70mph RPM

[marki]

Hi All.

  Just checking a few things out on my newly purchased 4a, can someone confirm the RPM at 70mph in 4th OD.

cheersMark

[RobH]

Revs depend on a number of factors -

what tyre size

what diff ratio.         4.1 or 3.7   (normally 4.1 on overdrive cars but not always)

what overdrive ratio. ( the standard A type is 22%  but a 28% is also possible )

 

Edited September 11, 2023 by RobH

[John Morrison]

I run 165 x 15 XAS, 22% ration, and 70mph is about 3100

John.

[marki]

Thanks guys

[ctc77965o]

Something around 3200rpm

[keith1948]

Hi Mark

Just deleted my reply because I think my calculations were wrong.

Maths eh

Keith

Edited September 12, 2023 by keith1948
not sure I have given correct info

[BlueTR3A-5EKT]

Do we not like Minty Lamb then?

http://mintylamb.co.uk/gearspeed/

[ianc]

Thanks, Peter.

Minty Lamp's figures agree with the original tests of TRs from the 1950s and 1960s, and what I have always experienced in my TR2 and in 4VC, both of which I have driven on the tacho, rather than my wobbly speedo (which is in front of the navigator on 4VC, so not easily read other than for the mileage figure when filling the tank).

Additional thoughts:

What makes the sums so simple on a TR with 3.7 axle, 165/80 tyres and A-Type (22%) overdrive, is that the mph/1000 rpm are (apart from 1st gear) in multiples of 5 mph:

10, 15 20 and 25 (just under) in 2nd, 3rd, 4th and O/D 4th.

At 5000 rpm, one has speeds of 50, 75, 100 and 124 mph.  Overdrive 2nd, an exceedingly useful gear as it slots halfway between 2nd and 3rd, means one can exceed 60 mph at 5000 with only one change of gear (1st to 2nd).

Ian Cornish

Edited September 13, 2023 by ianc

[keith1948]

17 hours ago, BlueTR3A-5EKT said:

Do we not like Minty Lamb then?

http://mintylamb.co.uk/gearspeed/

I had forgotten about Minty Lamb - thanks for reminding me. Mark you should be able to input your car data onto Minty Lamb to see what your revs should be at 70 in OD.

Problem on my 4A is that the speedo is the 1184 one as fitted in 1965. Looking at the Moss catalogue with 165/80 tyres it looks like I should have the 1152. My speedo reads about 10% high. I have 3.7:1 diff and A type overdrive. The odometer also reads wrong. At 60mph in OD the rev counter reads just under approx 2400 which is roughly double 1184. That doesn't really mean a lot since the speedo needle can be 'adjusted' to this figure. According to a tyre cross reference chart I have found, the 165/80 tyres do 795 revs per mile whereas the closest equivalent to the original cross ply would be 165/75 (815 revs/mile) or 175/70 (819 revs per mile). However more revs per mile to my mind would mean the speedo would read higher still.

I could get the speedo recalibrated I guess but is there an option to fit a slightly different tyre size to bring it back to 1965 reading. It must have been correct once. I'm getting brain fatigue trying to figure out why my speedo/odometer is high. I have learned to live with it but would be better if it was more accurate. Could I use Minty Lamb and set the speedo to read exactly 60 at 2410 revs. At the moment it will read above 60 at these revs.

Any help appreciated please

Keith

Edited September 13, 2023 by keith1948