Tire Size vs. Mileage

[Doughboy21kd]

I know on the 06's that DC can only recalibrate the speedo up one or 2 tirre sizes from stock... . So my question is i know the biggest they can go is 285's i am just wondering how far my speedo/odometer will be off if i am running

33 x 12. 5 x 18? I was told also that the faster you travel the more it will be off... . If thats true or not i dont know... . Thats why i am asking you TDR members for help..... And if so how much will it throw it off or whats the easiet way to figure it out?



If it does throw it off does that mean my mileage would be higher when hand calculated or lower since it throws off the odometer too?



Anyway please help... . THANKS

 

[makelemen]

If the tires are off the truck, measure the circumference and compare the ratio between them. When the tires are on the truck, for me, the best way to gauge the accuracy is to use the highway mile markers. Run a 10 mile test more than once and compare the actual highway miles verses the odometer. The Dodge is 1% slow. My 88 Toyota truck with bigger tires is about 10% slow ( In 10 highway miles, I record only 9). So when the speedometer says 50 I am doing 55. 5. (50 X 10/9 =55. 5) 70 is 77. 8 mph. Assuming that the speedometer and the odometer are in sync. My wife got pulled over for speeding and I used the tire story on the cop. Didn't work, ignorance is no excuse. When I gas up at 300 miles, I will add in 33 miles before computing gas milage.

 

[PKitzman]

Both hand calculated and overhead milage will be lower than actual. The most accurate way to check is to use a GPS to compare.

 

[speedracer76]

When I had my Toyo MT 295/70/17 tires on for the winter I used the GPS and calculated the percetage it was of. It ended up to be just a hair over 5% so I just added 5% to the trip calculations for fuel millage.

 

[Doughboy21kd]

Ok so what do i do? Everytime i fuel up i check my mileage both hand calculated and overhead..... So after said and done not adding in the oversized tires and wheels my overhead(god for bid if thats ever right) reads between 15. 5 and 18. 5 depending on how i drive, and for the hand calculation if the the overhead reads 18. 0 hand calculated comes out to be about 14. 5..... So with that being said would it be higher than that or lower than 14. 5????

 

[Vaughn MacKenzie]

Your mileage would be better than 14. 5. Once you figure out what percent your odometer is off you can calculate actual MPG.



So if your odo is 10% slow, then multiply your MPG by 1. 10, if it's 5% slow then multiply by 1. 05. Make sense?



Bottom line, when going bigger than 265 tires your MPG starts to suffer, corrected or not, especially bigger than 285s.



Vaughn

 

[Graphite 6]

You need to figure the rolling diameter. You mark the tire where it contacts the ground and mark the ground at that point. Roll the truck forward until the mark on the tire is at the point of contact with the ground again. Measure this distance to the mark made before on the ground. This is the rolling circumference. Do this with both tire sizes. With these figures, divide the larger diameter tire number into the smaller one. This is the percent of difference.



Example: 285s rolled 85" 265s rolled 78"

78" / 85" = . 92



The 265s roll 92% the distance as the 285s, so if the milage on the odometer reads 100 miles with the 285s, you actually went 108 miles.



If you are doing 60 mph by the speedo, you will really be doing 64. 8 mph



To get your final figures when you calculate your milage or speed, multiply by 108% to give you the correct number.



You should have the same load and tire pressure for both tire sizes when doing your calculations. I would suggest you use the front tire for you measurements since the load variance is minimal compared to the rear





This was given to me by a member named Hoefler? It should help you out. Call me w/ any questions, Brad

 

[JWilmer]

ORRRR forget all the math... .



Drive down the highway for a spell... .comparing your trip meter to the mile markers... . if you drive 50 miles and the trip meter shows 45 (just for explanation)... . you simply ADD 5 for every 50 you drove... .



500 miles driven + 50 for the tires = 550 total miles... than use THAT number to figure out mileage... BJ

 

[lawdog]

Or forget hand calculating the difference and go to the below listed link and use the tire calculator... .



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

 

[Jeremiah]

If would be helpful if Miata.net offered information in half-inch increments. I hadda divide the difference between 19" and 20" by two to come up with a figure for 19. 5" wheels. Oh, well ... :-laf

 

[RMarkey]

Back up a few there...

Whoa, Whoa, Whoa... I had DC adjust for my tire size, and I run 37's. My speedo is dead on! They only give 2 options in their computer, but that isn't what they need to set.



First, go to the tire rack.com or somewhere similar and find the rev per mile that your particular tire has. Go to DC and tell them to adjust the pinion... ... pinion something anyway. They didn't know how to do it @ my dealership either, I think he was a little peeved when I told him the settings to put in. :-{}



Anyway, then they'll tell you that you might lose your cruise control over 65mph or something, again, not true. It might be possible, but I haven't heard of it happening. PM me if you have any other ?'s



Ok, I just realized you have an 06, and mine is an 04. But They told me the exact same thing on my 04, so it might still be the same thing, and they just don't know what they are doing, or they might be right for once.



