Sixtysixdeuce

Let us know how it works out.

You can calculate how it will theoretically affect your acceleration & top speed. Find out what your transmission ratio is, then multiply that by your pinon/spur ratio. Then simply apply your tire circumference and you will know how many inches of travel you get per engine revolution.

For instance, the RC10GT has a 2.6:1 transmission ratio and comes stock with 15/66 clutch bell/spur, which is 4.4:1. So the final drive ratio is 11.44:1, or 11.44 engine revs to one wheel rotation. If the stock tire is 3.5" OD, it will cover 11" per revolution. So, each turn of the engine moves the car 0.96". If we go to a 4.5" tire, it will now cover 14.13" per revolution, roughly a 28% increase in distance covered per revolution. To maintain our 0.96" travel per engine revolution, we must increase the final drive ratio by 28%, bringing it to 14.64:1. We have to account for the 2.6 multiplying factor of the transmission ratio, so we divide 14.64 by 2.6, thus telling us we need a 5.6:1 pinion/spur ratio. If we mean to retain the 66T spur, then we divide 66 by 5.6, getting 11.78. So a 12T pinion would be as close as we get. To make the change with spur gear, multiply pinion tooth count by the desired ratio to find the correct spur size, which would be 84 in this case. Of course, neither of these sizes are usually available in nitro, so we have to use combinations, and often settle for a change in grunt & top speed.

Having said this, you can see where your change in clutch bell will probably not fully compensate for a 1" increase in tire OD. The Firestorm 10T is basically an RC10GT clone, except that the transmission ratio is 4.4:1 and the OEM clutch bell/spur is 19T/50T. Your stock final drive ratio is the same as the 10GT at 11.44:1, but they change ratio in spur or pinion are very different. Going from 19T to 17T while retaining the 50T spur is only a 12% change in ratio (2.94 vs. 2.63), which will not compensate for a 25% or 30% increase in tire rolling circumference. To maintain, you'd need a ~3.3:1 ratio, which would require a 14T or 15T bell.

The other thing to remember is that larger tires add unsprung weight and rolling resistance; even if your final drive gives you the same distance per engine rev, you will have slower acceleration and lower top speed (unless your engine is powerful enough to reach maximum RPM regardless). This is a more significant factor in full sized cars, but it does matter with 1/10 cars, too.

In my case, the primary purpose of going to the larger tires was a top speed increase, since that OFNA .32 has more than enough power to spare. I currently have a 9.88:1 final ratio, and cover 1.44" per engine rev. The engine still easily reaches max RPM and I want more speed, so I'll be going from a 14T bell/54T spur to a 20T bell and 50T spur, giving me a 6.5:1 final ratio and covering 2.18" per engine revolution.

For future reference, if you have an SG shaft engine, they are standardized, and you can make just about any clutch bell work. It may require trimming the back a tad or shimming bearings to get proper clearances, though. However, if your car runs a more proprietary set up (like the older AE 3/16 adapters), you may have to stick with bells designed for your car or get creative with adapters.

Spur gears are also pretty easy to adapt if the dedicated part isn't available. My 10GT pictured above is fitted with a Team Durango 54T mod. 1 spur.

Up to the point that you run out of adjustment, yes. If you make a drastic change, it will require both.