Sunbeam Alpine Owners Club

Paul King · Posted: Fri Jan 08, 2010 2:35 pm Post subject: Ratio's

I know this topic has done the rounds a bit, so please excuse me trying to get it straight in my head as figures are not my best subject. My asking is because everytime I use an online tyre size site to get the ratio's as near as, I get different results. And purchasing trial and error can start to become a bit expensive

Using an O/D gearbox, 3.89-1 diff, 1504 tpm speedo and 175/70x13 tyres.

Would this set up give me a close to 'actual speed', or would another combination be better suited without having to change the speedo drive gears in the gearbox?

Thanks

Chris Barker · Joined: 15 Dec 2006 Posts: 439 Location: Somerset

Only registered users can see links on this forum!
Register or Login on forum!

is excellent because it gives you the data for any tyre size you feed in.
175/70 x 13 tyres convert to 919 turns/mile.
Standard Alpine tyres - 165/80 x 13 are 889

Your diff is 3.89:1, and AS YOU HAVE OVERDRIVE your speedo cable drive gears are 5:12.

So the 175/70 x 13 will give 1488 turns/mile
The combination of 165/80 x 13 and the 3.889 diff (the non-overdrive ratio incidentally) comes to 1440 tpm.

On the face of it, 1504 tpm is close to ideal, but most speedos over-read.
When I changed from the OD 4.22 diff to 3.89, I found that with 165/80 x 13 tyres, my speedo became virtually accurate, though the odometer is, as calculated, 4% slow. I can live with this and I do prefer the higher gearing.

two more points:
- you may find that the 175/70 tyres, being 3/8" les in radius, mean you hit the centre of the cross-member more often, especially if your springs are tired.
- for other owners with a non-OD box, all the above calcs are about 50% wrong! The non-OD speedo was 1020 tpm.

So the answers to your questions are 'yes' and 'no'!

_________________
Chris Barker
SAOC Information Officer

Paul King · Posted: Sat Jan 09, 2010 9:01 am Post subject:

Thanks again Chris, I think I've got it sorted now and will go for the 165/80x13 as this combination is tried and tested and works for you.
This is the beauty of the forum, you can rely on established wisdom.
Paul

Paul King · Posted: Sun Jan 24, 2010 6:03 pm Post subject: Ratios speedo head to rev counter

Hi Chris,
Sorry to bother you with this again, but I am having fun and games trying to figure out if the new speedo head I've got is showing a true reading.
I have gone up a local motorway to check my speedo against the side markers 0 to 0 (=10ths of mile) and found the odometer is about 0.5 slow. Not too much of a problem, I can live with that. But I'm not so sure of the actual speed and would appear to be going faster than other cars especially through areas where average speed cameras are in operation.
My set up is:-

Tyres......165/80x13
Gearbox..SeriesV O/D
Diff.........3.89-1 non O/D
speedo....5324/12A 1504 (which I was assured was for a seriesV)

My only checkable constant is my rev counter as I have two meters that when connected up when out testing prove that it is calibrated correctly.
Would you be able to tell me what speed I should be doing in relation to my rev counter in 4th gear or O/D 4th gear or how I would go about finding out.
Thanks
Paul

Chris Barker · Joined: 15 Dec 2006 Posts: 439 Location: Somerset

First, the markers on a motorway are at 0.1km intervals, so one mile is 16 of them, not 10.

With the set-up you describe, 70 mph is about 3200rpm in OD 4th, so 40mph will be 1800. In direct 4th, the figs are 3950 and 2250.

The formula is 60000/axle ratio/tyre turns per mile = mph per 1000 rpm.
Overdrive increases the result by 1/0.803

Your car set-up is the same as mine, and the gearing cancels out my speedo error, so my speedo is accurate, but the odometer is 4% under. If the speedo were accurately adjusted, it too would under-read by 4% (so 70 would be an indicated 67)

_________________
Chris Barker
SAOC Information Officer