thatguy

Here is a tire size calculator that I found online..<br><br>enter your old and new tire size and it gives you the speedo difference..<br><br>http://www.miata.net/garage/tirecalc.html<br><br><br>Hmmm... my truck has the std tire listed on the door jam as the 245/75-16 but it came from the factor with the 265/75-16 on it.. do you think the speedo is corrected for the larger tire size? when speedo says 60 actually doing 62.3<br><br>any thoughts

SmokinShaker

Sounds like mine. I'm only about 4mph off and I'm running 35's

BIG&BAD

my brother lifted his truck and got 35's droped his grears down to 4:56 and his speedo is all messed up <br>its like 20 some mile over <br>he'll be going about 20 and his speedo says 40 <br>he thought his truck was super fast when he first got it back<br>till people started passing him but thats a furd driver for u<br>dont get me started on furd driver i'll go on for weeks

bdog

I am putting 35's on my truck. It is my first dodge. On my ford you could ground a wire under the dash and then recalibrate your speedo. What is involved in calibrating the speedo on the dodge? Mine is a 2001 diesel 5 spd

doug

Here's the man behind the curtain for tire size calculators: <br><br>spedometers, odometers, rear end gears, tranny gears, and tire diameter are all related by simple percentages. examples:<br><br>We know that if your rear end gear changes from 3.54 to 4.10, then you take the difference (.56) and divide by 3.54 and get 15.8 percent. so your spedo will be 15.8 percent off<br><br>same for tires: if you change from a 30&quot; tire to a 31&quot; tire then take the difference (1) and divide by 30. thats about 3 percent. so your speedo will be 3 percent off. <br><br>You can easily figure out tire diameter by understanding a few simple things about the p-metric tire sizing:<br><br>265/75 R16 means that the section width of the tire is 265mm from side to side, and the section height above the rim is 75 percent of the section width. And of course the rim diameter is 16 inches. <br><br>armed with that information, and realizing that there are 25.4 mm in every inch, check out how easy it is to calculate tire diameter and predict the difference between 245/75-16 and 265/75-16: <br><br>245mm converted to inches is 245/25.4 = 9.65&quot;. ok, so this tire is just over 9 1/2 inches wide. 75% of 9.65 is 7.23. ok now we know that the rubber height above the rim is 7.23 inches. We know that the rim is 16&quot; in diameter, and that there are two sections of rubber (2 x 7.23) to account for, so 16+7.23+7.23 = 30.47. So this tire is 30 1/2 inches tall (theoretically, and without a load on it). Here's a visualization of the total tire diameter:<br><br>[ ] section height of 7.23 inches<br> []<br> [] rim diameter of 16.0 inches<br> [] <br>[ ] section height of 7.23 inches<br><br>Well, 265/75 is a 31.6&quot; diameter tire (theoretically). So the difference between 245/75 and 265/75 is approximately 1 inch, divided by 30.5 which is a touch over 3%. so the spedo will be off by 3%<br><br>But two things to keep in mind. First, if the truck came from the factory with 265/75 tires on it (mine did) then the spedo is probably calibrated to that tire size already (mine is). Second, even if the spedo is not calibrated to the larger size, you'll get 3% more miles to go before your warranty runs out ;D. <br><br>Seriously, the consequences of a 3% error in this case are in your favor, and the manufacturer may have built in some error in their favor anyway. I personally would not loose a wink of sleep over 3% or even more. Just be aware of it and measure your error directly at one of those spedometer check places on the freeway. For example, suppose over a 5 mile stretch you notice that your odo reads between one and two tenths off. well, thats the same as three tenths out of 10 miles or 3 miles out of 100 -- bingo, there's your 3% error.

Haulin_in_Dixie

Mine came with 245/75 16 and I run 235/85 16. The speedo is off about 2 to 3% or 2 mph at 70. Not from the math but from the actual speed compared to the speedo. 68 mph indicated gives me just about 70. Interesting at 155000 on the odometer the actual miles are right at 159650.

doug

there are a couple of things at work. The math predicts that your new tires are 4% taller, which would translate to a 4% error in the spedo. But this is only an approximation -- tire sizes are like SAE oil viscosity ratings -- neither one are direct measurements, they are designations that contain some variability in actual measurements.<br><br>I'm asssuming you've had a gps in your vehicle since zero miles? the difference between your odo and actual miles is right at 3% and so is your spedo error-- but both spedo and odo both work from the same raw data, so they will always agree.<br><br>Anyway, differences in actual riding (loaded) tire diameters will easily account for your observations.