JST Posted December 27, 2005 Share Posted December 27, 2005 I posted this on the old forum (which i cant find the ref to now) and as some of you have asked about the unit in the past i though i would post my findings again: 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 72,200 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: Note: a. The figures in Red show theoretical engine rpm. b. Green likely main usage area. c. 1.4 T box and 235/85s 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: Calculated with 0.889 fuel cost at time of fitting. Estimated savings of fuel at 20% - 40,200 miles 15% - 50,800 miles 10% 73,700 miles 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) Sockets Spanner Screwdrivers Allen sockets Torque wrench Extra cable ties About 3L ATF Nimble hands Sense of humour A helper for actual fitting of unit Other I told my insurance company about it (NFU) and it made no difference to the premium. The unit has now been fitted for over 50k miles and since the first 1000 miles has been completely trouble free. I change the ATF in it every 36k 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 More sharing options...
Ben Jordan Posted December 27, 2005 Share Posted December 27, 2005 I have no need or plan to have an over drive but thats a reaaly good indepth write up. What do you plan to do a report on next ?? I would read it. Link to comment Share on other sites More sharing options...
Ben_J85JTY Posted December 27, 2005 Share Posted December 27, 2005 Out of interest, Whan you mention about the disco ratio gearing, was that before or after you fitted the O/D? Only I have a Disco ratio box from ashcroft at the moment, and want to fit a GKN as well. Don't suppose you know if it is possible? Cheers, Ben Link to comment Share on other sites More sharing options...
western Posted December 27, 2005 Share Posted December 27, 2005 Out of interest, Whan you mention about the disco ratio gearing, was that before or after you fitted the O/D? Only I have a Disco ratio box from ashcroft at the moment, and want to fit a GKN as well. Don't suppose you know if it is possible? Cheers, Ben IIRC a Disco Transfer gearing is 1.2 to 1, with a o/d fitted you wll be overgeared & I believe you have problems pulling in the o/d gears, my 110 runs the standard 1.4:1 gears & 255/85 tyres & GKN o/d, which I use as a splitter on 4th & 5th & as a 6th gear on A roads,DC's & Motorways. I'l have to record some figures to enable a comparison with JST's numbers above, my 4th o/d is almost the same as 5th as well. Link to comment Share on other sites More sharing options...
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