# RPM's



## grs1961 (Oct 23, 2012)

Depends entirely on which engine you have, and which transmission.


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## 406 (Apr 5, 2019)

grs1961 said:


> Depends entirely on which engine you have, and which transmission.


Engine doesn't matter, just tire size, differential gear ratio, transfer case ratio (which we don't have), and transmission gear ratio.


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## jblackburn (Apr 14, 2012)

406 said:


> Engine doesn't matter, just tire size, differential gear ratio, transfer case ratio (which we don't have), and transmission gear ratio.


Ratios are different between different engine options on these cars.


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## 406 (Apr 5, 2019)

jblackburn said:


> Ratios are different between different engine options on these cars.


I get that, but the engine is at no point involved in rpm calculation. I'll do my best to type the formula for rpm based on km/h, tire height, and gear ratios...

Pre calculation to simplify a bit...
Tire Circumference [km/rotation] = (Pi * Tire Height [cm]) / 10000 [cm/km]
Put another way, this is also the distance per rotation of the tire.

RPM [rotations/minute] = (Speed [km/h]) * (Axle Diff Ratio [dimensionless]) * (Trany Ratio [dimensionless]) / (Tire Circumference [km/rotation]) / (60 [min/h])

Fill that in with the proper values/units and you'll know what your engine rpm should be for both scenarios.

Dimensional analysis.


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## jblackburn (Apr 14, 2012)

406 said:


> I get that, but the engine is at no point involved in rpm calculation. I'll do my best to type the formula for rpm based on km/h, tire height, and gear ratios...
> 
> Pre calculation to simplify a bit...
> Tire Circumference [km/rotation] = (Pi * Tire Height [cm]) / 10000 [cm/km]
> ...


Yep.

Generally, you can say:
I have a 2011 1.8L automatic.
I have a 2014 1.4T LT manual

and have someone tell you general RPM their same car is running on the highway at a certain speed. They were different between almost every engine/trans combination the Cruze was offered with.

As they may still be different in other countries than most of the US models that make up the bulk of the forum, an RPM calculator might be a good fallback if the ratios for that specific engine/trans can be looked up online somewhere.


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## 406 (Apr 5, 2019)

jblackburn said:


> Ratios are different between different engine options on these cars.


To elaborate on this, the ratio of engine crank rpm to transmission input spline rpm is 1:1. Given a specific vehicle speed, the engine plays no role in calculating transmission input spline rpm, which to clarify, equals engine rpm.


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## 406 (Apr 5, 2019)

jblackburn said:


> Yep.
> 
> Generally, you can say:
> I have a 2011 1.8L automatic.
> ...


I agree, with that information, I could hunt down the numbers for the guy and crunch it out. This just introduces some independence so anyone can figure it out on their own.


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