350i Diff ratio - mph/1000 rpm

[Terence]

Hi All. I test drove a late (1989) 350i the other day and the gearing seemed rather low at an indicated 20mph/1000 rpm in 5th. I am sure my last 350i (1987) was taller than this. Looking through previous threads I have seen quoted 25mph/1000 rpm and 28mph/1000 rpm. Has anyone else got a 350i with an apparent 20mph/1000 rpm ratio? Thanks in advance Terence

[grahamw48]

Are you sure you were in 5th ?

Have dyno sheet for my 400 here, and it says 21.33/1000rpm in 4th. Should be accurate.

Standard diff should be 3.54, as you probably know, but tyre and wheel size can have big effect too (mine all standard).

[TVRleigh_BBWR]

All the 400SE was 3.54 as they was limited slip, but I think some of the 280's and 350's may have been fitted with taller ratio's

if it was a 3.07 5th would be 28MPH at 1000rpm
if it was a 3.31 5th would be 26MPH at 1000rpm
if it was a 3.54 5th would be 25MPH at 1000rpm

Leigh

[Terence]

Thanks for the replies folks. It would appear so far then that this is a bit of an oddity. I did check it at both an indicated 60 and indicated 40 mph and both speeds matched 20mph/1000 rpm. It did not feel as though the speedo was under reading however the engine did seem very free revving. Has anyone come across a rev counter over reading? I guess if a V6 rev counter had been fitted by mistake it would be over reading by a factor of 1.33 which would equate to 27 mph/1000 rpm. Interesting stuff.

[grahamw48]

I use my satnav as speedo in the the Tiv, and if you hold steady speed it will give you an accurate enough reading. Just compare the indicated speed with how many revs you're showing in 5th.

On the rev counter (number of cyls) theory, I did actually have a 4cyl electrical counter in a car that I'd fitted a 6cyl into, and the counter just went bonkers high before I fitted correct one.

[JR]

grahamw48 said:

Standard diff should be 3.54, as you probably know, but tyre and wheel size can have big effect too (mine all standard).

TVRleigh_BBWR said:

if it was a 3.07 5th would be 28MPH at 1000rpm

if it was a 3.54 5th would be 25MPH at 1000rpm

I had the 28mph/1,000 diff with LSD in my 350i but nearly all 350i cars have the 3.54 25mph/1,000 diff.

As Graham mentioned I suspect something amiss, maybe 205/50 tyres or even 205/45 in place of the standard 205/60.

[grahamw48]

Checked satnav to revs in 5th this morning.

It was showing 38mph at 1500 rpm.

Approx 25mph/1,000 revs.

On Sunday I will let you know what it shows at 150...when the brakes fade at the top of the runway.

[JR]

As a footnote to my earlier post: even very low profile tyres are only going to knock off a couple of mph/1,000 so it must be a speedo related problem or an aftermarket diff that we haven't heard of on this forum.

[Terence]

I had the satnav in my car when I went to see the 350i I should have thought to take it with me. I am still thinking along the lines of an over reading rev counter as the speedo read 30 mph when I was keeping up with traffic in a 30 zone and the same on a stretch of open road at 60 mph. Thinking back it did not seem to be idling over quick but the rev counter was showing around 1000 to 1100 rpm. If I end up purchasing the car i will investigate further.

[red tasmin]

As the speed increases in tandem with the rise in revs 3000/60, 4000/80 if the tacho was out the revs per mph would vary. I believe the car to be fitted with a lower ratio gearbox or diff.

[grahamw48]

Or slipping clutch ?

[red tasmin]

I think I know enough to tell when a clutch is slipping!!!!!!!

[Wedg1e]

Some tachos have a switch on the back to select 4, 6 or 8 cylinders. Could be as simple as that.
Otherwise it could be a component out of tolerance within the instrument itself.
...or a different ratio diff, obviously.

[grahamw48]

red tasmin said:

I think I know enough to tell when a clutch is slipping!!!!!!!

Oh sorrrry.

How's the sale of the Wedge going ?

[red tasmin]

Sold to a guy from Yorkshire!