engine and drivetrain Rpms At 5500 At 65Mph?

[McKinney]

First off I have tried searching and found many topics about rpms at certain speeds, but I haven't found one pertaining to my issue. The most common rpm numbers I have seen is 4000rpm @ 65mph and some are even lower. At 4000rpm i am at 50mph and at 65mph i am at about 5200rpm. I have also seen that people talk about getting up to 100mph. I was very surprised to read that because I hit redline on the rpm gauge at around 80mph. My 02 is a 1976 with a weber 32/36 and four speed manual. I have been researching how to check which gear ratio I have but I am having a hard time telling whether is it stock or not. Is there a way to find out? Could this issue be caused by anything else besides the gear ratio? I commute to work daily about 60 miles I don't want to strain the old motor more than I need to.

[Conserv]

Indeed, your numbers do not make sense unless: your speedometer is wildly inaccurate (possible), your tachometer is wildly inaccurate (possible), your clutch is wildly slipping (possible, sort of), you have mounted 10"-diameter rims and tires on your car (possible, but highly unlikely as I'm being a bit facetious), or your car has a 5.45:1 differential (again possible, but highly unlikely, as I'm once again being facetious).

Do you have access to a GPS to assess your actual speed, rather than relying on the speedometer. Does your speedometer reading match the GPS's results? There's your speedometer test. Next, if you choose a given speed -- say 60 mph -- your tire diameter, and your differential ratio (assume 3.64:1 for a CA-only '76 until you learn more), you can use one of many Web-based utilities to determine your theoretical engine speed. Does your tachometer reading match that theoretical engine speed when the GPS says you have reached 60 mph? There's your tachometer test. Conversely, and without a GPS, if you plug your tire diameter, tachometer reading (perhaps asking a garage or friend with a diagnostic tachometer to compare RPMs with your tachometer), and differential ratio into one of these utilities, you can see whether your speedometer's reading makes any sense, given that top gear on the factory 4-speed is a straight-through 1:1 ratio.

Start with basics: are your speedometer and tachometer vaguely trustworthy? Then you can move onto the differential ratio, but the most likely alternative ratios (3.64, 3.90, or 3.91 -- common transplant from e21) cannot account for the dramatic effect you're seeing, or think you're seeing. This exact scenario was the subject of a thread a year ago, or so. Unfortunately, I don't recall how the story ended, or if we ever learned how the story ended. My first suspect would be the speedometer!

Good luck,

Steve

Edited April 30, 2014 by Conserv

[Mike Self]

A '76 will have a 3.90 diff (unless it's a California specific model)--the only year 2002s were so fitted--so your engine will be turning higher revs than other year cars with the more normal 3.64.  But I think the revs vs speeds you're quoting seem too high.  With a 3.64 diff and a 4 speed, 4000 rpm is 74 mph; with the overdrive 5 speed, it's about 3200.  

 

It's very unusual to find an 02 diff in the US that isn't either a 4.11 (1600), 3.64 (most 2002s) or a 3.90 (76 2002s) so it's really unlikely that you have one of the racing diffs with higher than a 4.11 ratio.

 

Many folks fit the 3.90 limited slip to a car with a five speed conversion to gain additional acceleration through the gears, but end up with about the same speed at 4000 rpm in 5th as a 3.64 has in 4th--about 74 +/- rpm.  With your 3.90 and a 4 speed you'll be a bit less than 74 mph, but certainly not 50.  

 

Something isn't reading correctly--either your speedometer (check with a GPS) or your tach.

 

mike 

Edited April 30, 2014 by mike

[Preyupy]

Dbl post

Edited April 29, 2014 by Preyupy

[Preyupy]

There are a number of ways to check the final drive ratio:

#1: jack up the car and put it on jack stands, crawl under and look on the left side of the differential housing gust behind the input shaft flange. You will likely need to clean it off maybe even use a wire brush to clean it. There should be two numbers stamped into the housing (ie 10 39) decide the larger # by the smaller # and you will get the ratio ( the numbers are the # of teeth on the pinion gear and the crown wheel gear)

#2: with the car in the air put a mark on the driveline or input shaft flange and marks on the output flanges. Count the number of times the input shaft turns for every 1full turn of the output flanges (if you have an open diff you need to make sure BOTH output flanges turn at the same time, they BOTH have to turn 1full revolution for you to be accurate)

#3: Remove the Diff and take the back cover off and count the teeth on each gear and do the math.

The most likely ratio you should have in the car should be a 3.64:1. There is the possibility you could have a 3.90:1 or less likely a 4.10:1 the car is 38 years old and there is a good chance that the diff could have been changed at least once in its life.

