fabric

Sorry, i couldn't find this in a search, and the manual didn't seem to enlighten me.


What should my engine RPM be in 4th gear at 60 MPH? Or some other permutation. Specific situation isn't important, I'm just trying to get a feel if my speedo is close to accurate.

Thanks!

the flyin' scotsman

Chris..........60mph equates to 100kmh in this part of the world and my engine is spinning at 1800 rpm in drive.

For better driveability I have the gbox in 3 at this 'low' speed for awesome slam back in the seat instant motion............gotta have the engine in the high 3k rpm range for major giggles!!!

IcemanG17

If I remember correctly (a challange) for a US spec S4 auto (2.2) it should be around 2050-2100 rpm or about 34rpms per mph? With stock size rear tires of course!
Brian

hinchcliffe

I'm wearing 255/40r17's and at 110 MPH I'm running just over 3600 RPM

mark kibort

The real answer is:

rpms divided by the total gear rato, times the circumference of the rear tire.

most rear tires are between 24.5 and 25.5 (245x50 to 315x35s)

remember the relationship between rpms and MPH is FIXED, however the actual speed measured by a radar gun may change.

5th gear would be: 80" / 2.2 x 3000rpm = speed in inches per second, divided by 12 for feet/sec divided by 5280 for miles per second x 60 for MPH.


mk

Dave H.

90 mph is 3100

6.0-928S

In my 86.5 with 275/40/17 rear tires I'm spinning 2000rpm at 60mph. I also have the 2:20 rear diff.
Hammer

mark kibort

At 25.5" diameter, 60mph is 48mph in 4th gear. decrease that by 3% for a 24.5" diameter tire (ie 275x40x17, or 275x35x18). your actually closer to 60mph at 1750rpm in 5th

4th being 1.45 x 2.2= 3.19:1 total ratio

at 3000rpm that would be 71mph also with the 25.5" diameter rear tire.

at 110mph that would be 4600rpm in 4th gear too, also with the 25.5" diameter tire.

90mph is closer to 3700rpm, while 3100rpm is about 74mph

take the above information and change for the 24.5" diameter of a 275 tire.
if you are off, your speedo calibration is off, or your RPMs are wrong on the tach.


MK

m21sniper

Just checked mine tonight while out repo-in' on the way to da hood.

Cruise RPMs, top gear:

60mph 1800
70mph 2100
100 mph 2900

80 and 90 went by really fast on the way from 70 to 100.

Tires are 245/45/16 all the way around, no idea what my final drive ratio is, but the tires make my speedo read 0.7% too fast, so add 0.7% to every rpm listing, lol.

hinchcliffe

See.

mark kibort

sounds about right.

why would those tires make your speedo read too fast? i thought 245x45 and 225x50 were pretty much the same tire.

as far as the other guys above, and the picture, that looks like top gear, not 4th as the first post was talking about.

mk

mark kibort

4th gear right?

anyway, i gave you the rpms vs actual speed equation. But its not going to matter what the RPMs read, unless you are able to calabrate the speed by radar or time /distance. no matter what tire you put on the the rear, the RPMs vs indicated speed willl be fixed. (nothing you can do about that) however, your actual speed can change.)

mk

fabric

4th gear in an auto, which is the same ratio as 5th in the manual. Looks like I'm pretty close, I was mainly worried that it was reading really low. Which is probably safer for me, but I like to know the truth.

I'm doing a long drive this weekend, i'll try timing myself on some highway mileage markers or time intervals between mileage delineated exit ramps.

fabric

What year is yours? You should be able to determine final drive based on that, unless somebody did something wacky with your car:

https://www.928gt.com/t-928trans.aspx

hinchcliffe

That is 5th gear in my '87. 255/40 rears.

255/40r17 are 533.8mm tall
245/45r16 are 516.65mm tall