Tyre size, Diff ratios etc etc

**rrturboD** · 11th July 2007, 10:55 PM

GDay all, I am looking for some help investigating the engine speed impact that 4.11 diff ratios will have if fitted etc etc.
My RR is fitted with a Tdi300, Diesel ZF Auto, 1.4:1 Defender transfer case, currently std diffs, 31x10x15 tyres.
I have the opportunity to fit locking diffs, currently fitted with 4.11 CWP combo. I understand that the 4.11s will raise the engine speed at all road speeds. (I like to travel with the traffic on the highway, and do not want to over stretch the Tdi300)
Is there a calculator or formula that will give me some idea of the engine speed at various road speeds?

**Xtreme** · 12th July 2007, 06:49 AM

Difference in final ratio (3.54 compared to 4.11) is 16%
Does that mean a 16% increase in engine revs for same speed or is that too simple?

**wovenrovings** · 12th July 2007, 07:17 AM

Nope it is that simple. (it is actually 14.8%) The engine revs will rise 14.8% in all gears for the same road speed. Also your speedo will be out 14.8% low. What some people do is fit 14.8% bigger tyres or the reverse, change the diff cause of the large tyres or change the high range ratio so that it is 14.8% higher, and keep the extra gearing for low range. So multiply your current engine revs by 1.174 to get the revs with 4.11 diffs. So if you change you transfer case to 1.19:1 you will be back to standard gearing (in high range). Or you could get the same effect by taking off your 31"(eg) tyres and put on 36" tyres. Does that help?