Diffs and speed in top gear

[cormorant]

Hi here is a little table which I found on the web somewhere giving the different speeds in top gear for different diffs. I added a 3,500 rpm bit to the original table. Though it might be of use.....11/41 is Riley I guess and virtually the same as 10/37. Having fitted an 11/41 I can confirm that 62mph feels pretty much like 51mph used to! Haven't checked whether 6,000 rpm really is 106mph, will let you all know


RPM in top gear and MPH

RPM 3.727 3.900 4.222
......... 11/41 10/39 9/38
1,000 17.8 17.1 15.5
2,000 35.6 34.2 31.0
3,000 53.4 51.3 46.5
3,500 62.3 59.8 51.2
4,000 71.2 68.4 62.0
5,000 89.0 85.5 77.5
6,000 106.8 102.6 93.0

[linearaudio]

Can anyone else confirm these figures??

They seem a tad low to me, having using various speed checks, "smiley face" signs, 56mph rolling road blocks etc, I had figured my speedo was fairly close, allied to rev counter cross-checking over the top gear range, but doesn't seem near these figures!

[cormorant]

My standard speedo must have been way out - I fitted a 3.7 diff and left the standard speedo in. Checked using a handheld GPS, an indicated 60mph was in fact a true 63mph. Must have been reading way over with the 4.22!

[PSL184]

Wouldn't the tyre size make a difference to actual road speed?

[rayofleamington]

Must have been reading way over with the 4.22!

A standar speedo will read high - also on modern cars.
However standard radials (/80) give a smaller effective circumference than their crossply predecessors, which adds to the speedo error.

[Stig]

Wouldn't the tyre size make a difference to actual road speed?

I recently checked my odometer against the km marker signs on the M4 (km from junction 1 I assume) - it was late and I was bored and trying to stay awake.

I calculated that my odometer reads 3.2% low with 155/80 tyres and a standard diff. So with 145/80 tyres it would be spot on.

Next time I'll take a stopwatch to keep me amused and check the speedo.

[bmcecosse]

My 3.7 diff/155x14 tyres with 1248 tpm speedo - reads spot on! I've no idea about revs - I dont count them !

[linearaudio]

all

My 3.7 diff/155x14 tyres with 1248 tpm speedo - reads spot on! I've no idea about revs - I dont count them !

Pray tell us- how did you check your speedo is spot on?
I haven't yet been on a quiet enough stretch of road to keep a steady speed, count Km markers and watch what all the other loonies are doing for a decent period! Or have you one of those new fangled gps gadgets?

Whilst on that- are they known dead accurate? I only ask as an article in this months MG club mag about the "Road Angel" camera detector mentions its speed display being "very accurate when compared to another gps system", inferring that they are not all neccessarily dead right!

[win]

my speedo reads 8% fast with 1376 speedo 4.22 diff 145x14 michelin XZX.
might want a longer prop shaft.

[linearaudio]

my speedo reads 8% fast with 1376 speedo 4.22 diff 145x14 michelin XZX.
might want a longer prop shaft.

I calculate an extra 2" on the prop shaft will probably sort your speedo out, but it's worth double checking

[Peetee]

Checked using a handheld GPS, an indicated 60mph was in fact a true 63mph. Must have been reading way over with the 4.22!

Interesting. My 948 speedo reads nearly bang on with a 4.22 diff (should be a 4.55)

[win]

whats your turns/ mile on the 948 speedo,

[bmcecosse]

My speedo confirms exactly with the flashing speed signs (why do they always show a 'sad' face ??) - that's as accurate as I need it to be ! 948 speedo should be 1508 for 4.55 diff. 1098 speedo should be 1408 for 4.22 diff.