Working out Gearing.

[wanderinstar]

There has been quite a lot of discussion on here recently re diffs, gearboxes and tyres.
Is there a simple formula to work out what the speedo will read with a 3.7 diff and running on 185/70 x 14 tyres. I have already found out that using the tyres I intend to use I will be doing 73.21mph when the speedo shows 70mph. So what will the 3.7 diff do to these figures (it replaces a 4.22).

[bmcecosse]

Well - the ratio is 4.22/3.73 = 1.13
So -if your speedo was showing 100 mph (and was correct before) you would actually be going at 113 mph with the 3.73 diff. That's before you factor in your tyres of course. I'll leave you to do the math!

[mike.perry]

Does someone with a brain fancy producing a chart of speeds / 1000 revs. for the normal range of diffs (5.375 -3.7) based on standard wheels?

[PSL184]

Not really cause brain hurts too much, but if you have a rev counter then use this...
Speed x Axle Ratio x 336 divided by tyre height (in inches) = rpm
You can use this with varying tyre sizes to get overall speed at any given rpm.

[morrisman1]

google it, very cool little app

[wanderinstar]

But what do you put into Google??

[Matt]

not *quite* what you want... but you can use it to compare speeds at given rpms

[Mogwai]

this gearcalc program is good just imput ratios/ tyre size etc & it calculates speeds in all gears to engine rpm
http://www.locost7.info/gearcalc.php

[badobsession]

so if i have a 9/38 stamped on the diff what ratio is it ???
ta .

[Welung666]

so if i have a 9/38 stamped on the diff what ratio is it ???
ta .

4.22:1

[badobsession]

so if i have a 9/38 stamped on the diff what ratio is it ???
ta .

4.22:1

and will that be out a van (low ratio)

have an other one in the shed it has 9/41 stamped on it is it for a car?

and if i was looking out for a 3.9 what no. is stamped on them .

[Welung666]

9/41 = 4.55:1

9/35 = 3.9:1

[bmcecosse]

It's simple math - the 35 divided by 9 = 3.89!

That 4.22 diff - is the standard for all 1098 cars/travellers - it's not a 'low ratio' as such, although a 3.89 and then a 3.73 are certainly 'higher', although numerically 'lower'.

[IslipMinor]

Wanderinstar,

The answer to your original question is that 185/70x14 tyres and a 3.7 diff will give 82.83 mph instead of the 'standard' 70 mph.

This website gives details about any tyre size that you are interested in:

http://www.powerdog.com/tiresize.cgi

I am pretty sure that the 3.9 diff is 10/39 rather than 9/35?

Based on 145/80x14 radial ply tyres, which have a slightly larger rolling radius than the 'original' 5.00x14 cross plies, the mph/1000rpm are:

5.375 12.8 mph/1000
4.555 15.1 mph/1000
4.222 16.3 mph/1000
3.900 17.6 mph/1000
3.727 18.5 mph/1000

[wanderinstar]

thank you for that Islipminor. That is a hell of a difference, no wonder I keep getting these little speed warning smilie things flashing at me. Think I had definitly get speedo recalibrated.

[morrisman1]

when i said google it, i mean to google gearcalc, i missed that out!