So, I did a bit of research now
First I found: I couldn't find a "Cat 350D" ADT, but there is a John Deere 350D.
http://www.deere.com/wps/dcom/en_US/...series_ii.page
EDIT: Okay, the truck is a Cat D350 or maybe a D350D...
Anyway, both use 26.5R25 tires:
http://www.bridgestone.eu/upload/album/AP_429.pdf
These have an outer diameter of 1736mm.
http://www.bridgestone.eu/upload/album/AP_429.pdf
The RC4WD tires have a diameter of 131mm.
->Scale 1736/131 = 1:13.2
The circumference of the original tire is 5.45m
This means that from the John Deere pdf (I didn't find a D350D one but from
http://www.ritchiespecs.com/specific...&modelid=91901 we can guess that the D350D as a speed like the Volvo 350D without its 6th gear) we can convert their speeds to rpm at the wheel:
1st gear = 25rpm (5 mph)
2nd gear = 52rpm (11 mph)
3rd gear = 74rpm (15 mph)
4th gear = 113rpm (23 mph)
5th gear = 144rpm (30 mph)
(6th gear = 165rpm (34 mph) only John Deere)
If you intend to do only (very) short hauls, 35rpm seems maybe acceptable, if the 35rpm of these motors is the load speed. If it's the no-load speed, I think that even then it would be too slow.
Cheers
Jens