Tire Size--how Does It Effect Your Driving And Mileage?

h2ofwlr · July 26, 2010

I am thinking of getting LT235/85R16 VS 245/70r16. Basically the 1st tire is 1" longer in circumferance. What would happen?

The reason I ask is becuase I was reading that guys gas mileage went DOWN with the bigger tires. I would think it would go up. So trying to figure this out.

Also FYI, my speedometer readings about 5-6% low, meaning if it reads 30, I'm actually going 28, if it reads 60, I'm going 57 per radar. So thinking if a tad biger diameter tires it should read dead on. But do not want to loose gas milaege either...

Mckenzie · July 26, 2010

Yea my mpg went down to 10-11 when I put 285/75/16's on my truck, but the tires you want shouldn't mess up anything

dlstewart01 · July 26, 2010

Just plug the numbers in here and see...

http://www.miata.net/garage/tirecalc.html

dewfpo · July 26, 2010

1" change in circumference won't change anything since the diameters of your wheels/tires are the same. However, if the new tires/wheels are heavier, then you will notice a negative difference directly proportional to the change in weight. Small change, probably not noticeable.

Tread pattern affects mpg numbers as well. Generally, mudders (MT's) get worse mpg's than all-terrains, and AT's get worse then highway tires.

When you go to a larger tire diameter, you are effectively making your rear axle ratio numerically smaller (less revolutions for the same distance covered). If all you do is highway driving this might help with mpg's slightly depending on how much bigger you go. However, in town it will hurt because you have to use more pedal pressure to get the truck moving now.

DEWFPO

zeven13 · July 27, 2010

Unsprung weight... nuff said

Cmps8n · July 27, 2010

if you are concerned about gas milage ....dont run a MT, or an AT, stick with the "city rubber"

Doug_Scott · July 27, 2010

1" change in circumference won't change anything since the diameters of your wheels/tires are the same. However, if the new tires/wheels are heavier, then you will notice a negative difference directly proportional to the change in weight. Small change, probably not noticeable.

Tread pattern affects mpg numbers as well. Generally, mudders (MT's) get worse mpg's than all-terrains, and AT's get worse then highway tires.

When you go to a larger tire diameter, you are effectively making your rear axle ratio numerically smaller (less revolutions for the same distance covered). If all you do is highway driving this might help with mpg's slightly depending on how much bigger you go. However, in town it will hurt because you have to use more pedal pressure to get the truck moving now.

DEWFPO

If you do not recalibrate your speedo for the new tire size, it will affect your computed mileage. The larger tire will get worse mileage do to the slower speedo reading. In other words, you will travel further than the odometer says. If you want to get better mileage on paper, run a shorter tire.

FargoZ71 · July 27, 2010

Better resale though due to less shown milage

1" change in circumference won't change anything since the diameters of your wheels/tires are the same. However, if the new tires/wheels are heavier, then you will notice a negative difference directly proportional to the change in weight. Small change, probably not noticeable.

Tread pattern affects mpg numbers as well. Generally, mudders (MT's) get worse mpg's than all-terrains, and AT's get worse then highway tires.

When you go to a larger tire diameter, you are effectively making your rear axle ratio numerically smaller (less revolutions for the same distance covered). If all you do is highway driving this might help with mpg's slightly depending on how much bigger you go. However, in town it will hurt because you have to use more pedal pressure to get the truck moving now.

DEWFPO

If you do not recalibrate your speedo for the new tire size, it will affect your computed mileage. The larger tire will get worse mileage do to the slower speedo reading. In other words, you will travel further than the odometer says. If you want to get better mileage on paper, run a shorter tire.

jro909 · July 27, 2010

anybody going to mention it will effect shift points? could result in the trans working harder

Crazy Canuck · July 27, 2010

I am thinking of getting LT235/85R16 VS 245/70r16. Basically the 1st tire is 1" longer in circumferance. What would happen?

The reason I ask is becuase I was reading that guys gas mileage went DOWN with the bigger tires. I would think it would go up. So trying to figure this out.

Also FYI, my speedometer readings about 5-6% low, meaning if it reads 30, I'm actually going 28, if it reads 60, I'm going 57 per radar. So thinking if a tad biger diameter tires it should read dead on. But do not want to loose gas milaege either...

Bigger tires generally cause your mileage to decrease mostly due to weight, but there are other factors such as final drive ratio, etc. that come into play. The 235s that you are looking at are actually 6.99 inches bigger/longer in circumference than the 245s. (683.57 vs 635.63 revs/mile) which will cause you to see about a 7.5% difference in your speedometer rating. The diameter of the tire is 2.23 inches taller. You will most likely take a small hit in milage by going this route. That said, my opinion is if you wanted a vehicle for mileage, you would probably be driving a prius.

Crazy

J-man67 · July 27, 2010

I got better mileage when I put my 35's on.

...b/c when they weren't on, I couldn't drive my truck im bored.....

Paul58 · July 27, 2010

I went from the stock 245/70-17's to 255/75-17's and my speedometer is actually dead on now. I'm also assuming my gas mileage calculations are more accurate because of it as well. I can get 20-22 mpg highway if I stay at 65 mph with the cruise set! More realistic highway mileage is around 19 mpg. I can get 16-17 mpg city very consistantly with normal driving. Unfortunately, I swapped to these tires when the truck was new, so I really have no data on what my mileage was before the swap. I am running street rubber and keep them inflated to about 44 lbs. They are heavier than the OEM tires, so I'm sure the weight is a factor... I've considered filling them with hydrogen to off set the added weight, but fear of what would happen in a blow-out has kept me as bay...

Made_N_USA · July 28, 2010

anybody going to mention it will effect shift points? could result in the trans working harder

However, if you have tech2 or have dealership update ABS module with new tire size then the TCM or PCM (not sure for gmt900) will be adjusted.

Paul58 · July 28, 2010

anybody going to mention it will effect shift points? could result in the trans working harder

However, if you have tech2 or have dealership update ABS module with new tire size then the TCM or PCM (not sure for gmt900) will be adjusted.

This raises an interesting question... Being as the larger tires I put on my truck corrected my speedometer and it's actually showing the same speed as my GPS, I assume my shift points are now where they should be. So, assuming the PCM or TCM control the speedometer and transmission shift points, are all the trucks still running OEM tires going to have problems because of their shift points being off?

kdixer · July 28, 2010

anybody going to mention it will effect shift points? could result in the trans working harder

However, if you have tech2 or have dealership update ABS module with new tire size then the TCM or PCM (not sure for gmt900) will be adjusted.

This raises an interesting question... Being as the larger tires I put on my truck corrected my speedometer and it's actually showing the same speed as my GPS, I assume my shift points are now where they should be. So, assuming the PCM or TCM control the speedometer and transmission shift points, are all the trucks still running OEM tires going to have problems because of their shift points being off?

The believe the shift points are dictated by throttle position and engine rpm's. I am not sure the TCM looks at vehicle speed except during engine braking on the 6l80e. But I could be wrong.