well i had 225/75/15 originally on my xj then went o a 255/70/16 that which is 30"x10.5" tires,how much could my speedo be off or be off at all??,i just cant get a ticket since im only 17,i be screwed and im stock hight

 

I hear ya about the ticket thing. I'm 16 so I REALLY can't get a ticket. Right now my speedo is Running 4 mph faster than what GPS and radar tell me. I bet that after I get the rest of my 31s on in auto shop on monday, I'll be about 10 mph off. My excuse for right now will be that I have a non accurate speedometer and I'll be fixing it soon (if a cop is nice enough to think I'm not BSing him I'll be alright). Anyways since you probably don't want to hear about me or my XJ's problems lmao, I'll tell you that 30s will definitely mess with your speedo. You'll probably be reading 5-6 mph over what you're actually doing which is a good thing. You can pretend you're doing 5 over the speed limit when you're actually not breaking the limit lol. Also if you'll be running 30s at stock height prepare to get a sawzall and some paint because you're in for a nightmare of trimming lol

 

i expected i was 5mph or so off and as for now my 30's hasn't rubbed once yet,but thanx

 

Wow you're making me wish I got 33s with my 3 inch lift if you can clear 30s at stock height.

If you have a gps or a phone with one, just get the speedometer reading pulled up on the screen and base your speed off that. It's even more accurate than radar.

 

i would put 33's on it lol but i reckon ima just borrow my moms gps for a lil bit and find out

 

Try this site http://www.miata.net/garage/tirecalc.html

 

it said theres a 6.2% difference, is that meanin 6mph off?

 

It would be 6.2 mph off if your speedometer read up to 100mph. Since our XJs read up to 85mph it'd be 5.27mph off.

 

ok,well thanx yall i appreciate it

 

No, when your speedo reads 60 your going 63.7 mph, if your speedo says 30 mph your going 31.8 mph

 

% is based off 100

 

ahh i hate thinkin about math lol so would i be safe by just stayin with what my speedo says then?

 

yes

 

thanx

 

Ditto this. The absolute mph your speedo is off depends on how fast you're going (think about it this way - it doesn't read 5mph when you're at a stop).

The math:

The speedo determines its information from the number of revolutions your tires make in a given period of time (actually, it's how many revolutions the rear DS makes in a certain amount of time, but it translates to the wheels/tires). This means we've got to consider the circumference of the old and new tires... Stock XJ tires are 28.3", and you've installed 30.1".

C = pi * diameter
C28.3 = 88.907"
C30.1 = 94.562"

Our reference is the stock tire size, so...

94.562 / 88.907 = 1.0636

...in words, the 30" tires have 106.36% of the circumference of the 28" tires. (You get the same number from 30.1 / 28.3 since the formula for circumference is linear, but the critical value is circumference, not diameter.) Consequently, this means that you're actually going 6.36% faster than what your speedo tells you... (Following values are rounded to nearest mph.)

0 mph = 0 * 1.0636 = 0 mph
10 mph = 10 * 1.0636 = 11 mph
20 mph = 20 * 1.0636 = 21 mph
30 mph = 32 mph
40 mph = 43 mph
50 mph = 53 mph
60 mph = 64 mph
70 mph = 74 mph
80 mph = 85 mph

I'd say as long as your speedo doesn't read over the posted limit, you won't get pulled over - cops usually don't care about 5 over.

If you want it to be correct, there are a couple options... The simplest is to get a converter that will adjust the PWM signal from your speedo gear based on the correction ratio you give it (determined by your change in tire size). These can be a little pricey though, and there'll be a bit of a lag between when you attain a given speed and when your speedo reads correctly (couple seconds at the most, but still).

The other option is to get a new speedo gear with the right number of teeth for your tire size. This involves a little dinking around the transfer case, but it's not too hard, and the gears aren't real expensive (as long as you don't get them from a dealer). For your tires, I believe you'd need a 32 tooth gear (as opposed to the stock 33 tooth gear), and you shouldn't need to modify the gear housing any. You will want to get a new o-ring for the housing, as well.