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aardvarcus · 06-02-2015, 07:11 PM

Yes you can use larger tires to reduce RPM, however you need to very careful in tire selection because if you pick a tire with high rolling resistance or wind drag those detriments will outweigh the advantages of the gearing. Practically you aren't going to get more than 4-5% gearing increase before you run into issues of tire fitment or tread pattern selection. Also the most popular large diameter tires are also wider, which very quickly leads to fitment issues. Also weight of larger tires must be factored in as well.

I have tried changing tires for MPG many times for several trucks and have posted my results before, so I found some of my old Tacoma related posts and put the relevant excerpts and a bit of new information here:

Tacoma 4x4/Prerunner stock has P245/75R16 or P265/70R16 both of which are roughly 30.5” diameter. (Note Tacomas can come with 17” and 18” wheels have similar diameter tires, but I don't consider those wheels advantageous when discussing MPG.) Taller sized 16” tires that fit the truck without modification on stock wheels are 265/75R16, 235/85R16, 255/85R16. Any tires larger or wider than these sizes will not fit without modification and even these tires might not fit if you are using aftermarket wheels with different widths or offsets. The 265/75R16s and the 235/85R16 are both roughly 31.7 tall. The 255/85R16 are roughly 33.4” tall, but only available in very few tread patterns, none of which are advantageous to MPG (off road mud type tread).

I run 235/85R16 Michelin LTX M/S2 on my 2005 Tacoma on factory 16” TRD alloys. I also have a set of 255/85R16 Cooper Discoverer S/T on a second set of factory 16” TRD alloys for off-road use. On my truck both fit with no lift and don’t rub (note I am on stock wheels). I later added Bilstein 5100’s on the front at 1.75” and they still fit and don’t rub.

The 235/85R16s are only going to be available in LT rated sizes which are heavy but more durable for off-roading (versus P or SL). The weight of the tire will affect the ride slightly (sprung vs un-sprung ratio increases). My mainly highway MPGs stayed the same (+/- 0.5 or less) between cheap brand 245/75R16 on steels to 235/85R16 Michelin LTX M/S2 on alloys when adjusted for diameter. If I put the off-road 255/85R16 Cooper Discoverer S/T on the truck, the highway MPG goes down approximately 2MPG adjusted for diameter, mainly due to the aggressive tread pattern. The sound of the mud terrain drone is the sound of aerodynamic drag. Look at my fuel log to see the actual tanks; I usually make a note on the log when I am running the different tires. (Note I think this gap was wider prior to my front air dam.)

The 235/85R16 Michelin LTX M/S2 are awesome on pavement (wet, dry, whatever) and do ok (better than you would think by looking at them) offroad, except in mud where they quickly get loaded up. The 255/85R16 Cooper Discoverer S/T aren’t that great on the pavement, but seem to do better off the pavement, especially in mud/soft soil. I would not want to run mud terrain tires on the highway all the time, maybe it is the lack of siping but the handling just isn’t the same. I don’t care how cool it is, I hate driving with mud terrains on pavement. Period.

My suggestion is to not overlook the importance of tread pattern. My recent tire purchases have focused on picking a tread pattern first, and a size second. I have found that other features of a tire (tread pattern, sipes, compound, etcetera) are much more important than size.

06-02-2015, 07:11 PM	#42 (permalink)
aardvarcus Master EcoModder Join Date: Apr 2012 Location: Evensville, TN Posts: 676 Deep Blue - '94 GMC Suburban K2500 SLE 90 day: 23.75 mpg (US) Griffin (T4R) - '99 Toyota 4Runner SR5 90 day: 25.43 mpg (US) Thanks: 237 Thanked 580 Times in 322 Posts	Yes you can use larger tires to reduce RPM, however you need to very careful in tire selection because if you pick a tire with high rolling resistance or wind drag those detriments will outweigh the advantages of the gearing. Practically you aren't going to get more than 4-5% gearing increase before you run into issues of tire fitment or tread pattern selection. Also the most popular large diameter tires are also wider, which very quickly leads to fitment issues. Also weight of larger tires must be factored in as well. I have tried changing tires for MPG many times for several trucks and have posted my results before, so I found some of my old Tacoma related posts and put the relevant excerpts and a bit of new information here: Tacoma 4x4/Prerunner stock has P245/75R16 or P265/70R16 both of which are roughly 30.5” diameter. (Note Tacomas can come with 17” and 18” wheels have similar diameter tires, but I don't consider those wheels advantageous when discussing MPG.) Taller sized 16” tires that fit the truck without modification on stock wheels are 265/75R16, 235/85R16, 255/85R16. Any tires larger or wider than these sizes will not fit without modification and even these tires might not fit if you are using aftermarket wheels with different widths or offsets. The 265/75R16s and the 235/85R16 are both roughly 31.7 tall. The 255/85R16 are roughly 33.4” tall, but only available in very few tread patterns, none of which are advantageous to MPG (off road mud type tread). I run 235/85R16 Michelin LTX M/S2 on my 2005 Tacoma on factory 16” TRD alloys. I also have a set of 255/85R16 Cooper Discoverer S/T on a second set of factory 16” TRD alloys for off-road use. On my truck both fit with no lift and don’t rub (note I am on stock wheels). I later added Bilstein 5100’s on the front at 1.75” and they still fit and don’t rub. The 235/85R16s are only going to be available in LT rated sizes which are heavy but more durable for off-roading (versus P or SL). The weight of the tire will affect the ride slightly (sprung vs un-sprung ratio increases). My mainly highway MPGs stayed the same (+/- 0.5 or less) between cheap brand 245/75R16 on steels to 235/85R16 Michelin LTX M/S2 on alloys when adjusted for diameter. If I put the off-road 255/85R16 Cooper Discoverer S/T on the truck, the highway MPG goes down approximately 2MPG adjusted for diameter, mainly due to the aggressive tread pattern. The sound of the mud terrain drone is the sound of aerodynamic drag. Look at my fuel log to see the actual tanks; I usually make a note on the log when I am running the different tires. (Note I think this gap was wider prior to my front air dam.) The 235/85R16 Michelin LTX M/S2 are awesome on pavement (wet, dry, whatever) and do ok (better than you would think by looking at them) offroad, except in mud where they quickly get loaded up. The 255/85R16 Cooper Discoverer S/T aren’t that great on the pavement, but seem to do better off the pavement, especially in mud/soft soil. I would not want to run mud terrain tires on the highway all the time, maybe it is the lack of siping but the handling just isn’t the same. I don’t care how cool it is, I hate driving with mud terrains on pavement. Period. My suggestion is to not overlook the importance of tread pattern. My recent tire purchases have focused on picking a tread pattern first, and a size second. I have found that other features of a tire (tread pattern, sipes, compound, etcetera) are much more important than size.