Heather & Ron
I have an older 4-runner with a 3.4 liter v8 "183 hp" and also tow a 3124KS.
Your 2008 Toyota Highlander is most likely a 3.5 V6 at 270 hp with a 5 speed trans. From what I understand, towing with the overdrive on uses a planatary gear which gears it more than 1-1 ratio. 4th gear should be a 1:1 ratio, but I may be wrong because I am not familiar with the Highlander.
I constantly go up and down 1-70 from Denver which is a 7% grade in about 3 places. I always leave my overdrive off increase my braking on the trailers electric brakes and use my engine compression downhill when I can. The brake rotors are fairly thin on the 99 4runner and they will warp fairly easy when overheating them.
Going uphill -- 6% grade 55mph, 7% grade 25-30mph.
Happy camping.
