Diff Ratio Versus Engine Revs

[FB MAD]

Hi All,

Was having a fiddle last night................................. hang on I'll have another go at this

Was working on some calculations using an online Diff ratio versus Engine Revs calculator and came up with some theoretical engine speeds using various diff ratios.

Might help those contemplating changing the original grey motor diff centre ratio ( 3.89 ) to a better ratio for economy ( red motor diff centre ).

The original 13 inch crossply tyres were 24 inches diametre but most 13 inch tyres available today are around 22 inches so I used that tyre diametre in the calculations.

Here goes,

3.89:1 @ 60 mph = 3565 rpm
3.55:1 @ 60 mph = 3255 rpm
3.36:1 @ 60 mph = 3080 rpm
3.08:1 @ 60 mph = 2825 rpm
2.78:1 @ 60 mph = 2550 rpm

For those old farts like meself who find it just a bit too much sitting on the 60 mph I did some 50 mph calculations.

3.89:1 @ 50 mph = 2975 rpm
3.55:1 @ 50 mph = 2720 rpm
3.36:1 @ 50 mph = 2570 rpm
3.08:1 @ 50 mph = 2355 rpm
2.78:1 @ 50 mph = 2125 rpm

These figures would have some error margin of maybe a few percent but would give anyone contemplating a ratio change something to go on.

I could throw up some more calculations with 14 inch wheels if someone wants them but I'd need to know the tyre diametre to do it.

Cheers,

Terry.

[rosco]

Hi Terry,
Glad to see that I'm not the only one who enjoys specifics.
I had a play with this twenty five years ago - to find suitable ratio when fitting the red into mine.
Put in a 3:55 to start with then jumped up to a 3:08 a little later on.
I found the tyre size to be a substantial factor when it came to speed.
Not so much low down, but at highway speeds there were apparent discrepancies.
I have since purchased a GPS unit and found most of my original calculations were fairly good, fitting 185/70 x 13 tyres has opened the error margin again - the 175 x 13's were fairly close to the original 6:40 x 13's.
You have done very well with your figures. They are sometimes perplexing to work with and if one figure used as a base is out, the other follow suit in a parabolic sense.

I mailed out my chart a week ago or so to some members of the forum.
I don't believe you were on the mail-out list but if you want it - I'll need an email address.
The chart I put together is a complete set of engine rpm v speed for the four gears of an Aus using diff ratios of 3:89, 3:55, 3:36, 3:08 & 2:78.

The chart indicates associated speeds at engine revs from 500 to 4500 in increments of 100 rpm.

It was calculated using the following data from the EJ/H Workshop manual :-
829 wheel revolutions per mile
20.03 mph per 1,000 engine rpm using 3:55 diff ratio
6.40" x 13" 4 ply tyres.

If anyone is interested, just pm me with an email address...........
I will spreadsheet it one day - but not yet.

Of course, GPS units and tacho's are a lot easier.............

frats,
Rosco

This is the blurb I posted with the chart

As promised – please find attached sheet for diff/gearbox speed vs rpm as calculated circa 1982
Gears/diff ratios across the top, engine rpm’s down the left side – figures quoted in boxes are in mph.

They will be fairly accurate using 6:40” x 13” tyres – I am led to believe this is approximate to 175/13’s I do not have conversion figures for latter day ie 185/75 x 13 – all I know is that the “new” figure ie 75 is the percentage of wall height compared to width of tread.
I do know that my 185/70 x 13’s are approximately 3 – 4 mph out – reading high on the speedo – ie 60 mph shown on the gauge is only around 56-7 mph actual.

I worked on the reference datum for a 3:55 ratio diff centre (standard EH centre) at 829 tyre revolutions per mile equating to a speed of 20.03 mph per 1,000 rpm- this came from the EJ – EH workshop manual.

If we play around with that by altering the ratio aspect – that’s where the data base was created for figures quoted.

I haven’t played with km/s hour yet but here is an example – we know that there are 1.609 metres in a mile – therefore we multiply 20.03 times 1.609 gives us 32.23 km/h per 1,000 rpm..............

If we increase the ratio from 3:55 to 3:89 (standard grey diff centre) that would reduce the km/h to 29.4 km/h per 1,000 rpm so therefore at the rated maximum engine revs for a grey being 4,400 we multiply that by 29.4 and get 129.41 km/h or 80.43 mph – of course this is at 4,400 rpm – I know they will go a fair bit higher.



To get the equivalent rpm using a grey centre and creating a ratio to how many wheel revolutions per mile on a grey diff centre we divide 829 by 0.912596401 (that is the formula between a 3:55 and a 3:89 diff centre) we now have 908.3971831 equivalent revolutions per mile of the same wheel using a grey 3:89 centre.

To get how many equivalent wheel revolutions per k/m we divide 908.3971831 by 1.609 ( that’s the formula for km/s per mile) = 564.572519 wheel revolutions per kilometre.


To create your own vehicle's speed v rpm formula you need to work out your own specific speed at rpm – you need to find out how far your wheel covers per wheel rpm – best done inflating your tyres to whatever pressure you normally use and whatever average load you have in the vehicle.

