Final Drive Ratio calculations ...

Gary Stockton

Super Moderator
Supporter
I am in zimbabwe
Sep 10, 2012
3,924
38
48
Edinburgh
Garage
Quick question - how do I calculate final drive ratios?

If I have a diff ratio of whatever and a gear box model A343F, and going from 265/60x17 (factory standard) to 285/70x17 Coopers I was curious to know what impact this will have on driveability - would I need to alter diff ratios etc. Anyway, a bit of googling found this site:

http://www.grimmjeeper.com/gears.html

which is pretty damn good I suspect (although I expect Dr Jon will have his own calcs up his sleeve in the Tardis somewhere :cool: )

Anyhoo - the above link shows I'll have taller gearing, as to be expected, with road speed going from 60 to 64mph at 2250 rpm. One to watch out for - I'll have to recalibrate the on-board GPS and also watch out for speed cameras a bit more :lol:

So - do I need to alter the gearing now, or is it still in in an OK zone?

Cheers

EDIT: Actually it's the A750F gearbox, which does this:

2250 revs goes from 62 to 67 mph
70 mph comes down from 2527rpm to 2361 rpm

Interesting stuff .... maybe better for economy, slightly, but what about rolling resistance etc.?
 
Don't like the adverts? Remove them by becoming a supporting member.   Click here

Crispin

Administrator
Staff member
Guru
I am in great_britain
Feb 24, 2010
5,970
576
113
springs, gauteng
GPS would not need recalibrating as it works of satellites and just calculates your position and movement. The speedo though counts the revolution of the drive shaft and assumes you have the standard tyres on. If the circumference of those changes, you would have a sight error. Seeing as all speedo's are, by law apparently (May be wrong, it's what a birdie told me) , optimistic, you would only have a truer reading on the clocks.

Sat-nag would know no different.
 

Gary Stockton

Super Moderator
Supporter
I am in zimbabwe
Sep 10, 2012
3,924
38
48
Edinburgh
Garage
Crispin said:
GPS would not need recalibrating as it works of satellites and just calculates your position and movement. The speedo though counts the revolution of the drive shaft and assumes you have the standard tyres on. If the circumference of those changes, you would have a sight error. Seeing as all speedo's are, by law apparently (May be wrong, it's what a birdie told me) , optimistic, you would only have a truer reading on the clocks.

Sat-nag would know no different.
Errr - no. The LC5 GPS runs off satellites and wheel rotations. It uses that (including delta btn wheels to account for turns) to figure out where you are after an extended period of no satellite cover, e.g. Long tunnel, tree cover, etc. The owners manual had a whole bit on recalibration - I'll go read it :).

So I'll go calibrate after I change boots (but I need to get 17" x 8" steel rims first ;) )

Additionally, as I found out when looking for the speedo pulse plugs to fit the tripmeter data logger for the France trip, there are three takeup points - gearbox and transfer case, so recalibrating wheel sizes should help a bit, I guess. I'll have to recalibrate the trip meter thing too, I expect. Just another excuse to go for a nice drive!!
 

Crispin

Administrator
Staff member
Guru
I am in great_britain
Feb 24, 2010
5,970
576
113
springs, gauteng
Fancy bloody LC5s.

Before driving in Italy and Monaco, I would have said needing a GPS in a tunnel is daft. They have some very complex intersections in those long tunnels.
 

Rob

Well-Known Member
Mar 1, 2010
3,019
16
38
Bristol
Garage
Useful site, saves a few minutes having to do that in excel. Now all we need is a link to a site with the diff and gear ratios.
 

Jon Wildsmith

Super Moderator
Supporter
Guru
I am in england
Feb 24, 2010
6,729
156
63
Stourbridge, West Midlands, England
Garage
You don't need drive ratios to work out % gearing change at the wheels caused by tyre size changes, just the % change in wheel/tyre combo diameter. I'm sure most people know but for the benefit of those who haven't got the hang of tyre profile numbers yet, for the smaller size, 265 is the nominal width in mm, 60 is the side wall height expressed as a percentage of its nominal width so that's 60% of 265mm, and 17 is the wheel diameter in inches :mrgreen: So a 265x60x17 is 159mm+431.8mm+159mm=749.8mm overall diameter. The 285x70x17 is 199.5mm+431.8mm+199.5mm=830.8mm overall diameter. You don't need to work out the circumference of the tyre because it has a linear relationship to the diameter so a 10.8% increase in diameter is also a 10.8% increase in rolling radius so no pi is needed :) Being as both sizes are for a 17" rim you didn't even need the diameter, just the side wall heights (159mm & 199.5mm). Sorry if I missed the point and you knew this.
 

Jon Wildsmith

Super Moderator
Supporter
Guru
I am in england
Feb 24, 2010
6,729
156
63
Stourbridge, West Midlands, England
Garage
Gary Stockton said:
Interesting stuff .... maybe better for economy, slightly, but what about rolling resistance etc.?
I think they'll be quite a bit heavier, have greater rolling resistance and remove an mpg or 3. It's no good thinking about mpg if you're thinking about big tyres in my experience.
 

Gary Stockton

Super Moderator
Supporter
I am in zimbabwe
Sep 10, 2012
3,924
38
48
Edinburgh
Garage
Jon Wildsmith said:
[quote="Gary Stockton":yp5276ow]Interesting stuff .... maybe better for economy, slightly, but what about rolling resistance etc.?
I think they'll be quite a bit heavier, have greater rolling resistance and remove an mpg or 3. It's no good thinking about mpg if you're thinking about big tyres in my experience.[/quote:yp5276ow]

I think that applies to LCs in general ;) As soon as we get one, we start to mod it, most of which add frontal resistance or rolling resistance or something else that screws with fuel economy in one way, shape or form. Live with it ...

I enjoyed reading your earlier post BTW - very useful (but very confusing to my currently addled, lurgy-impacted brain).

I guess where I was going was, hypothetically, if you fitted larger wheels etc. and wanted to keep RPM : MPH the same, would you need to re-gear somewhere along the line if the final drive ratios were going to become so extreme that you'd never get out of 3rd for example? But as that's not going to happen, and the % change I'm looking at is minimal in the overall scheme of things (drive train play, engine wear, etc etc) I'm not even going to pain my brain by thinking about it - and I guess the more that's left alone the better? As long as the stresses introduced are not out of deign tolerance we should be OK ... :?

Cheers
 
AdBlock Detected

We get it, advertisements are annoying!

Sure, ad-blocking software does a great job at blocking ads, but it also blocks useful features of our website. For the best site experience please disable your AdBlocker.

I've Disabled AdBlock    No Thanks