There is also the possibility that the tach is not accurate and I can assure you the speedo is off by at least 5mph at 60 mph if it has the Correct final drive for that speedo. What tire are you running? With the correct tire ( 165sr13 which is about 23.5" tall)

3000rpm=58mph with the 3.64 final drive

3000rpm=54mph with the 3.90

3000rpm=51mph with the 4.10

If you are running a shorter tire (for instance a 195/50/15 is 22.5-22.7" tall) engine speed will go up for the same mph

[Conserv]

A '76 will have a 3.90 diff--the only year 2002s were so fitted--so your engine will be turning higher revs than other year cars with the more normal 3.64.

Mike,

The 49-state '76 cars, indeed, came with the 3.90 differentials (my '76 has a 3.90), but the CA-only '76 cars retained the 3.64 differentials (and Thermactors, and E12 head) of all '75 U.S. cars. Since the OP had a Santa Monica, CA address, I suspected he had a CA-only version. I don't know that for a fact; I just supposed. The specs below, showing the 49-state and CA-only differential ratios, are in the January '76 versions of the '75-'76 brochures.

Regards,

Steve

Edited May 1, 2014 by Conserv

[dcmackintosh]

Without a GPS you can always measure your actual speed the old-fashioned way, by timing yourself between mile markers.  Divide 3,600 by the time in seconds to get your average speed over the mile.

[williamggruff]

You may find some value in the spreadsheet linked in this thread:

 

http://www.bmw2002faq.com/topic/152750-transmissions-differentials-rpm-and-top-speed-spreadsheet/

[Conserv]

Without a GPS you can always measure your actual speed the old-fashioned way, by timing yourself between mile markers. Divide 3,600 by the time in seconds to get your average speed over the mile.

That is so "old school." I like it, but I forgot that that's how we did it pre-GPS, with varying accuracy!

Steve

Edited April 29, 2014 by Conserv

[John_in_VA]

There are a number of ways to check the final drive ratio:

#1: jack up the car and put it on jack stands, crawl under and look on the left side of the differential housing gust behind the input shaft flange. You will likely need to clean it off maybe even use a wire brush to clean it. There should be two numbers stamped into the housing (ie 10 39) DIVIDE the larger # by the smaller # and you will get the ratio ( the numbers are the # of teeth on the pinion gear and the crown wheel gear)

Edited April 29, 2014 by John_in_VA

[McKinney]

Thank you for all the replys and help. I meant to double check the tire size when I arrived at work this morning but it skipped my mind. I'll be able to double check them in a few hours. I was able to use an app on my phone called SpeedView to check my mph via the gps. Granted I have no idea how accurate this is but my results were even worse than expected. When my rpms were at 5500 in 4th the gps said I was at 58mph. When I was at 65mph the rpms were around 6000.

I need to figure out the gear ratio in order to do some math and see where I should be based on my tires sizes but I won't be able to do that until Thursday.

So my other question is let's say my gear ratio is either the 3.64 or 3.90. Is there any other issue that could be causing this such as tuning, low oil level in diff, weak motor, etc? This Thursday I'm planning on changing the trans and diff fluid to redline, do a valve job, and some carb tuning so hopefully this helps or miraculously fixes the issue.

Edited April 29, 2014 by McKinney

[Preyupy]

The only way you can have a change in speed vers rpm is to have slippage in the drivetrain. The differential can't "slip" if it does it is because you have broken teeth in the gears and you would know it. Is it possible your clutch is slipping? I know this might sound insulting but just checking, you do know where 4th gear is? 6000rpm@65mph sounds like you are driving around in 3rd gear with a 3.90 diff. It might be a good idea to check your tach too.

[Mike Self]

double post...

Edited April 29, 2014 by mike

[Conserv]

I know this might sound insulting but just checking, you do know where 4th gear is?

+1

Sorry, McKinney, but I had the same thought...it was fleeting, of course... (I've met more than one person in my lifetime who thought "4-speed transmission" meant 3 forward speeds and 1 reverse.)

Steve

Edited April 29, 2014 by Conserv

[McKinney]

'll pretend I'm not insulted!!

I know where 4th gear is! But I see why you had to ask. Another question at what speed are most of you getting into 4th gear? I am easily into 4th by 35-40mph

The previous owner said he just installed a new clutch but I'm sure it's possible that it is slipping. The clutch holds well during all other driving :/

I was just able to check my tire size and they are

195/60 14 on what I believe are E30 bottle caps?

Edited April 29, 2014 by McKinney