OK, here is what I know... before with the old tires, 285-70-17's, worn but still had tread left (can post pics if needed), I was able to coast down all of the "flatter hills" between Tacoma & Tukwilla on I5 with a 2~3 mph drop in speeds. Now with the new tire, 285-75-17's, there is no way. Even the steeper drops have lost a noticeable amount of coasting capabilities. Being that the tires are new, heavier (5# comparing new old size tire to new tire according to my son who works at the shop and had access to the #'s) and taller (by 1", same width). The new tires have a noticeable increase in grip (also expected) but to me feel like they are better suited for snowy conditions. The rubber is pretty grippy and I already see a good sized drop in the number of miles driven vs what the fuel gauge says (speedometer corrected). FWIW to anyone, I will post mpg #'s for this tank at the end of the week, last tank was 22.4 mpg.
I did see an approx 75~85 rpm drop from the bump in size. Also, tune needs to be tweeked... Not mentioned on this forum but when I did the driveshaft swap I had to adjust the timing to get the turbo spooled back down around 1000~1100 rpms. Now with the tire swap, I will need to re-tweek it again as I now have some timing rattle (mpg killer) in certain area's of the tune.
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Originally Posted by skyking
I'm thinking 275/70 for my truck with the tall gears I have.
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Look for a weight reduction from your current tire. I think your problem with the lower mpg on the Seattle trip is the hill climbs. Next time you take that trip, pop it in neutral on all the down hill area's on I5 (if you dont take I5, take it) and report back how well it did on the downslopes... Our trucks are near identical so you should be able to coast down most of them with minimal drop in speed.
Also, did you do an aluminum DS? If not, do it... its worth it.