Tyre size, Diff ratios etc etc

[rrturboD]

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]

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]

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?

[Bigbjorn]

The formula is (RPM x 60) divided by (overall gear ratio x tyre revs. per kilometre)

[Defender200Tdi]

Gear Ratio Calculator

http://forums.lr4x4.com/index.php'showtopic=4103&hl=gear+ratio+calculator


Paul:)

[Xtreme]

Gear Ratio Calculator

http://forums.lr4x4.com/index.php'showtopic=4103&hl=gear+ratio+calculator


Paul:)

Excellent spreadsheet.
Thanks for the link Paul

[Sandtoyz]

Hi,
Try this simple program out,
Just unzip it into its own folder,
instructions are in the doc file,
We use it on our race car :)

[rrturboD]

Thanks for the help, and the links, just what I needed, so now to decide!

[squaddiefox]

do it! fit the 36" tyres! :D

[rovercare]

Extra 600rpms at 100km/h, you'll be on the governor:eek:

These diff's dont happen to be a maxi drive rear and an ARB front:angel:

[rrturboD]

These diff's dont happen to be a maxi drive rear and an ARB front:angel:

nope these are maxi drives F & R (ex disco buggy)

[rovercare]

nope these are maxi drives F & R (ex disco buggy)

Righto, just i sold and sent front and rear 4.11 locked diffs to a bloke in canberra a while back, thought they may have been them:)

[rrturboD]

Righto, just i sold and sent front and rear 4.11 locked diffs to a bloke in canberra a while back, thought they may have been them:)

I had been looking for lockers for ages, got mine from Newcastle, then you advertised yours, and a couple more have changed hands since.

I think I will revert the CWP to 3.55 for now, and if I decide to get the engineering done etc for larger than 31" tyres, then I can go back to the 4.11s. Probably should check the $ to change the Transfer case to 1.2 instead and at the same time could add Maxi lower low kit.

Decisions decisions!

[Bigbjorn]

The formula is (RPM x 60) divided by (overall gear ratio x tyre revs. per kilometre)

7.50 x 16 tyres are 408.5 revs. per kilometre & 31 x 10.5 x 15 are 417.4 revs. per k.