nickster

I have a 08 528i with manual transmission. At 80 mph my rpms are about 3000. I got a 328i with automatic transmission today and noticed that my rpms at 80 mph were 2500. I thought my rpms seemed high and was wondering what other engine/transmission combos are at 80mph.

kscarrol

Not an expert by any means but the 528 is a bigger, heavier car which requires more power to run at the same road speed as the lighter 328. Just my layman's view which I'm sure others will confirm or deny! Will have to get back to you with my engine but I want to say my 545 is running around 2,700 rpm at 80 mph.

redwhiteblue

Woa how about telling us which gear you are in. I am assuming 5th gear or 6th? In automatic it may simply be that it shifted to 6th and in manaul you were in 5th-or what gears are we talking here. Shift manually with the auto into the same gear and compare it to the other one.

Otherwise the weight of the car has no impact and they should run the exact same rpms

Black BMW530iA

But... do you have the same ratio in the manual gearbox as you have in an automatic? Just becouse you drive in 5th ovh 6th gear with one type of gearbox doesn?t mean that the wheels are spinning in the same speed as with the other. Or have I misunderstood the question?

BrianTx5

If you do some research you can find final drive ratio of each car, which includes the ratio of the top gear of the transmission and the ratio of the rear differential. The 528i might have bigger gears in the rear to improve the performance of a heavier car with the same engine as the 328i. Magazines like Road and Track include this information when they do a thorough review of a car.

redwhiteblue

Sorry I did not read close enough-thought you were comparing 2 manuals. Either way any time you have an auto v. manual in the same model of car the auto gets shorter gears (higher final drive ratio) in order to somewhat compensate for the deficiency the torque coverter causes in decreasing power in the car to the wheels. When you have different final drive ratios and basically different size gears throughout-will account for the small difference in rpms at each level. So basically having a higher ratio in first will allow you to have some gearing advantage in first over a car with a lower ratio and such through each gear. That is why many people change out their gears for a final drive with a higher number and create the torque multiplication effect as much as 20 percent increase with even small increaes in final ratio.

EIther way here are the numbers for the final drive ratios

528 auto-3.73
528 man-3.36

328 auto-3.73
328 man-3.23

So there is your difference

nickster

Thanks for the info RWB. I am just really surprised at the high rpms in 6th gear. I have caught myself engaging the clutch and shifting to make sure I was in 6th gear. Now before you all start to run me down for not knowing what gear I am in, I only have 600 miles on the car and I am coming from a car I had for 16 years. It is going to take me some time to know my new baby inside and out like I did the old one. A handful of time I shifted into 6th to go in reverse becasue that is where reverse was in my old car. I guess I asked the question about rpms originally to make sure I was where I needed to be.

BrianTx5

Another factor might be the tire size; if they have different wheel diameters or if the profile of the tire is different or both.

By the way, I am currently driving a 328i loaner car with the automatic and I also was at about 2500 rpm at 80 mph, maybe it was closer to 2600 rpm.

aznunknownx

Mine's about 2,400 to 2,5000 RPM at 80 MPH for a 550i.

My guess is bigger engine = enables to reach higher speed with lower RPMs, whereas smaller engine = same speed with higher RPM.
Car size and weight also plays a factor in the RPM. The bigger the car or the heavier it is = higher RPMs to reach the same speed. (This is probably your case since you drove the same engine, 528i and 328i.)

Correct me if I'm wrong...

redwhiteblue

Engine size actually has absolutely nothing to do with what RPMS a car runs at. It is completely all gearing and engine design. FOr example the e46 inline 6 m3 had rpms up to 8000 and a vette with a huge v8 goes to 5500 for a crude example. That is top rpm but just to illustrate the range of rpms a car has is an engine design but not necessarily due to size.

Anyway it is not tire size-standard tires are 17s and 18s for sport just like the e60 is and same profile tires I have been in both.

The difference is simply the gearing that I illustrated-The rpms a car runs at in each gear is dependent on gears.