Theoretical question


If you had two identical cars, identical from front to back with one exception. Same rear ends, gear ratios, transmission, tire and wheel size, etc…the only difference being that one was powered by a four cylinder engine and the other by a V8 engine. Using only the information given in this question, if both were on the highway turning 4000 RPM, would they be traveling at the same speed? Thanks for the help in settling this, rapidly becoming acrimonious, bet.



Normally the final gear ratio would not be the same for both engines so the V8 would be turning slower.


Yes. The number of cylinders makes no difference.


Unless you hook the tachometer up wrong…


Yes, and note that the V8 would be firing at half the rate of the 4-cyl. engine.


Same speed.


If both are turning at 4000 rpm, both are are firing at the same rate. All other things being equal, the V8 will burn more fuel because it has double the internal pumping losses and friction.


The relationship between engine speed (rpm) and road speed (mph) is determined by transmission gear ratio, differential ratio and tire size. Since these parameters are the same for both cars, they would be traveling at the same speed.