I'm running on almost new 20" Michelins, and had a blowout on left rear. I mounted the spare, which I found to be a new stock 19" Goodyear which came with the vehicle. It was fine to get home on, but I've got to drive through the mountains to pick up family in Denver today, a 3 hour trip each way. I can't get a replacement tire for a couple days. Will driving on a smaller wheel damage the transmission?

 

I wouldn't drive it bud, the wheels are out of balance and you run the risk of damaging the axle, differential, and possibly (but doubtful) the transfer case and transmission. I would be wiling to bet the rover will make it there and back fine, but you will be doing damage that will catch up to you later. To play it safe, just don't drive it until you get the right tire on there.

 

I once had occasion to use my spare which was 3/4" smaller in rolling diameter than the oversize rubber I was running at that time. It was bloody scarey. The rig was all over the road. DON'T do it.

 

whell size doesnt matter... rolling radius it what counts. if you sized the 20's right when you got them, youll be fine. what sizes are the 20s, the 19s should be 255-55... the 20s should be 285-45, depending on rim width, but thats about the perfect rolling radius