What pinion and spur gear sizes should I get for 17.5 and 13.5t? I want to run 48p as thats what I have but im guessing I need to go with 64p.

For 17.5, run a 72t spur, in 13.5, 76-84t. Roll out will be from hi 70's to mid 80's in 13.5, low 90's to mid 100's in 17.5. So, depending on tire size, will need pinions from 43-58 to cover both classes and all tire sizes. This is with 64p gears, not sure with 48p.

Thank you! Im not very good with all these charts and calculations. Your help is appreciated.

If you have an android phone, there are a few good free rollout calculators out there..

With 17.5t and the 72t spur, what pinion variation would I need then, that is the only class they run in 1/12. I know this is frowned upon but I plan on starting with full size tires. It makes me cringe to think I'd need to shred half the tire off, plus I don't have a truer. I believe they use Jacos as spec tires. I appreciate the help.

I do! What do you recommend?

The concept of taking new tires and truing 3/4 of them off is hard to get used to. However, there is a reason for it. Besides the performance difference, which is significant, often times, the bigger tires chunk and end up not lasting longer anyhow.

At our track, you HAVE to true them down, because a car without them cut down will do nothing but traction roll due to the sidewall flex.


The benefits and fun of 1/12th scale outweigh the negatives of having to do this.

Welcome to 1/12th scale!

more than likely your side springs...check that and make sure your tweak is good. if not it could be your front king pins

I had a similar issue and it turned out the bearing inside my spur gear was too wide and the diff rings weren't making good contact with the diff balls. It felt tight when it was put together but there was just enough play in there that it slipped under acceleration and one side of the car pulled more than the other.

So what pinions in 17.5 with a 73 spur gear? It's the lowest I could find at amain.

47-60 should cover you. The bigger the tire, the smaller the pinion needed for the given roll out. If you are just starting out, aim for 95mm roll out, this should get you going on the track, may not be optimum, but you can adjust from there. You will also need a good set of digital calipers to measure stuff with, tires, shims, springs, etc.

To calculate roll out, a bit of math is required. the formula is;
(3.14*tire diam in mm)/(spur/pinion)=Roll out

So, if your tires are 46mm, spur is 73, and pinion is 48 it will look like this;

(3.14*46mm)/(73/48)=roll out. solve in the brackets first;

144.44/1.520=roll out;

95.02mm roll out.

As the tires get smaller, the pinion goes bigger, approximately add one tooth to the pinion for every .9mm of tire wear.

For the same sized tire, the bigger the pinion, the higher the roll out.

Roll out is the distance in mm the car will travel with one revolution of the motor.

This should get you started.

If the motor is getting too hot, above 150 deg F, lower the roll out, and or lower the timing on the motor.

My man!

Or to simplify it even further, multiply it all together then divide by the spur ;

3.14x46mm(tyre)x48t(pinion) then divide by the spur / 73t

Or since this is the year 2016... you could just use an app such as RcRatio to do the work for you, and save multiple sets of data

If your talking about 5280 then:

The tires for 1/12 are CRC Spec - Purple Stripe
People at the track can help you cut them
41.5/42.5 is the largest i would go with at our track
53-57 is all you need w/ 72-73 spur if you do cut them

Your going to have to cut them - especially when the new carpet arrives.