Blue88Comanche Posted October 16, 2013 Share Posted October 16, 2013 yesterday i discovered my charger has shorter than stock tires and its been bugging me knowing that my mileage is off. using current brand tires as example (General Tire, G-Max AS-03) current tires are 255/35ZR20 with 769 tire rotations per mile Stock sized tires are 245/45ZR20 with 724 tire rotations per mile I am not a math wiz, and have been thinking about this for the past several hours but can't make up my mind on how to solve the problem... again i suck at math lets say i drove indicated 100 miles due to the smaller tires my actual mileage is less than indicated by about 5.9%, putting me at 94.1 actual miles? Link to comment Share on other sites More sharing options...
92tanMJ Posted October 16, 2013 Share Posted October 16, 2013 "255/35ZR20 with 769 tire rotations per mile 245/45ZR20 with 724 tire rotations per mile" Isnt 255 bigger than 245, so wouldn't the larger tire have less revolutions per mile? And the way I looks at it with my MJ is I have 265s on it now, stock is 225..which makes my speedo slower (for example says I'm going 10, actually going 12) so since my speedo is off the odometer will be off as well and my logic tells me it should be less because speed is distance over time. For my MJ, 31s vs stock, (I used multiple checks and don't remember all of them now- but to get my actual mileage..I would take the trip times it by 4 and divide by 3..because for ever 3 miles I went on my trip I had actually gone 4 (more or less). So for me if I drive 100 miles (on my trip) ive actually gone 133. (Granted I know the difference changes more/less by the speed I'm going, but on average it seems to be 3 to 4) Hope that helps some, I kinda am speed typing and rushing threw this post cause I'm going to be late now cause I had to look at "one last post" hahaha Link to comment Share on other sites More sharing options...
87manche1 Posted October 17, 2013 Share Posted October 17, 2013 Use this http://www.tacomaworld.com/forum/tirecalc.php Link to comment Share on other sites More sharing options...
87manche1 Posted October 17, 2013 Share Posted October 17, 2013 Actually will read slower at 20mph its 1.6 mph off At 65 its 3 mph off not a huge deal. He probably wanted to get new tires but the oems size had fewer choices. So he picked a more popular size. I did the same on the Hyundai had to go slightly bigger to get a better choice. Link to comment Share on other sites More sharing options...
Keyav8r Posted October 17, 2013 Share Posted October 17, 2013 The first three numbers in the tire code are the measurement of the tire's width in millimeters. So, a 245 is narrower than a 255. The second two numbers are the aspect ratio, or the ratio of the tire's sidewall height to the tread width. So for a given tread width, a lower aspect ratio number will mean a shorter sidewall and therefore a smaller overall tire diameter. A 255/45 would be taller than a 255/35 and would cover more ground per revolution. The last two numbers are the rim size. The letter generally denotes speed rating. The speed you drive at has no effect on the speedometer error that is induced by the difference in tire diameter/ circumference. The ratio of the error will remain constant whether you are driving 30 mph or 90 mph. The 255/35 tire would have a sidewall height of 89.25 millimeters - 20" rim plus 89.25 mm plus 89.25 mm The 245/45 tire would have a sidewall height of 110.25 millimeters - 20" rim plus 110.25 mm plus 110.25 mm. Therefore it would be the "taller" tire requiring fewer revolutions per mile. Don't have my inch to mm chart handy but you can google one and calculate the overall tire diameter and multiply that pi for the circumference. Link to comment Share on other sites More sharing options...
mvusse Posted October 17, 2013 Share Posted October 17, 2013 am not a math wiz, and have been thinking about this for the past several hours but can't make up my mind on how to solve the problem... again i suck at math Have a dealership reflash the ECM to correct for the tire size difference. Link to comment Share on other sites More sharing options...
Incommando Posted October 17, 2013 Share Posted October 17, 2013 The first three numbers in the tire code are the measurement of the tire's width in millimeters. So, a 245 is narrower than a 255. The second two numbers are the aspect ratio, or the ratio of the tire's sidewall height to the tread width. So for a given tread width, a lower aspect ratio number will mean a shorter sidewall and therefore a smaller overall tire diameter. A 255/45 would be taller than a 255/35 and would cover more ground per revolution. The last two numbers are the rim size. The letter generally denotes speed rating. The speed you drive at has no effect on the speedometer error that is induced by the difference in tire diameter/ circumference. The ratio of the error will remain constant whether you are driving 30 mph or 90 mph. The 255/35 tire would have a sidewall height of 89.25 millimeters - 20" rim plus 89.25 mm plus 89.25 mm The 245/45 tire would have a sidewall height of 110.25 millimeters - 20" rim plus 110.25 mm plus 110.25 mm. Therefore it would be the "taller" tire requiring fewer revolutions per mile. Don't have my inch to mm chart handy but you can google one and calculate the overall tire diameter and multiply that pi for the circumference. Good post. This is so confusing for a lot of people even enthusiasts. I have heard more times then I can count that the first three digits are the height and the second two the width. If I could add an example that would mainly apply to jeeps: *Start with 265/75/r16. *Multiply the tire width by aspect ratio as a percentage : 265 x .75 = 198.5 *You must double this number as this is the height of ONE sidewall and you of course have 2 sidewalls = 397.5 mm. *Convert this to inches ( 1mm= .04 inch +/-) for 15.9". *Now add the rim diameter (16" +15.9") and you get 31.9" for height. *265 mm width = 10.6". *So this tire is roughly a 32x10.5 would probably be sold as a 32x11. Rarely ever do flotation size tires (31x10.5, 33x12.5 etc...) measure as tall or wide as advertised. True off road tires like Super Swampers are an exception. *Doing this same math on a 265/70/r16 to show the difference the aspect ratio makes in height: (265 x.70) x 2 = 371mm height for both sidewalls = 14.8". Add 14.8" to 16" =30.8" height. The width remains the same at 10.6". So the 265/70/r16 is 1" shorter than a 265/75/r16 and is as close as you can get to a 31x10.5 as I can imagine. But remember: A 32" tire only gives you .5" of more ground clearance over a 31" tire because at any given time half of the height increase is above the axle line and does nothing for you. Link to comment Share on other sites More sharing options...
Blue88Comanche Posted October 17, 2013 Author Share Posted October 17, 2013 Still haven't answered my question, do I subtract 5.9% from my reported mileage to get the correct mileage? am not a math wiz, and have been thinking about this for the past several hours but can't make up my mind on how to solve the problem... again i suck at math Have a dealership reflash the ECM to correct for the tire size difference. I have been thinking about that, I would just replace the tires but they look almost new, and I just spent my extra cash on new gears for my MJ. Just need to call the dealership to see what it will cost and how long it will take. I may be able to have that done today after work. Use this http://www.tacomaworld.com/forum/tirecalc.php that's a pretty cool calculator, I went the hard rout to get my numbers... lol Link to comment Share on other sites More sharing options...
92tanMJ Posted October 17, 2013 Share Posted October 17, 2013 Still haven't answered my question, do I subtract 5.9% from my reported mileage to get the correct mileage? If I read keyav8r post right (good post by the way) then yes you should subtract it since the tires are smaller...I believe lol Link to comment Share on other sites More sharing options...
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