The time is coming for me to swap tires on my Ram (2005 4x4 QC diesel, manual trans, 370k miles) and a question has come to mind which I need some expert input on...
RE: Current tires... I have currently 285-70-17's (33" tall) on there and can easily get 21's for mpg. This is to & from work, to & from trailheads (dirt roads) and minimal city driving. If I put a % on it, I'd say 90% hiway is what I do. Living in the Seattle area, nothing is flat, I get to leverage the hills quite a bit coasting in neutral.
Possible future tires...From past experience with a 285-75-17 (34" tall) on long road trips 22's were easily had. City mpg however suffered when mixed. I'd have to go back and dig up receipts but 18's come to mind for combined mpg during the warm months.
Currently my commute miles with all the cold starts, rush hour traffic and many trips, I can out perform straight hiway miles for mpg. Seems if I get on the longer road trips I'm down ~.5-.75 mpg vs commute mpg. I know if I go to the taller tire, the commute mpg may drop because of the added load on all of the take off events. But, the question is...
If I have a taller tire (+5# over shorter tire) vs a shorter tire, whats going to happen to the coasting performance going down hills? I will be going with the same tire which is an Toyo Open Country AT2. I know going up will suffer because of the taller tire and the added weight but can that be made up on the backside of the hills?
edit: adding mpg data from before and after tire swap.
https://ecomodder.com/forum/showthre...tml#post583520
Toyo Open Country AT2's 110k miles on them (speedo was within +/- 0.5 mph)
09/1/52018 425.0 18.432 23.05772569 100% eco
09/22/2018 453.3 19.624 23.0992662 100% eco
09/23/2018 216.0 10.366 20.83735288 DGAS tank
09/29/2018 491.7 21.142 23.25702393 100% eco
10/08/2018 263.4 13.004 20.25530606 DGAS tank
10/13/2018 400.8 17.87 22.42865137 100% eco
Totals 2250.2 mi 100.438 gal 22.40387104 mpg average
Mazama Tires (speedo updated to within +/- 0.5 mph)
10/18/2018 535.1 25.471 21.00820541 100% eco
10/21/2018 519.5 27.768 18.70858542 DGAS tank
10/26/2018 492.1 25.792 19.07955955 DGAS tank, test & tune
10/28/2018 496.4 25.279 19.63685272 100% eco
11/04/2018 406.3 20.693 19.63465906 100% eco
11/10/2018 357.2 17.832 20.03140422 100% eco tune change, went to previous best performer with 33's on there
Totals 2806.6 mi 142.835 ga 19.64924563 mpg <-Average