Either way if you ask about that, they might realize you know more than the average Joe and might deal level with you in the future.

 

[JMalone]

Okay... I'm learning, so bear with me...



RMarkey, what you have said has made my day! I have just got an '06 and looking to in the very near future to put 315/70-17s on... I wasn't really looking forward to speedo being off (I'm obsessed with figuring MPG and I'm not any kind of mathematician). One of my friends told me that I'd have to get a programmer to change it -- but I don't wanna do any of those mods for awhile.



Soooo (hopefully) I can go to the dealership and tell them that the new tire turns at 587 Revs/Mile and they should be able adjust the "pinion"?

 

[Jeremiah]

Obsessed with MPGs

Mr. JMalone, if you're obsessed with mpgs you might want to check out the 19. 5" wheels. Unlike even numbered sizes (17, 18, 20, etc. ), the 19. 5" wheel gets you a commercial tire, which runs tens of thousands of miles farther than over-the-counter auto and truck tires. And regardless of what you might read in this thread, you won't find information on these wheel sizes at tirerack.com or miata.com either.



Commercial tires have H and G ratings and haul something like 4300 lbs per tire. They may even have steel plies in the tread and side walls. 80-120,000 miles per tire are being reported. Yes, there are non-commercial tires with H and G ratings but you won't get 100K from 'em, guaranteed.



Rickson Wheel is the only company I know that sells wheels in commercial sizes. I'm sure there must be others. But check 'em out before you buy even size wheels and tires. I did and I'm glad I did. The initial hit in the wallet is huge ($2700 in my case) but you recoup the cost with the second and subsequent set of tires. Plus, the ride and handling of the wheel and tire combination I bought is far superior to the OEMs.



Re the speedo correction, you are correct: furnish the dealer with the revs per mile (in my case it was 622) and they can adjust accordingly. My dealer didn't know how to do it either but they figured it out. And charged me $60 for the priviledge of learning! I think they do it on a computer.



Good luck!

 

[JMalone]

Thanks for the awesome advice, Jeremiah!



As far as MPG go for me, I just like knowing exactly what I get out of a tank. I drove a 1/2 F-150 before getting this truck, so anything I get will be better than what I had!! LOL



You're right about the wheels and tires... . That would definately be the smarter, more economical thing to do... . but... I wanna big, mean-lookin' truck. (Not alot of chicks run around here in diesels w/ 35's ha ha) Hopefully in a couple years I can doll it up with some serious stuff, so for now I'd like to get by with just a leveling kit and and tires. The good stuff will come later.



Thank you for the awesome advice and I'll need all the luck I can get!!!



Mrs JMalone (or Megan)

 

[Jeremiah]

Big and mean

JMalone, login to Rickson and find Customer's Truck photos or something like that. Look for the light blue truck (if I remember correctly) with 265 wheels and 2" lift kit. Neatest looking truck I've ever seen. Gotta be a chick-grabbin' machine! It may even have a hood scoop, which makes me wonder if it's not the Hemi package. I like the scoop but not enough to make me buy a gasser. Arrg!

 

[Doughboy21kd]

Befor eyou guys get all gun hoe about not being able to change the speedo for bigger TIRES... . They can change them on the 05's and older... Its just the 06's that they cant change cause DC took that option out of the computer on the 06's... .

 

[hensign]

About 5k miles ago I purchased a set of used Hummer wheels/tires off e-bay. Tires had 40-90% left and I got em for $250. Sold the wheels for $90 and put the BFG KO's on my stock wheels. It was just an experiment. My milege on the highway is the same(16. 5) at 70 mph indicated or about 77mph or better(18. 5) at the true 68 I used to drive, the speedo is not corrected. With the speedo error and my weekly 700 mile run I drive I have simply recalibrated in my head and the speedo is off by just under 10%, this also means my milege is off by about 10% so I am actually getting about 20mpg if I go the same speeds I used to. I have minor rub on the front control arms, better ride than before and much better traction than the 265 michelins in the wet. PS the Hummer wheels will bolt on but WILL NOT clear the hub centers. To run those wheels I would have needed to get them machined out. Now for the bad news, before the install my mpg was 13. 5 with trailer in tow now it is 10-10. 5 and the truck is definitly working harder. If I pulled more I would definitly go back to the original sizes.

 

[JMalone]

Thank you very much for the information, hensign. That's exactly what I've been planning on doing. I don't want the wheels, but I've got a friend that does. I'm planning on putting a 2" leveling kit on, so that should keep the control arms from rubbing. Speedo off 10% and MPG off 10%. I can handle that kind of math !!

 

[JMalone]

I picked my truck up from having the cab lights put on at the dealer. I was talking to the guy that put them on and I asked him if he could change the speedo for 315/70/17 tires (which I bought some H2 take-offs at Tulsa today ). He said that he would just need the revs per mile and it won't be a problem. I'll let you guys know how it turns out ... .

 

[hensign]

I would be interested in knowing if anyone has tried this swap on an auto, my late model 6 speed final gearing ends up like a 4. 10's I think that auto's have a more preferable final ratio.....