R.p.m. at 60 mph with 3.9 compared to 4.22

[Michiel]

Could anybody tell me what the engine rpm is doing 60 mph with a 3.9 and with my current 4.22 diff. (1098 cc with ribbed case gearbox and 145/14 tyres)

Thanks.
Michael

[Peted7202]

4.22 = 3640 RPM. 3.9 = 3380 RPM

[Michiel]

So the benefit is 260 rpm at 60. Thanks Peted. I'll check what the difference is in 4th and 5th gear on my VW to get an idea.

[Michiel]

A reduction of only 260 rpm at 60 mph seems unwise with the current 3.9 diff prices.

[oliver90owner]

It will, of course, depend on the rolling diameter of your wheels, too.

Do remember, also, that speedometer readings are notoriously high, in comparison with the truth (the legal requirement of speedo accuracy is +10% and -0%). That, also depends on whether the gearbox and/or speedo head is/are original.

The selection of differential ratio should not be taken on fuel prices alone.

[Michiel]

I did not have fuel consumption in mind. I my my opinion our cars are one of the most fuel efficient of oldtimers. It was about the health of my engine and my ears.

[oliver90owner]

Simply reduce your ‘indicated’ speed to 3.0/4.22 times any indicated speed you care to choose. That is the way to compare (mainly) the difference to your hearing. A reduction in rpm at any given speed might not necessarily aid the health of your engine - the way it is driven is far more important - and fuel consumption may not change. It may even be worse.

Energy required per unit distance travelled is fairly constant - relying on rolling resistance and drag factor. I don’t personally consider a Minor to be particularly outstanding, for cars of the period. There were better, and worse, examples.

[Harryhotrod]

Here is what we did with 1098
Now have 5 speed type 9 60mph = 2600rpm all speeds taken off GPS not speedometer

[oliver90owner]

That simply demonstrates the errors that are possible/ If 1900rpm was correct for 30mph, 60mph should have bee 3800rpm.

Alternatively, if the 4000rpm, at 60mph was correct, the 30mph should have been 2000, not 1900rpm an error of + or - 5%, so results within 10%?

One thing you can say is that it was much quieter at 30mph than 60mph. The other is that fuel consumption would (well, should) be much lower at 30mph.

There is an 8% increase in speed from 4.22 to 3.9 diffs. 4.55 diffs to 4.22 is about 8% difference, too. It is a straightforward maths calculation.

Morris Motors didn’t fit the 4.22 diff for no good reason. My 1970 1100 Mk1 Escort was fitted 4.2 diff IIRC. By the time I had changed the motor (and “warmed it up “ considerably) it easily pulled full rpm with a 3.78 diff and would likely have coped with a 3.54.only a 20% difference betwee OEM and the high ratio diff - but the engine power was increased by a lot more than that.

[MikeNash]

Thanks Harry for those real world figures.

What you actually get varies quite a lot from car to car so can you tell us what yours is - convertible (with hood up or down), 2 or 4 door or Trav? The weights vary as does the air drag. And your companion's weight?

I note that in "Pearson's Illustrated Car Servicing for the Morris Minor" the max speed is given as 76mph and 0-50mph as 17.2 secs while "Hayne's Morris Minor 1000" gives 78mph and 0-50mph as 15.8sec. Your 0-50 in between which seems reasonable; what book do your "book specification" figures come from?

If 60mph ties up with 4000rpm then the peak engine power at 5100 will give you about 76mph which is what my 1098 Trav will do - if I can find enough road! Yours would seem a little down.
Regards from MikeN.

[MikeNash]

Harry,
Just read your paper more carefully and see you say you have the 4.55:1 diff ratio on a 1098cc engine. Really? Is that not a typo for 4.2:1?
MikeN.

[kevin s]

When referring to the original spec it's worth remembering thet would have been of 5.20/14 crossplies which are about 3% bigger than the 145/14 most people run now, the differnce between 4.22 and 3.9 is 8% so even the original cars would have been almost half way to a 3.9 diff.

If you are worried about your engine logeivity at high speed the best thing you can do is fit a thermostatic oil cooler and/or use something like millers 10W50 fully synthetic oil. (or perhaps just drive at 60).

[Michiel]

When referring to the original spec it's worth remembering thet would have been of 5.20/14 crossplies which are about 3% bigger than the 145/14 most people run now, the differnce between 4.22 and 3.9 is 8% so even the original cars would have been almost half way to a 3.9 diff.

If you are worried about your engine logeivity at high speed the best thing you can do is fit a thermostatic oil cooler and/or use something like millers 10W50 fully synthetic oil. (or perhaps just drive at 60).

Thanks Kevin. Not planning to exceed 60 often. The advantage of no sound deadening is that your hear when the engine is working hard

[Harryhotrod]

So mine is a 61 but fitted at the time with a later 1098, the rear diff was 4.55 can send pic to confirm, speed was off GPS, my passenger lets say is a big man. Tyresare 155x14 .
I'm not a number cruncher but thought this would give people a real time idea.
The car is now 1275 with a 5 speed box .