Hi O,
The diff ratios you quote for each gear box are as per my spread sheet so I list the actual gear ratios for each gear box below. If you have some different ratios then please let me know and I will pop them in to the spread sheet and report the results. I honestly believe that my calculations are correct for the quoted figures but obviously I am fallible. The rolling radius for both calculations is 74 inches.
I am happy to send anyone the spread sheet so that the calculations can be checked.
You state ‘As standard both auto and manual are similarly geared’ could I respectfully ask where you obtained that information so I can cross check it.
Sorry Malc. I should have said similarly geared ‘in 6th’
ie, both manual and auto at say 3000rpm in 6th will be at a similar road speed.
The actual 1st to 6th gear ratios are different, with 6th(man) being 1:1 (pass through, good for minimising transmission losses) and 6th (auto) being 0.53:1) This is from memory, so I’ll dig out where I got my figures from and post the web address.
As an aside, the auto with a manual diff should be useable, as at c2000rpm the engine can produce about 125ft lbs, and 50 ish bhp which is enough for most situations.
My calculations at 3,000rpm shows M=73mph and A=101mph.
Is there any body out there that can help here with an actual rpm in 6th gear at 70mph in an automatic so I can cross check my calculations. I know the manual figures are correct but there seems some discrepancy on the auto figures.
Hi again.
Just looked at this site and the manual is exactly as I have it. The auto is a little more confusing. Looking at this info Specs 2021 Mazda Roadster RF VS automatic (aut. 6) (automobile-catalog.com) The final drive is shown as 3.583 but my UK brochure shows 3.45. This is a 2.0 litre with 17 inch wheels. There is some data for lower powered autos with 16 inch wheels which may be confusing the matter.
Having said that my calculations are correct for the 3.45.
From the site you directed me to they show for the 3.583 diff the mph/1000rpm for the auto are 5.9, 10.1, 14.9, 20.9, 29.3, and 35.9mph. These are what I get if I put 3.45 in my spreadsheet.
So at 3,000rpm you are doing, 35.9mph times 3 which is 108mph in the auto compared to a manual which is 73mph.
The site you referred to shows that the auto is considerably higher geared than the manual which is what my spread sheet is showing.
If you are seriously considering an auto please drive one to make sure the long leggedness is for you.
As far as the diff is concerned it is my understanding that the auto has the NC diff in it not the ND one so swapping the internals may not be as simple as at first appears.
Yep, I’ll have go back to my original calcs, as from what I remember the fact that 5th and 6th gear in the auto are effectively overdrives, (as 4th is the 1:1 direct gear) was why they fitted the shorter diff in the auto, so the overall ratios, manual and auto, were similar.
The site I gave you may have wrong info.
I think, though I’m not certain, that the rear subframes in both cars are the same, so there shouldn’t an issue swapping over the whole diff assembly.
The automatic box in the ND is the same as the NC by the way.
Appreciate your input. It could be a little project for me over the summer!
