15 replies to this topic

[Alex Kay]

Hi Guys,

3.65 diffs are some what common in the UK from what I understand, so I was just wondering if someone could answer a question for me?

At 60mph or 100km/h, how many rpm will your engine be sitting at?

Big thanks in advance

[Asphalt]

Hi Guys,

3.65 diffs are some what common in the UK from what I understand, so I was just wondering if someone could answer a question for me?

At 60mph or 100km/h, how many rpm will your engine be sitting at?

Big thanks in advance

10" wheels: 3995rpm
12" wheels: 3853rpm
13" wheels: 3835rpm

[Alex Kay]

Asphalt you little beauty thanks mate

[liirge]

you can use this:
http://www.guess-wor...nical/ratio.htm
to have a fiddle!

[Alex Kay]

Thanks Liirge, I'll save that one. I'm just trying to pick a diff for an engine I have just built, its running a 3.2 at the moment which is waaaaaay to tall for the cam I reckon so its just behaving like a bit of a pork chop.. I was thinking about going to a 3.44 but i didn't think it would be a whole lot better.

Thanks for your help

[AndrewJ530]

I get 3500 rpm at 60 mph and 4000rpm at 70 mph (998 stage 1 mini with 3.65 diff and 12" wheels).

[danrock101]

what diff would I have if 3k rpm = 70 mph?

[liirge]

what diff would I have if 3k rpm = 70 mph?

depends ,
it works out about 64@3000 on a 2.7fd using 12's
Are you sure your speedo is accurate?
what size wheels you got, and what gearbox have you got?

[Guess-Works.com]

what diff would I have if 3k rpm = 70 mph?

I think your speedo is telling lies... Reading some of the history, I believe your mini has a 1275 out of a metro, which probably means it has a 3.4, maybe even 3.6 final drive, and the speedo drives are configured for a metro with bigger tyres than you have and a different speedo..

[mister bridger]

I'm glad this question has been asked as I've been meaning to do so for ages! My 64 mk1 on 10" wheels does indeed read 4000rpm at 60mph. This is obviously pretty frantic at open road speeds. What would be a better diff to go for and whats involved in changing it? Its a stage one tuned A+ with remote type gearbox by the way and has plenty of grunt to handle higher gearing though obviously this would compromise acceleration.

[Asphalt]

I use a 3,21:1 diff at the moment.

That's 3500rpm @ 60, and 1760rpm @ 30 in 4th.
But I can't compare with other dif ratios.
It's the shortest I ever had I'll try a 3,65 next.

Cheers,
Jan

[Burnard]

I'm glad this question has been asked as I've been meaning to do so for ages! My 64 mk1 on 10" wheels does indeed read 4000rpm at 60mph. This is obviously pretty frantic at open road speeds. What would be a better diff to go for and whats involved in changing it? Its a stage one tuned A+ with remote type gearbox by the way and has plenty of grunt to handle higher gearing though obviously this would compromise acceleration.

I am also haveing the same problem, so and answer to this would be great.

[minimender]

What I have found with diffs on road cars is that using the same engine (let's say 998) and different diffs the cars maximum speed is around about the same .

High diffs, 3.1, 2.9 tend to be more economical on motorways if cruising speeds are kept below the legal limit and acceleration is acceptable round town but sluggish on the open road. If you can push the revs up to around 3500/ 4000 in top then you are doing very well. (e.g. 2.9 diff at 4000 is about 80 mph)
Medium diff 3.4 will be good all round with good open road performance and not too busy at motorway speeds if the speed limits are adhered to. (e.g. 3.4 diff at 4000 is about 68 )
Lower diffs 3.7, 3.6 are brilliant for performance when you need it and are also economical if you are cruising at arround fifty miles an hour. These low diffs allow the motor to rev out to near max in top gear but a bit revvy at motorway speeds though very little exrta fuel will be used if driven on a light throttle.
The very low diffs say 3.9 are great for accelleration but 1st gear isn't much use and second only brings about 35 mph flat out, great diff round the lanes though.
I used to go for the 3.6 when building a road car, whether it was 998 or 1300 the diff seemed to suit both engines.
Did you know that an 1100 (1098) with a 3.6 diff is as quick all round as a 1275 with a 2.9 even on the motorway. I know because I once challenged a mate to a duel and beat him on accelleration and top end on the motorway.
The lower 4.2 and less diffs are useless with a standard box and need different gear ratios to be of any use.
so you pays yer money and takes yer choice really, be good if you could have a two speed diff like the old trucks had, then you would have the best of both worlds.
Changing the diff from a high one to a low one is as good as paying for loads of horse power from tuning companies.

[danrock101]

I've got 13" wheels, I know the speedo is accurate (ish) and I don't think the rev clock is too far off but I have no way of telling, most it's revved to is 6.5k then I chickened out, not sure on the gear box, It's possible it's the city e gear box, but it's a metro engine fitted by the prev owner, I would of thought it's a metro box but can't be certain

Edited by danrock101, 24 December 2009 - 11:22 PM.

[minimender]

I've got 13" wheels, I know the speedo is accurate (ish) and I don't think the rev clock is too far off but I have no way of telling, most it's revved to is 6.5k then I chickened out

If you stand a ten inch and a twelve inch and a thirteen inch side by side there is very little difference, some difference but not a lot.
You can check your speedo on the motorway, it's a minute exactly between the phone boxes at sixty miles an hour. Not many rev counters are accurate so not to worry it's only a rough guide anyway and who watches it when pushing along a country lane anyway? I listen to the engine and keep my eyes on the road.
A standard motor will not rev to 6500, anything over 5500 is a bonous, MG will go 6000 if you push it.
I had 50 profiles on 13 inch Revolutions and a 3.4 diff and that was doin about 50 at 3000 which was pretty accurate according to the figures we had on tyre sizes etc.