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Calibrate with Bigger Tires? Really Necessary?

14K views 15 replies 12 participants last post by  txtumbleweed 
#1 ·
I had my Jeep in the dealership the other day and I happened to get talking about gas mileage and such and stated that I'm really getting better than it reports because it thinks my tires are stock and in reality they're 35's. Thus, a stock tire rotation say travels 2 feet while my 35's travel three feet, so I'm going father/faster than the odometer thinks I am.

I have a 6 speed stick manual transmission

They mentioned to me that I should have it calibrated, they can do it for 70 bucks otherwise overtime it can ruin certain components, they mentioned "hubs" and stuff. But to be honest, I seriously doubt this is true.

I know you guys probably know the "shizzle" on this so I eagerly await the experts chiming in ... thanking you in advance ...

Scyth
 
#2 ·
I'd do it.

Not only your odometer reading will be off (albeit, to your advantage), but you will be driving faster than your speedo shows, which can get you a ticket. It also throws off your MPG calculations, etc.

Instead of paying the dealer $70, spend twice that amount only ONCE and get an AEV ProCal: it will not only allow you to calibrate for tire size, but it will also adjust the shift points in the automatic transmission, lets you activate DRLs, etc.
 
#5 ·
If you let the dealer do it, they won't be able to invalidate any warranty issues you may have left. Otherwise, if they see you have written to the ECU, regardless of why, you may end up with an argument on your hands.

Personally, I'll do it myself in about 8000 miles but going to leave it alone until then.
 
#3 ·
I'm no expert, and haven't calibrated mine yet either, but the only real pitfall would be ODO and speedo accuracy. I can't imagine how wear and tear would be relative to your speed/tire size...

Pick up an AEV procal and DIY; forget the dealer. If you find that the Procal doesn't suit your needs, you could likely sell it for close to retail price.
 
#7 ·
Not having it accurate won't "hurt" anything on a manual...that was just stealership talk. The 35's would be the only thing that would hurt the hubs.

But being that you alread have 35's, I can't see why one would not update the speedo and odometer.
 
#10 ·
With a stick, this is nonsense. The Jeep just "thinks" it's going a bit slower than it really is. Once you see how far off the speedo is, compared to a GPS, you'll find it's about 5% or so and can add that to your mileage, say 3 MPH at 60, for example.

There is no safety issue. What accident are you referring too???????

As far as mileage is concerned, your Jeep is actually traveling farther per tire revolution than it thinks it is. You've traveled farther than it realizes, so more work has been done. That means the mileage readout will be lower than if the stock tires were on it. But you can add back into the distance traveled, that extra 5% of distance I mentioned, to correct for the difference. Then you'll find your mileage is actually lower than expected because the tires are larger and have more rolling and wind resistance. The net mileage will be lower, compared to stock tires, even though the engine is running slower. In fact, the mileage may be quite a bit lower if the engine is dropped below it's efficient operating range. For the 3.6 it should be at about 2100 RPM.
 
#9 ·
It will be impossible to know how many miles you've driven. That could be a factor in knowing when to change oil or other required service. I don't see how it could cause premature wear. It is just that all components have more miles on them than the odometer shows.

My Sport, bought used, left the factory with stock 29" wheels and tires. Prior to me buying it someone had put factory takeoff 32" wheels and tires on it and I was off by about 12-15%. It isn't rocket science to calculate actual speed or miles driven, but was worth the cost to me to pay the dealer.

I requested a build sheet from Jeep and was informed at the time that mine had not been in for a recall. When I took it to the dealer for the recall I asked about recalibration. Since they already had it in the shop, and the recall involved re-programing the computer, they only charged me for 1/2 hour labor, about $40 I think.
 
#11 ·
Yes even a standard is affected. Tire size change affects tire rotational speed differences which in turn affects the ABS, Traction control, etc. This can cause erratic ABS activation most noticeable on higher speed tight cornering such as clover leaf on off ramps.
 
#14 ·
Then you simply have never driven a jeep with a big enough tire difference to prove otherwise. The effect is incremental, the bigger the tire the bigger the effect.

We have 5 Jeeps in the family, believe me it makes a difference when you are switching to 35s or bigger.
 
#15 ·
A slight (5% or so) difference in tire diameter will have no affect on this calculation because the computer has no way of knowing if the tires have been changed, or the actual speed over the ground.
If it were 5%, you might have a point. The difference between a brand new set of tires and a set with 40,000 miles is about 5%. But when you go up to a set of 35's from a stock set of tires the difference in circumference is almost 20%
 
#16 ·
Handy Reference: Tire Size Calculator

Circumference = Pi × diameter or 2 × Pi × radius. The circumference of a 35" tire is ~3.14x35=109.9". The circumference of a 32" (stock JK) is ~3.14X32=100.5. The circumference of a 35" is approximately 9.5% larger than 32".

Without accounting for tire size corrections, the actual vehicle speed for a 35in tire at 60mph indicated speed is actually ~5.6 mph faster. So, my recommendation is to have the tire size corrected in the computer or slow down to account for the difference in registered vs actual speed. Don't know what impact larger tires have on non-corrected speedos in regards to stability control systems, ABS, etc. But (IMHO) it is reasonable to assume there is. The question then becomes whether systems such as PROCAL simply fix the speedo readout or do more.

Regarding the comment about new vs old tire circumference. The typical tread depth on a KM is ~18/32". Assume a loss of 16/32 (1/2") = a "worn out" tire. Running the calculator you would find that the difference between a new and worn tire = ~1.5% difference in circumference and ~1mph difference. Not something I would get to excited about.

So....I'm not trying to throw rocks at anyone. The math is pretty simple. Just trying to help inform the discussion.
 
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