Mark a line on a strip of level hard surface and also your tyre at bottom centre where the line on the ground is.
Rotate the wheel ten times – mark the second line then measure the distance between lines.
Finally divide this measurement by ten = that will give you a very accurate distance per wheel rpm.

We now have a formula with which we can accurately measure engine rpm to miles/km’s per hour for your specific vehicle.
We know there are 1,000 metres in a km................ so divide that 1,000 by the distance your wheel travels in one full rotation – that will give us how many wheel rpm per km.

Multiply this by 3:89 (or what ever diff ratio you are using) will give us engine revolutions per kilometre.

Finally multiply the speed you wish to use - by that figure then divide by 60 ( 60 minutes in an hour) and it will give you how many engine revs per minute for the speed variable required. ................ that wasn’t hard was it.................?
If you want the equivalent in miles per hour – divide the input (variable) speed by 1.609.

My chart was calculated using 175 x 13’s at the time – it will be marginally different now................

I find using a GPS and tacho – this gives exact land speed versus indicated tacho rpm as opposed to calculated figures.

If you want to play with speeds in each gear – just divide by the gear ratio - 1st is 2.99, 2nd is 1.59 – these are for a grey crunch box.
For an Aus M20 - 1st 3:05, 2nd - 2:19, 3rd - 1:51)

This chart will illustrate the difference in diff ratio’s – there are also figures
calculated for the different ratio Aussie boxes (M20 & M21) and also one for a standard grey crunch box.

All top gear speeds are soley dependent on diff alone...........

[reddo]

Thanks for that Terry and Rosco.

This is a topic that i have been trying to sort out in my head for a while.
I own a vintage caravan and a few of us are heading to Mt Gambier Next easter. towing our vans.

I am taking my HR Ute with M20 4 speed. I drove to sydney on saturday and the car sits happy at an indicated 70mph. gauging by the cars sitting on thier cruise control at 110KPH ( 65 mph) my speedo is reading 70mph. which pretty much is in line with what you 2 have said about the 3 to 4 mph inaccuarcy at those speeds with 60 series 205 radials.

I have towed the van a few times and the car seems to die in the arse a bit off the lights but once up over 1500rpm it is very happy ( triple Su's on gas). I am converting it back to a single holley hoping to improve low down torque. If that dont work i might have to drop back to a 3:36 or even a 3:55 for the trip so i dont stress the clutch and box so much with the load hanging off the back.

I know most of the trip will be top gear stuff but i am prepared to wash 10kph off my cruising speed to drop engine revs ,save a bit of fuel , make the car easier to manage in traffic and therefore have a more enjoyable trip while driving. .
I know all this stuff is splitting hairs and there is a world of difference between towing a van and every day ( unhitched) driving .

What a difference a few shifts in numbers to the right of the decimal point can make
Reddo

Reddo

[mrs ratbox]

something else to considder reddo re: engine revs v fuel consumption really only applies to people with modified motors bigger cams etc, cams have a rev range which is probably nothing new to most, you will get better fuel consumption with yor motor revving in the rev range rather than under, so say yor cam rev range is 2200 rpm to 5500rpm and yor car cruises at say 2000 rpm in top you should get better consumption if you change yor rear ratio to get it revving at say 2500 rpm

[FB MAD]

Thats where Dyno's can help.You can find out where your engine is putting out its best figures at what rpm and adjust your diff ratio to get as close to the best rpm figure as the diff ratio will allow.

I had a 350 GTS HQ Coupe years ago when I was at Tech college.Put it on the dyno and found the 3.08 diff in it was a little under the engines best power figures so changed to a 3.36 and found a marked difference.Obviously the 3.36 gave better pickup but the engine didn't really use much more fuel than with the 3.08 diff.
I sort of kinda like got something for nothing!

The 3.36 diff with the 350 chev was actually a bit too much for the dyno rollers as when it was pedal to metal the car would actually climb up onto the front roller on the dyno.Luckily it was chained down.


Fuel was much cheaper back in the '80's and I didn't have a mortgage or a family to feed so it was leadfoot days for me back then...........just stop and flatout was about it........ no trying to get the best economy from an engine for me!!

Different times now... at least for me that is.

Cheers,

Terry.

[FB MAD]

Reddo,

I'd say your HR has a bit of engine work done to it that is, without hearing or driving it. The tripple SU's and the nothing under 1500 RPM statement would indicate this.

Like you said, maybe the change to a 3.36 or 3.55 would be better especially for towing with a modified motor as it might keep the engine within its best power range at the speed you want to drive in.

Before taking the tripples off, try a ratio change and have a tow and see if it makes a difference.

I know I'm probably telling you something you already know

Cheers,

Terry.

[rosco]

Hi all,
spent this afternoon finally getting that spreadsheet done.
It is in Excel format - office 1997 - 2003 or Office 2007.
I have only done the M20 4 speed with 3.55, 3.36, 3.08 & 2.78 diffs.
It is also using the workshop datum speed of 20.03 mph per 1,000 rpm.
I have charted it every 100 rpm up to 5,000
I will also do a grey with a crunch box and 3.89 diff - maybe tomorrow......

pm me if you want it.

frats,
Rosco

[rosco]

Hi all,
grey motor gearbox/diff spreadsheet is done.
It's in excel 1997 - 2003.
pm me with email address if you want it.

frats,
Rosco