I had a 2002 F250 V0 with 3.73 rear axle. 7 to 9 towing 15 not towing about right. No problems with the engine or spark plugs. Sluggish towing horse trailers on steep grades with 3.73 rear axle. I put over 100k miles on that PU. I have a 2008 F250 V10 with 4.1 rear axle. 7 to 9 towing 12 not towing. No problems with enginge or spark plugs. I have about 60k miles on that PU. I towed a 4500 lb TT. I think it would be very slow up steep grades with 4.10 axle towing 10k. A 4.8 axle range would be much better. Keep in mind MHs use 5.3 rear axle and higher in larger V10 MHs. I would be more concerned with axle ratio not the engine.