Help in knowing the climb on rt 40

Needles to Kingman, gradual climb 60 miles from 500' to 3350'
Kingman to Seligman, gradual climb 65 miles to 5250'
Seligman to Ashfork, fairly flat 25 miles to 5160'
Ashfork to Williams, moderate climb 18 miles to 6765'
Williams to Flagstaff, fairly flat 30 miles to 6910'

Needles will be hot in summer, 5 degrees warmer than Phoenix.
Kingman typically will be 15 degrees cooler than Needles.
Williams and Flagstaff typically 30 degrees cooler than Needles.

You can't miss the heat, unless you hit monsoon. You are going through the Mojave desert and then the uphill from Needles to Kingman will likely be just as hot as Phoenix.

I want to thank you all for your response and now I feel much better I really didn't want to go through Phenix with the heat I am sure it will have by the middle of June.

According to the 'Mountain Directory for Truckers and RVers' (highly recommended, by the way):

"About 10 miles east of Ash Fork there is a brake check area for westbound traffic near milepost 155 1/2 and some warning signs "trucks-vehicles pulling trailers check brakes and equipment - use lower gear" and "6 percent grade next 6 miles". The first 2 miles are 6 percent followed by about 1-1/2 miles of 4-5 percent and 1-1/2 miles of 6 per cent."

As noted above, that's for westbound downhill. It doesn't give any warnings for eastbound uphill.

We didn't feel it was an issue. If you can do the Grapevine you've got it made.

NMDriver wrote:
The longest steep part is just east of Kingman. The whole climb to Flag is really a series of steps from the Colorado River up to Flag. You go up a step, level off maybe even go down a bit and then up another step. None of the climbs are as long or as steep as the Grapevine.

H E R E - Needles to Flagstaff is a graphical representation of these steps. While initially it may look daunting, keep in mind that the distance is 209 miles, so spread it out accordingly.

The longest steep part is just east of Kingman. The whole climb to Flag is really a series of steps from the Colorado River up to Flag. You go up a step, level off maybe even go down a bit and then up another step. None of the climbs are as long or as steep as the Grapevine.

I've driven I-40 many times coming home to Sedona with no problems.

Have a look at this website, which I discovered on these forums:
Flattest Route

We have driven it several times without problems. It's more of a gradual grade with a few steeper areas, but absolutely nothing tough. Not nearly as steep as the Shasta area. You may want to check the Mountain Directory West to get more details.