Well I just finished a trip picking up my M931A1 from Gulfport Miss and drove it 490 miles home pulling a M1009. This spring I got a M818 and drove it back home from Columbia SC ( 600 +/- miles) pulling a M1031. Both trucks have under 3500 miles and both were in like new condition and were basically stock. The M931A1 has Combat Wheels and 14.00 R20 tires which did increase the road speed. Since I own both and they both took such similar trips I thought I would do a side by side comparison.
The M818 has a bit less power pulling hills and the road speed is less even when you take out the advantage of the larger tires on the M931A1. The truck doesnt seem as big as the M931 and you can actually see behind you because the spare tire isnt in the way. The handling and driving is very similar. The ride quality isnt all that different. The M818 is much closer to feeling like a M35A2 driving than the M931 is. Average MPG on my trip was around 8mpg with an average speed of 53 mph at around 1900 RPM. I slowed down lots on the hill but I think some of it was me needing to change the fuel filter. Maybe not a big issue because the flats pulled great but may still be an issue on hills. The brakes being air-over-hydraulic feel like a M35.
The M931A1 is taller than the M818 and the automatic transmission is really great. The shift is smooth and doesnt run the RPM's real high before shifting up or down. Visibility from the cab is the same as the M818 except you have to use your mirrors you cant check your blind spot off the right rear tandems through the window like the M818 because of the spare tire. You need a convex mirror on the passenger side. Pulling a M1009 I averaged about 9 mpg but I had a ton of JP8 in the tank when I started where the M818 had straight diesel. I could maintain 65 mph on the flats and 55 @1900 RPM on pretty good hills. The brakes arent overly sensitive bob-tail on either truck. There seems to be more power available with the M931 than the M818. The truck runs and drives more like a civilian Semi than a military truck. I cant explain the differences but it didnt feel like a military vehicle to me. Except for the lack of AC anyway, that was a dead giveaway. The ABS airbrakes were nice, but Im used to them from driving semi's on and off for years. They take some getting used to if you arent familiar with them.
Summary if you are a collector and want a good military collector vehicle get a M818. The components and maintenance will be familiar to you from other mil vehicles. The Allison Automatic and air-brakes will be more expensive to fix when you have problems. If you do any work with your vehicle or take long road trips with shows and rallies get a M931A1. If your pulling a load over any distance the M931A1 is probably a better bet with the ABS airbrakes and more power.