engine rpm at 100kmh

[martym00se72]

Hi,
I've seen plenty of number wrt final drive ratios etc but am after a plain english isection of this - at 100kmh how many revs will the engine be doing for 3.5 final ratio transaxle and also for the 4.1 transaxle? I was doing a test using gps to check speed as my speedo seems to be 5kmh slow at 60 and 10kmh slow at 100. At that time I noted that I was doing about 3300-3400 rpm in 5th - thought that was a bit high for my 83 GTV6 which is suppoesed to have the 3.51 final drive. Little confused...?
Any thoughts?
Cheers

[stradale]

you also need to consider aspect ratio of your tyres 60 55 50 45
85 GTV6 195/60/15 as standard

[martym00se72]

Of course! Forgot that aspect - running 205/60R15 stock wheels - based on that would expect my rpm to be lower as those tyres should make for taller gearing - I would think?

[stradale]

You need to measure the difference between the two to determin the rolling radius
the larger the rolling radius the higher the gearing
in theory a 205/60 should stand taller than a 195/60 of the same brand of tyre
the 205 should roll futher for one rotation
There are others on this forum with better knowledge on this subject than me, who should be able to answer your question

[stradale]

I run pirelli 205/55/15 my speedo is out at least 10 kph up at freeway speed

[stradale]

The owners manual for an 85 GTV6 with 4.1 diff reads as follows

at 5600 rpm

1ST  =  43.7  kph
2ND  =  78.2
3RD  =  121.7
4TH  =  161.8
5TH  =  205

[alfagtv58]

Download the attached spreadsheet.

Fill in your tyre and ratio details in the yellow cells, i have already plugged your info in, but you can try the different final drive ratio and see the difference it makes.  I have also attached a list of transaxle ratios available which I got of the BB a while ago, its not a very good copy due to the size restrictions but it may be of use?

I found this really useful when trying to establish the best ratios for racing application (together with a theoretical top speed calculation).

Cheers,

[martym00se72]

Thanks Phil - very handy sheet indeed. The reason I asked this question in the first place was that I am thinking that my engine is revving too high for a given road speed if my diff is the original diff for the car (83 GTV6). Going by what you have in the spreadsheet I should have the final drive at 3.42, not 4.1 (assuming that the final drive stayed the same from 82-85, my understanding is that 83 had the 3.42 final). That would yield that cruising at 100kph in 5th should have me at approx 2300rpm. What I actually saw was approx 1000rpm higher than that. Any thoughts? I will recheck my measurements in the next day or two. Are there any markings on the diff/gearbox housing to indicate what is inside?
Cheers
Marty

[stradale]

I know I will be corrected if Im wrong but I think Australian delivered GTV's and 6's all had 4.1 diffs
only the yanks had 3.42 in 83

the only way to find out for sure is to take the diff cover off the transaxle and look on the crownwheel
the final drive number is stamped on there
sorry about picture quality is says 10/41

[alfagtv58]

Marty,

Dont dismiss tacho inaccuracy, the standard one in my 2L is woeful....particularly in the mid range.

[Brad M]

Dont dismiss tacho inaccuracy

I was wondering how long it would take for that to be raised as an issue. Sometimes the obvious gets overlooked.

[ACP]

Use a GPS to check the speedo.

Simple....

Anyways I have had both diff's in different 116's, and also I seem to recall some different gearbox ratio's as well(wasn't one a close ratio box?).

Also check that the tacho is accurate, they can be out.

Cheers,
AP

[martym00se72]

Hi AP,
I was using a gps to check my speed - already knew my speedo was off the mark. Tacho is a different matter though. I have just recently aquired timing gear from my Uncle incl tacho so will check that too I guess. Don't really want to pull the cover off the diff if I don't have to - just changed the oil ony a few weeks ago...
Cheers

[dehne]

correct me if im wrong here but all the older alfa's ive had when ive been doing 100 the revs are at 3000

[dehne]

come to thing of it, it might have been when in 4th at 3000 i was doing 100 and then changed to 5th mmm, might go and check that today