I'm not sure what gear ratio would be best for your monster truck since I don't know what size the tires are.
Here is how you can calculate the gear ratio to at least get you close. Using the original tires/gears this is the mini-x original final drive ratio.
The stock tire is approximately 28mm tall so in one revolution of the tires the car moves (28mm * 3.1416<pie> = ~88mm). So with the stock gear ratio (35/8 = 4.375) it takes 4.375 engine revolutions to travel 88 mm. Therefore, each revolution of the engine makes the car travel ~ 20.1 mm (88/4.375).
As a starting point you should try to achieve that same ratio ( 1 motor revolution equals 20.1 mm travel distance). Since I don't know the exact diameter of your new tires I will just give you an example and you'll have to plug the actual numbers in to figure out what gears you need. If your monster tires have a diameter of 75 mm then one revolution of the tires will move the car 236 mm. You are running through the stock differential plus a second differential which will multiply the ratio a second time. So here are the calculations:
s1 = original spur gear
p1 = original pinion gear
s2 = added spur gear
p2 = added pinion gear
td = tire diameter
dis = distance traveled per motor revolution
Pie = 3.1416
First we'll use a formula to calculate the original ratio:
dis = (td*Pie)/((s1/p1)
dis = (28*3.1416)/(35/8)
dis = 20.11 mm
Now to calculate the correct pinion gear for your new combo we'll use this formula:
p2 = s2/{[(td*Pie)/(s1/p1)]/dis}
p2 = 34/{[(75*3.1416)/(35/8)]/20.11}
p2 = 12.7 teeth
So, if your monster tires are 75 mm tall you will need roughly a 13 tooth pinion gear to maintain the same approximate overall gear ratio as the mini-x originally had. This would only be a starting point and you would probably want to experiment from there to get the best possible acceleration.
I'm sure there are easier ways to come up with the correct pinion gear but this was the best I can do. Hope it helps and was not too confusing.
Michael