200 Series Common Tire Size Mileage Adjustment Table (1 Viewer)

[TheGrrrrr]

I did a quick but not thorough search and didn't find anything quite like this so I figured I would post it for quick reference. Obviously I didn't capture EVERY size that folks run, but certainly most of them. I'll add more if I realize I missed an obvious one. (I already realize I haven't accounted for the LX crowd... I'll work on adding those sizes. Sorry.)

I guess there are a few ways to read this (using 275/70R18 as an example):

1. Trip Odometer on the truck says 250 miles, I actually travelled 263.29 miles
2. Distance To Empty says I have 250 miles left, which calculates to an actual DTE of 263.29 miles
3. I have travelled 250 odometer miles since last fill-up, and just filled 20 gallons for an MPG of 12.5, but my actual MPG is 13.16

The basic calculation is based on OEM revs of 641/mile, so a 275/70R18 at a circumference of 104.1 turning 641 times per odometer mile is actually traveling 1.0532 miles.

ie. 104.1*641/12/5280=1.0532

To help with calculating actual vehicle mileage, if you switched from OEM size to the 275/70R18 at 50,000 miles and now have 100,000 miles on the odometer, your truck has actually travelled 102,657.9 miles.

I tossed in the other tire specs for convenience, not for calculation. Let me know if I screwed up so I can fix it promptly. Obviously different manufacturers deviate from the sizing in the real world, so literally YMMV.

 

[sdnative]

Nice. May want to add 265/70R18 as well. I don't think too many people run this size, but I do see it occasionally.

You probably don't need the 1-300 columns, just show the adjustment factor to the factory size. Say 5.32% for the 275/70R18 so you would know your speedo and MPG calcs are ~5% low.

 

[TheGrrrrr]

Nice. May want to add 265/70R18 as well. I don't think too many people run this size, but I do see it occasionally.

You probably don't need the 1-300 columns, just show the adjustment factor to the factory size. Say 5.32% for the 275/70R18 so you would know your speedo and MPG calcs are ~5% low.

Yeah, lots of ways to show the data. I just wanted a quick reference, so I added a bunch of mileage columns and the additional tire specs. Will add a few more sizes with the LX data.

 

[Sandroad]

Might be a candidate for the FAQs when the spreadsheet is done? Nice to have the data "pre-calculated".

 

[mcgaskins]

The only issue I see with this method is it assumes all tires for a stated size are actually the same size as each other in real life, but there are often size differences from tire to tire. BFG KO2s run "small" whereas a Falken AT3W often runs "big" in the same size. The way I do the back of the envelope calculation is I get up to a pretty good speed like 75mph registered on the speedo, and then I use a separate GPS speedometer app to compare what my real speed is. If the GPS app shows 78mph when the speedo shows 75mph, I calculate (78/75)-1 which is 4% off. That presumes the speedometer was accurate when the vehicle was on stock tires (not always the case), but for me it's close enough. If you're really bored on a road trip, you can reset your trip meter at a mile marker and then after 10 miles check to see how far off the trip meter is. If the 4% off factor is accurate, you should see 9.6 miles on your trip meter after 10 miles driven, and of course you could use GPS to check instead.

 

[TheGrrrrr]

The only issue I see with this method is it assumes all tires for a stated size are actually the same size as each other in real life, but there are often size differences from tire to tire. BFG KO2s run "small" whereas a Falken AT3W often runs "big" in the same size. The way I do the back of the envelope calculation is I get up to a pretty good speed like 75mph registered on the speedo, and then I use a separate GPS speedometer app to compare what my real speed is. If the GPS app shows 78mph when the speedo shows 75mph, I calculate (78/75)-1 which is 4% off. That presumes the speedometer was accurate when the vehicle was on stock tires (not always the case), but for me it's close enough. If you're really bored on a road trip, you can reset your trip meter at a mile marker and then after 10 miles check to see how far off the trip meter is. If the 4% off factor is accurate, you should see 9.6 miles on your trip meter after 10 miles driven, and of course you could use GPS to check instead.

Yeah I largely agree, but the goal here was general applicability and quick reference. For anyone needing to get more accurate for their specific manufacturer, GPS and the Milemarker Method are both going to be inherently inaccurate because they are both approximations (one electronic and the other eyeball at 75 MPH). Are the tolerances of those approximations greater than the tolerances between manufacturers for a given size? Accuracy of one GPS to another or one persons eyes to another probably accounts for as much variation as the manufacturer deviations. Either way we get to the same place, where we are approximating a more accurate data point than the one being provided by the vehicle. At least that was my thinking. I'm always open to better ideas.

 

[TheGrrrrr]

The most accurate technique would be to take a cloth measuring tape and wrap it around each of your tires and get an average circumference then multiply that by 641 revolutions, then divide by 12 to convert to feet and then divide by 5280 to convert to miles. This would account for manufacturer differences as well as impacts of tread wear.

 

[grinchy]

275/80r18 is exactly 10% slow. 60 speedo = 66 actual. All figures are sandbagged by 10% (distance to empty, odometer, etc). 12mpg = 13.2 mpg