I plan on putting together a new rifle and am debating which chambering I should go with, 25-06 ackley or 6.5-06 improved. I am looking to use the round for long range coyotes. Any advice on the subject would be appreciated.

The 25-06 AI will handle the 115, 117 and 120 gr bullets with great accuracy - at least the rifles in the standard cartridge I've owned (3 of the Ruger #1's). With ballistic coefficients ranging from .453 to 391, these have long range ability.

The sole 6.5-06 (not AI'd because even P.O. Ackley said this really did not improve the performance of this cartridge) shoots the 120 and 129 grain bullets best. Here, the coefficients range from .458 to .445, depending on manufacturer.

While the 120 gr custom loaded for my particular 6.5-06 produce around 3250 fps avg and just under MOA, the 25-06's were stepping right along side them. Case capacity would be equal in each cartridge. Bullet weight is the same, so velocities should be the same. The .25 bullets will have a slightly longer bearing surface.

My nod would go to the 6.5-06 for long range work.

Sure do like my 6.5-06.

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Kdub's done a pretty good comparison. One thing I note is that all the bullets mentioned- and they're the best choices for long range- are not varmint bullets, but deer bullets.
I expect you'd get a pretty good sized exit wound; not an asset if you plan to sell the furs.

I can't comment on the other but I used to have a 25-06 AI
and it put the hurt on many hogs with 100 and 120 grain Nosler particans(Spelling) best day I had with ir was three hogs all in a row first one was a little over 400 yards the last right @ 500 yards the 2nd was in between those. It was stollen from me years back and I just got the bug to build another but this time I'm going with the .257 STW. The 25 Cals are great killers you just have to use the right bullet as low as I would go on Ballistic tips would be 100 gr my buddy used my rifle this one rainy afternoon and all I had loaded were 85 gr Ballistic tips he put four shots in him two in each shoulder and only one made it through the shield he went ALONG!! ways anywas it was a nice boar about 200+ and 2" tusks. Ahh one more for ya and then I"ll quit rambling
he went again the next winter same ranch same 25 Ackley and shot a sow standing looking staright at him right between the eyes at about 75-100 yards she went down in a hurry! he was using 120 particians and the butcher found the complete bullet mushroomed in her spine near her A$$. GOOD Penetration! anyways I"m out good luck on your decision
Later Keith