When you fill up at a higher elevation, like in Colorado, or Utah, you imght be picking the lowest octane, and getting 85 octane, instead of 87 octane.
Running 91 octane should provide much better fuel mileage, as the engine will adjust to the slower burning fuel, and lean out the fuel mixture, as well as advance the timing to the point it might start to ping, but will give much more power out of the fuel it is consuming!
I found that I would get more than 10% better mileage on 91 octane, and right now it only costs about 5% more than the 87 octane.
Also check your fuel filter. IF you had been running low fuel pressure while climbing the hill, then it is possible when running lean, that it will ping. Running rich, it will not ping. Back in the carb days, if you had the engine set up to run lean, it was subject to running warm, and hated cheap fuel that was 87 octane, but would make great mileage and power on 89 octane fuel!
Vans used to be set up to run a little more rich, so that it would not overheat. That extra fuel would cool the pistons a bit, and the engine would run cooler.
So a hot day, low fuel pressure, it is two things that can lead to pinging. 85 octane gas that is available in the higher altitudes, that will cause it too. If you 'might' have picked up some 85 octane fuel, then I recommend topping off with 91 octane fuel right away, and then use up as much of the 'bad' low octane fuel as possible, or at least 1/2 tank, then refill with 91 octane if you will be going up more hills.
Once you are back below 2,000' elevation, you will require 87 octane again. Above 4,500' elevation, normal cars do not require 87 octane, as the compression ratio is not high enough to cause pinging on cars going in higher altitudes.
Good luck,
Fred.
