A 31 to a 33 doesn't seem like much but I dont want a ticket.

 

Use this calculator. It will give you a fairly close estimate of how much your speedo will be off when changing tire size. http://www.miata.net/garage/tirecalc.html

 

Don't forget that incorrect speedometer information will also effect the odometers and the trip computer (estimate miles to empty, MPG readout, etc).

 

The dealership should be able to adjust your ECU for a 33" tire, I believe it's only if you go 34+ higher that the ecu can not be accommadated for unless you use the hypertech.

 

Or, for free, if you can, try this:

1) 31/33 = 0.9393, or, a ~ 6% difference in your speedometer and odometer/mpg gauge.

2) When you want to go a speed, say 65 mph....just drive ~ 5% slower on the speedometer (The bigger tires say you are going slower than you reeally are...)

3) That means, to be in ball park ticket avoidance mode, if you want to go 65 mph, drive at an indicated 62 mph....if you want to go 55 mph, drive at an indicated 52 mph, etc.

As you can see, ticket wise, you're NOT in crazy over the limit territory, just a bit higher than the speedo says you are.

An easy way to do 5% in your head is to look at the number on the speedo, and move the decimal point over one spot, so 65 mph = 6.5 mph, then take half of that = about 3 mph...

then go ~ 3 mph slower than the desired speed....IE: You want to go 65. take away 3, drive at ~ 62 on the speedometer, etc.



As its really more like 6%, you can fudge it down a little, but at that point, you are putting diapers on flies.



Of course, your mpg gauge will still read ~ 6% low, and your odometer will also read ~ 6% low...so if looking for an exit to come up after so many miles, it will come up about 6% sooner than the odometer says, etc.



And, of course, it you sell it, there will be fewer miles on the odometer as well.

In short, the easy answer is to get a recalibrator, and make it read right, so you don't have to do the math, or the time, etc.

 

They can only go up to a 32. However, you'll be pretty close if you get them to program that and you are running 33s.

 

Knew it was somewhere around there, 1" would be negligible unless you were going like 120 mph. My dealership did it for free.

 

How much do you think 35s change the speedometer? I realize that the speedometer can't be re-calibrated.

 

The speedometer CAN be recalibrated, as mentioned a couple of times, just not by the dealer for over 32's....You'd use one of the aftermarket recalibrators for larger than 32's...



To get the difference for 35's, just do the SAME THING you did for the 33's above.

Use 31/35 instead of 31/33.



31/35 = 0.8857, or, a difference of ~ 11.4 %

The change to the speedometer would be 11.4%...so 65 mph INDICATED on the speedo would mean you were REALLY going ~ 72.4 mph, and to DRIVE AT ~ 65 mph, you would drive at ~ AN INDICATED 58 mph, etc.

 

My Jeep came with 32s new and I upgraded to 33s. When I checked with my GPS this now brings me to the correct speed on my speedo. Check with a GPS to confirm.