’ roads goes down as you are in a lower gear e.g. 4th when you would have been in 5th however motorway/dual carriageway journeys the economy improves as you can now pull 5th gear.

The above is my experience with unit that I was supplied in 2000. I believe the breather problem/oil coming out has now been overcome. I have no connected with GKN I am just pleased with their unit and after sales service regarding advice on the initial oil spillage. Please feel free to contact me if you require any further info.

I hope this helps anyone thinking about getting the unit or similar design.

All the above are my own personal findings and I am not saying you will have the same experience/fuel saving if you fitted the device.

Link to comment
Share on other sites

Hi all

I worked all this out some time ago and in simple terms it works out like this. Fit 235/85 x 16 tyres on your defender with 1.44 ratio transfer box and the overal ratio works out to be the same as if you now had a disco 1.2 ratio transfer box.

With larger tyres I would not want to run the disco transfer box ratio as well because I think the overall gearing would be to high, especially if you do much towing.

My fuel consumption on open roads in my 300 Tdi 90 is around 22 to 25 Mpg best at 65 Mph cruising. 70 Mph + & it drops but at 40 to 45 I can get around 28 to 30 ish.

Link to comment
Share on other sites


thats the exact same conclusion i also came to.

ditto 255/85s on a 1.4 T i reckon is the same a 238/85s and a 1.22 t box.

interesting how Western can cruise with 255/85, 1.4 box (i think) and GKN over drive. mine wouldnt pull that, same engine as well. with 255.85s 5th is almost a mortorway only gear.

Link to comment
Share on other sites


after speaking to various gearbox type people, ashcrofts included, not all lt 77 gearboxs are the same, mine, as it turns out isnt a standard 200 tdi box, is a pre tdi unit, i.e 90/110 not defender, and the gear ratios are differant.

so anybody embarking on any gear ratios changes etc, make sure you have the serial number in front of you, and dont assume you have the correct box simply because its an lt77, somebody may have swapped it.

in my case i am going to fit military 1st and 2nd gear ratios, which solves the towing problem, 3rd is fine as it is, 4th you cant change as its 1.1 (one to one) and i can drop 5th slighly.

now all i have to do is plan the route to italy !

thank you for all the replies and advice.


Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...

Important Information

We use cookies to ensure you get the best experience. By using our website you agree to our Cookie Policy