Mixed calculations with integers and floats in C?

[nigelmercier]

I'm trying to do a calculation that should be simple, but I keep getting a spurious answer of zero. It involves the rate of fuel use of an engine, the formula I'm using is:
Rate of fuel use = (RPM / Max RPM)^3 * (Max Engine Power / 3.3)
where 3.3 is the number of kWh produced by 1 litre of fuel). My code is as follows:

// following in typedefs.h file
typedef unsigned short int byte;

int rpm ;              // current rpm 
float kwh = 3.3 ;      // Diesel Engine = 3.3KWh per litre
byte maxPower = 44 ;   // maximum power of engine in kW at ...
int maxRevs = 4000 ;   // ... maximum revs to give maxPower
...
litPerHour = pow((rpm/maxRevs), 3)*(maxPower/kwh) ;//

Library function:
double pow(double x, double y); //returns the value of x raised to the power y

I guess that I'm running out of precision somewhere, but I can't spot it.
Any suggestions?
Please note: I'm 50+, this is *not* homework! :)

[newb16]

rpm/maxRevs is calculated in integer, result is 0 if rpm is less than 4000.
(double)rpm/maxRevs will cast rpm to double first and will fix it.

[MartijnHoekstra]

i am concerned about your 'rpm/maxRevs' devision.
You are now working with rounded numbers, which is good if you only want fraction-less numbers.
for every number of rpm smaller than 4000, you will have pow(0,3).
If you want to perform this calculation with fractions, try casting to doubles.

litPerHour = pow( (double)rpm/(double)maxRevs , 3 ) * ...

same counts for the latter part: ((double)maxPow er/kwh)

[nigelmercier]

D'OH! Thanks for the replies, all working now. You can tell it's been a while since I did any math!