I know the whole recalibration thing has been discussed in regards to going to bigger tires and the fact that apparently the dealerships cannot flash anything larger than a 32" tire.

So my question is, can they flash for smaller size tires... say maybe something that works out to a 27" or 28" tire? Here's where this question comes from. And I may be way off on this, but my head hurts too much to think about it anymore, so I thought I'd throw it out to you guys. If I were to put 315/70/17's on my Jeep (I believe the true diameter is 34.6") and I went from 4.10's to 5.13's... From all that I can calculate, I'm figuring that what ever my speedometer says, I should really be doing about 14-16% slower than that. So if my speedometer says 70mph, I'd really be doing about 60mph. So my though is this... what if I have the dealership reprogram/reflash for a smaller tire... something around 27" or 28" in diameter... that would seem to offset the larger tires with new gears...

Let me know anyones thoughts and if a dealer can reflash for smaller tires???

Part of my though for the importance in this is that when my Jeep says it's got 120K miles on it... it might really only have 100K miles... not to mention my warranty will actually run out much faster than the true actual mileage I have on it.

Thanks guys.

 

Your thinking has got you going backwards. One other thing, changing gears in the JK does not affect the speedometer. Changing tires will cause it to read different. I went from stock Rubi 32's to 35's. My speedo is 9.3-9.4% off now. It reads slow. For example, when my speedo says 60-61 MPH, my actual speed (as clocked on my GPS) will be 65-66. If you had the dealer flash for smaller tires when you actually had bigger ones, the speedo would be even more off than before.
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Ditto

I had this argument with a friend recently and won

 

YES it does
tires = a overall change in ratio from the transfer case
gears = a overall change in ratio from the transfer case


I will break it down simple: we will use some generic measurements and numbers that do not necessarily have anything to do with available tires/gears.

gear on transfer case causes speedo cable to spin 100 times per 1 revolution of driveshaft.
4 to 1 gears in axle so tires go 1 rotation to every 4 driveshaft
every 1 rotation of tires = 400 rotations of speedo cable

tire is 50 inches in circumference so tire spins 1,267.2 times per mile
speedo goes 506,880 rotations per mile.

new 5 to 1 gears, now speedo goes 633,600rotations per mile so the speedo spins FASTER with LOWER gears.

ok last one:
new tires, old 4 to 1 gears, tires are 60 inches in circumference
tires take 1,056 rotations per mile, speedo goes 422,400 rotations per mile so with bigger tires the speedo will read slower

 

he's right....

 

I have had this argument before

 

I never did like math.

 

Ok, so here are my two calcs... I was only slightly jacked up in my first post.

1) If the gears don't affect it and only the tires do, the speedo will register below the speed I'm actually doing and the difference is around 8% maybe a little over. So if I'm actually doing 60, the speedo should register about 55.

2) If the gears and tires affect it, one will compensate for the other... in fact the gears will over compensate for the tire increase. The difference will be 3.9%. So my speed o will now register over what I'm really doing. So if I'm going 55, the speedo will register about 57. Not a big difference

So if #2 is correct... it should mean that I should be able to have the dealership recalibrate my computer for tires that are 3.9% smaller than 32's (~30.5" or so).

If #1 is correct, then one of the above postings is correct and I'm stuck w/it as is. Mileage wise though it benefits you because you'd actually have higher mileage than your odometer would state and therefore your warranty would last longer (sort of).

So is do both affect your speedo (gears and tires)... or is it just tires???

I'd like to think our friendly service doc (USMCDOC) is correct, along with Piginajeep... But, RedRock actually has this setup in his jeep (it appears anyway), so that is pretty convincing evidence!

 

I'm with you... it gave me a headache earlier, but once I had a beer, it made much more sense.

 

Anyone that wants to stop by my shop with their JK I will correct their speedo for free. I can go into the TIPM module with my laptop and set the speedo to read perfectly.
Dan
Burnsville Offroad