Gearing on different A8 engines
 

Hi,

Reading through posts I noticed someone commenting on top gear ratios of different engines.

Has my 3.7 D2 2000 got the same ratios as a 4.2 D2? I enjoy the relaxed cruising: 72 mph looks like 2000 revs.

Want revs do others run at 100 kph (or how fast are you going at 2000 revs!?).

Robert

5th in the S8 is about equivalent to 4th in the 3.7. I think the 3.7 is designed to have higher gearing than the other V8s to improve economy - its about 36mph per 1000rpm, whereas the S8 is only 27. Remembering how relaxed my 3.7 was is what bugs me about the S8 tbh. The 4.2 is probably in between.

Having done some research this evening, ElsaWin claims that the FL 3.7 is 28 per 1000 and the S8 is 24. Either ElsaWin is wrong, or my memory of my 3.7 is wrong.

What's really doing my head in is that the manual said the top speed of the 3.7 was only achieved in 4th gear, but according to the gearing in ElsaWin 4th runs out at 133 (6500rpm) so how was it limited to 155? It would need to be doing about 5600 in 5th to get to that speed if the stated gearing is correct, which leads me to believe it's not. Mine did an indicated 150 (probably 145 actual) without any trouble.

The S8 apparently needs to be doing 6450rpm to get to 155.

Sounds about right.

The 4.2 is running just over 2000 rpm to achieve motorway cruising, where the S8 will be running about 3000.

The 4.2 engine + gearbox is the best combination annoyingly - It stays lower on the revs, uses the torque and returns more like 30mpg on a long run. The S8 just revs all the time for the sake of a slightly quicker launch in first gear, which is rarely needed.

Can't comment on the 3.7 as not used one in anger, but would expect it to sit in between the two.

If I remember correctly the 5th gear ration for FL S8 is 0.804
The final drive works out as 3.73
(Ainarssems worked this out, I can't claim any credit :))

The equation for working out road speed or revs as per Elsa is:
Revs = kph * Final Drive * Gear ratio / (0.06 * Tyre circumference in meters)

or

Mph = Revs * Tyre circumference * 0.037 / (Final Drive * Gear Ratio)

For a 18" 245 45 tyre the circumference is around 2.12m

So theoretically a S8 at 7000rpm in 5th should achieve:

mph = 7000 * 2.12 * 0.037 / (3.73 * 0.804) = 183mph

probably not possible due to air drag and road friction.

My S8 was a guinea pig for the mpg difference, with its original 0.73 6th gear it was getting 24mpg on a motorway run at around 2700 rpm, with its 0.56 6th its getting 29mpg to 30mpg on a motorway run.
If we assume a linear relationship between ratio rpm and mpg, it almost works out 24 * 0.73 / 0.56 = 31mpg. But there are probably other non-linear losses so it cant quite achieve 31mpg at 70mph.

Thats a TDI 6th in an 01E is it, but still with the S8 final drive? How does it perform on the motorway? I've been looking at TDI 01Es for mine - the B6 0A3 I have is 39mph/1000rpm which is far too high. The C5 01E is 36 which still seems a bit high (0.56 6th but 3.889 final drive).

I've got the S8 down as 4.111 final drive so that may explain why my calculations are off a bit.

I'll have to look up what the 4.2 has. Maybe thats what I need to shoot for in a manual.

Edit - my S8 is 4.111 according to Etka and Elsawin. FBG gearbox and EUU rear diff. The DGV manual is the same.

Goran - what gearbox is in yours?

The A8 4.2 ECZ looks to be a fraction lower than the 3.7 in 5th - 27 per 1000, so 70mph is ~2600rpm.

There must be something wrong in the spreadsheet I'm using I think, since it doesn't conform to real-world experience. I doubt the data in Elsawin is wrong. I'll re-check calcs tonight.

72mph @ 2000rpm on my 3.7

Interesting to learn about w12 cars too.

So from this thread it seems that 3.7 will run lowest rpm on 5th driving at 70mph in comparison with 4.2 and S8. Correct?