Gilslandy Posted October 28, 2009 Share Posted October 28, 2009 hi guys and gals, I have a 2000 90 td5 with a remapped 2004 ecu. last month i changed the tyres to a set of insa turbos, 285's. from then i lost alot of the acceleration and torque i had on my 90. and i was consuming to much fuel. I had my speedo not reading well due to the change in tyre size. LAst night i fitted Aschcrofts 4.11 ring and pinion and i was so happy to get back my acceleration and torque, and hopefully less fuel consumption. I would like to ask, with the new ring and pinion, does that correct again the speedo reading please? and regarding top speed, will it be less with the new ring and pinion? btw i have the standard 1.4 transfer case. thanks alot. Quote Link to comment Share on other sites More sharing options...
FridgeFreezer Posted October 28, 2009 Share Posted October 28, 2009 You can work it out with maths as the speedo drive is from the output of the transfer box (basically propshaft rotations) so you can work out how many rotations your propshaft does per mile before & after. There is a very good gear ratio calculator in the technical archive which includes tyre size and allows you to compare side-by-side. Or you could check it with a GPS. Quote Link to comment Share on other sites More sharing options...
zoltan Posted October 29, 2009 Share Posted October 29, 2009 Haven't you said goodbye to fuel economy with the change of tyres? If the tyres are at a larger diameter and have lost you acceleration and torque, to have to change gearing to regain acceleration means you are going to have to spin the engine faster now to maintain the same cruising speed pre tyre change surely? Not to mention the possible increase in rolling resistance from the larger diameter tyres. Just a thought Quote Link to comment Share on other sites More sharing options...
Lars L Posted October 29, 2009 Share Posted October 29, 2009 Changing the gearing from 3,54 to 4,11 makes a difference of 17%. Changing tires from 235/85 to 285/75 is only a little over 3% (according to one of the calculators). So what you have got is quite a gearing down change. Meaning that the wheels have to turn faster than before at any given speed. Am I right? Quote Link to comment Share on other sites More sharing options...
q-rover Posted October 29, 2009 Share Posted October 29, 2009 When I had a 300Tdi 110 I fitted 255.85x16 (same diameter within a few mm) and never noticed any difference in performance or fuel consumption. So with a Td5 it seems even stranger. Although the speedo did read accurately after the tyre change. Quote Link to comment Share on other sites More sharing options...
Lars L Posted October 29, 2009 Share Posted October 29, 2009 Mine did too, but the engine at that time, 2,5 n/a, didn't really cope. Quote Link to comment Share on other sites More sharing options...
FridgeFreezer Posted October 29, 2009 Share Posted October 29, 2009 Changing the gearing from 3,54 to 4,11 makes a difference of 17%. Changing tires from 235/85 to 285/75 is only a little over 3% (according to one of the calculators). So what you have got is quite a gearing down change. Meaning that the wheels have to turn faster than before at any given speed. Am I right? Unless the 90 was running 205's to start with, in which case it probably would make a noticeable difference. It's a fair point though, 285/75's should not make a 90 struggle what with 235/85 being pretty much standard fitment on most 90's and all 110's. Quote Link to comment Share on other sites More sharing options...
callum Posted October 29, 2009 Share Posted October 29, 2009 i'd concur with that, i run 285's and recently changed to a 1.222:1 transfer box. different engine, but a td5 will proabbly be putting out more power than my home tuned isuzu so it really shouldn't be struggling. what might be the case is it came with 205's originally and you're still running that speedo drive gear. that will makes the speedo read well under and your mpg appear lower. Quote Link to comment Share on other sites More sharing options...
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