same ratio dfferent gears

The differance in the size of the gear can be crucial - too big can foul the chassis or reduce ground clearance - in anglewinders and sidewinders the right spacing to get the gears to mesh correctly (this is particularly a problem where the motor position cannot be changed.)
Some combinations mesh better than others, but this is an issue with particular makes and types rather than just number of teeth.
There is a slight tendency for more teeth to mesh better, but this is quite insignificant compared with getting the ratio right.

The right ratio is the one that makes the car go best in the conditions. For example on tracks with predominately long straights it can be better to gear for more top speed than on tracks with shorter straights where low speed acceleration matters.

Euro sport cars run better with gear ratios of 6:1 or 7:1 (Wing cars have fairly similar motors but usually run less extreme ratios because they need higher top speed.)
There are various reasons given (torque characteristics, gyroscopic couples, the motors rev so high that's what you need to reach full revs before the end of the straight etc.) Difficult to say how much a part each of these contribute to making the cars go better, most competitors main interest is that it does make the cars go better.
Incidentally, these ratios can usually produce more brakes than is wanted, so a controller with adjustable brakes is normally used.

QUOTE (Abarth Mike @ 24 Apr 2012, 09:33) <{POST_SNAPBACK}>Are all pinions the same diameter? In many makes they are not.
There are a few makes that do have a common size pinion with different numbers of teeth.
Usually the pinions that are near the correct pitch for the spur / crown mesh better than those where the tooth form has been compromised to get the "wrong" number of teeth on the pinion.

QUOTE (RikoRocket @ 24 Apr 2012, 09:13) <{POST_SNAPBACK}>Plastic teeth need a reasonable cross section to resist the high torques of modern motors. This can be achieved by increasing the depth of the tooth, but again that can lead to more rubbing. Look at the gears on the AutoArt cars, same pitch but really thin. Put a NSR motor in one of those cars and the gears are going to fail really quickly... Yes that is a good point but it doesn't seem to apply to all slot cars. The much more powerful cars generally use the 80 pitch gears which have considerably smaller teeth, they use a steel pinion and usually a plastic spur. These cars run ball races in the motor and axle, so there is no slop in the bearings - I think that's important with small teeth.

QUOTE (stoner @ 27 Apr 2012, 08:48) <{POST_SNAPBACK}>you go for the biggest gear set that will fit into the car and you achieve better gear mesh and less frictional loss.
Yes that's right, so is
QUOTE (John Cahill @ 26 Apr 2012, 22:38) <{POST_SNAPBACK}>So for a given ratio, I think clearly one would want to select the smallest and lightest combination of gears with good profile if one wants to stop and start briskly.
So you've got two effects - one recommends bigger gears, the other recommends the opposite.
So which to choose?
The answer is choose the best compromise, which may be different in different applications.
(Can take a bit of getting your head round that!)

If you can quantify exactly how much efficiency changes with gear size and you can quantify exactly how much acceleration is changed by the inertia effects of gear size then there's a neat mathematical way of working out what is best.
Or you can try it and see which works out best.
In my experience, slot racers generally try it rather than attempt to calculate it.