It Seems That Im Always Drive Fast Than Traffic When Im Just Doing The Speed Limit Or 5 Over. It Didnt Use To Be Like That. Do I Need To Recal My Speedo. How Much Would That Cost? Should I Be Expecting To Get Worse Or Better Gas Milage. I Would Think It Would Be Better Because It Take Less Revolutions To Stay At Highway Speeds. Ive Got No Lift.

It Seems That Im Always Drive Fast Than Traffic When Im Just Doing The Speed Limit Or 5 Over. It Didnt Use To Be Like That. Do I Need To Recal My Speedo. How Much Would That Cost? Should I Be Expecting To Get Worse Or Better Gas Milage. I Would Think It Would Be Better Because It Take Less Revolutions To Stay At Highway Speeds. Ive Got No Lift.

If you increase the size of your tires substantially, you will most definitely have to get the speedo recalibrated. As for the cost - not sure.

The milage probably will be worse considering the weight increase, and the larger rotational mass. The width of the tire also creates a larger contact area to the pavement which contributes more resistance to motion. Also, the road noise will increase with larger, more aggressive rubber on the truck at highway speeds.

Welcome to the board!

-Boof

ill add to that a bit and say that how the tire works in relation to the pavement/dirt is exactly like another gear, gas mileage will decrease overall, and be more drastic in the city, basically larger diameter tires have the exact same effect on acceleration, top speed, and mpg as gearing up would. you could have your speedo recalibrated, but i would get lower ring and pinion gears for your rear end/differential, that will make your at-wheel torque/hp just liek before, so the lower gears compensate for larger tire rotation bringing you back to factory spec. higher gear=more power lost between the engine and pavement/dirt

the easiest way to find out how much you speedo is off is to use a GPS. I am running 33's and my speedo is off 9 mph @ 60 mph.

thats a good idea, i wouldnt have thought of it, but i dont have gps either.
so 9+ at 60 is about 6.67% off

yep. And the faster you go, the more it is off also.

wow. I passed one of those radar signs leaving base the other day and it was reading 27 when i was showing 25 mph. Im sure its alot worse at 60. Oh well it sounds like i may be able to use this to talk my way out of a ticket knock on wood.

Or, just get a radar detector. Fuck the police.

or since you have the 32" BF A/T KO, when the cop tries to come after you just take the chase off road and get away

I wouldn't try to use the whole my tires are bigger excuse. In saying that they can nail for having a miscalibrated speedo. Which in some cases lead you to be driving a vehicle that's not "street legal". Which is real serious... and come on boofer that's real messed up. Police officers aren't bad. They're out there to protect people like you that seem to have a problem with authority from hurting everyone else that's doing what they're supposed to be.

If you have a scangauge it allows you to account for changes in tire diameter to calculate mph.

dude you bring up a post that is almost 3yrs old and you call out a guy that not even a member here anymore come on now.. lol and just for the record you cant use that as a way tl get out of a ticket.... and unless you know something i dont you cant get the speedo recalibrate yet.. thats why i use the scanguage, with my 33's its 9% off...

lol...so wait, you can't use that as an excuse?!?!? :mocking:

How do you re-calibrate the speed with the scangauge or at least adjust the speed on the scangauge?

I'll have to check the exact details but it let's you enter the difference in tire size from your OEM tires.

It then displays the actual speed on the scangauge.

You can also make the scangauge adjustment while driving using a GPS for reference speed until the scangauge matches the GPS speed.

But it doesn't recal the vehicle speedo. It just allows you to figure out how far off your speedo is.

You can adjust the speedmeter on the scan guage only, no dash speedo. With my BFG 32's I was 5% off.

I used my GPS to find my speed I then calibrated the scan gauge to read the same speed as my GPS. I believe it is spot on sccurate as I have driven past a few of those radar signs and noted the accuracy.

i maybe be wrong on this but doesnt tuning software like superchips or bullydog re-calibrate for a larger tire?

I believe so.

Plus, for free, you could just divide the OEM Tire Size by the new size, and get the percent off you are.

So if the OEM's were 29", and you mounted 33's,

that's 29/33 = 87.9 % or 12.1 %, depending on if you are getting your ground speed vs speedometer, or visa versa.

To make the math easier do do, call it 88% and 12%

So, if the real speed is 65 mph, the speedometer will read ~ 88% of that, or try to convince you that you are going only ~ 57 mph

If the speedo says you're going 65, add 12% to that, so that you know your real speed is closer to 73 mph.

If you want it even easier, round again to 90% and 10%...so all you do is move a decimal point....and just add, or subtract 10%.

That is less accurate, but after a while, your brain just looks at the speedo, and you sort of know what to drive at, etc.

So - with the 10% on the same example, the speedo says 65...so, add 6.5 mph, to get your REAL speed, which would give you ~ 72 mpg....close enough for most situations, etc...just fudge it a bit to adjust for the rounding...and you're good to go.

:D

Regearing the diffs will also correct the speedo, as it takes the signal off the output shaft to the rear diff out of the t-case...so the revs of the shaft are assumed to make a given tire size move you a given distance....regearing changes the number of revs per mile of that shaft, etc.

On the same note, you can get a device that you install to read the out put from that shaft differently....and use it to calibrate for different tire sizes.

The re-programmers are the most elegant solution though.


:wink-big:

a 33" tire is 9% from the stock 29's not only did i do the math that scanguage said in the book but i have also ran a gps while driving and the scanguage is dead on...

a 33" tire is 9% from the stock 29's not only did i do the math that scanguage said in the book but i have also ran a gps while driving and the scanguage is dead on...

Which 33" tire, This one:

Interco TSL 33x12.5/15 at 33.7" diameter

or This one:

Dunlop Radial Rover AT 33x12.5/15 at 32.3" Diameter?

Etc.

:D


They're both 33's...but, there's a 1.4" difference in their diameters.

You can't say there's a PARTICULAR percent change in going from a 29" (Which might be 28-30") to a 33" (Which might be 32 - 34")...becuase there are truck to truck speedo variations, and large variations between nominal sizes and actual sizes.

You have to use your own ACTUAL TIRE SIZES.

The speedometer had no clue what's on the sidewall...it ONLY acts relative to the ACTUAL tire size.

A good example, my truck came with 29's...I have 33's now...according to my GPS, and the endless cavalcade of mile markers going by, when the speedometer says I'm going 60, the gps and the mile markers say I'm really going 70 mph.

An earlier poster (Accordboy) had an example similar to that, with 69 mph actual, to 60 on the speedo, etc.

These are real numbers...if I add 9% to 60 mph, I get only 65.4 mph.

This is perhaps right for YOUR truck, but WRONG for Jeff or mine, etc.

I'd have to add ~ 16% to 60 mph to get 70 mph, not 9%

Accordboy would have to add ~ 15% to get from 60 to 69 mph, etc.

:D

So your truck might have been factory calibrated for a larger tire (Many were) so YOUR difference was 9% dead and true....

....but, others might be different, like mine, etc.

That's WHY the re-programmers are the most elegant solution...you can get it right, and not have to do math in your head.

:D

agreed i forgot about that... thanks..

No, you do not need to recalibrate your speedometer! You can just use it as it is with no problems! Just be aware that you are going a bit faster than it says you are going, and your mileage calculations will be off since your odometer will not be recording the actual distance you have gone!!!!!

You will get worse gas mileage with larger tires because they are heavier and take more power to turn at higher speeds. I have 33" tires and i generally try to keep it at 60mph or lower and i can get decent mileage if i do that!! Just take it easy on the skinny pedal.

wow.

lol, well he was yelling in his title so I yelled back lol