Given that the standard size tires (i.e. 31", 32" and so on) actually run smaller (30.7, etc), and tires do wear with time, wouldn't it be more accurate to buy a 33 tooth speedometer gear with 3.55's and 31's than the recommended 32 tooth gear? Any input would help, but I'm a stickler for accuracy, and the next step is finding a long stretch of flat road and doing a comparison with both and GPS as the judge.

WOW! How scientific can u get. Can't wait for your post back with results, sounds cool.....git 'er done......hurry up.....still waiting.......are we there yet. LOL

I'm hoping someone has sorted this out already, so I don't have to shell out a bunch of money to buy two new gears and three new o-rings (I tend to replace o-rings after each use. A little wasteful, but better safe than sorry when it comes to a dry t-case). If anyone has any 32 or 33 tooth long shafts laying around, I would accept donors for the experiment. I would give them back when I'm done and buy whichever was more accurate, of course.

THATS AN AWSOME QUESTION!! get tired of repeats but that will happen on a new member.

Wish I did, I'd sure help, sorry.

Did a search, but couldn't find anything on that specific part of it. Plenty of "what tooth gear to run with 31's", but nothing that clarified on which is more accurate when accounting for actual size and tire wear. Would you mind posting a link, so I can see what the results were and what methods they used to draw their conclusion?

here you go sir.

http://www.go.jeep-xj.info/HowtoSpeedoGears.htm

Great advice, I wouldn't be so quick to call someone a new member if I had only 92 posts and a join date of Aug 2010.

I appreciate it, but I've seen that chart already. It says 30.7 to make it easy for metric size conversion, from what I understand of it. However, it's supposed to be for 31" though. With 31's that have a few thousand miles on them, aren't they really closer to 30's? Also, don't manufacturers err on the safe side with these, which is why an indicated 65 MPH is usually more like 62-63 MPH? It probably seems a bit **** (and not the good kind), but if I am going to do something, I generally want to get it as close as humanly possible while I'm there.

I ran into this problem when installing 40x1350r 17s on my Blazer. I was going to have to build a speedometer gear box because of the different transmission and transfer cases that I was using when I remembered I had an old GPS a Garmen V ( spelling ) that I used on my motorcycle. I set it to show speed and direction of travel, after that when I changed to 39 inch red dot crawlers I always knew how fast I was going. From .2 mph to 80 mph it was always right on the money.

wasnt being mean. i really meant it was a good question. im tired of the how big of tires can i run with a 3in lift question. & i meant a new member with like 3 posts. guess i shoulda explained my post a little further so no-one gets defensive.

well the ones that are in parentheses like that are actual tire sizes of BFG AT's as he says in the description above the chart. the specs on this page agree with that.

I say try the one that he recommends on that chart and test it with the GPS. You can only really get so accurate, as there are tires that arent exactly 30.7 inches in diameter, like the difference between MTs and ATs.

Not sure if this makes it better or more frustrating, but both my '89 and my wife's '01s door stickers say that the car is supposed t have 215s on them. They both wear 235s at present. I have checked mine against the radar detectors that display your speed in big lit up numbers that they park before constructions zones or anywhere they need an "extra hand" keeping speed in check. In all the times I have driven by any of these things, they read dead on to what I show on the speedometer. So, I don't know how accurate my speedometer was to begin with if it was supposed to be "right" with 215s when I have run 235s and 30s and still got clocked at what the speedometer was showing.

After all, aside form keeping track of my mileage, being in synch with the law's means of measuring my speed is really the most economical path for me to follow.

Not curious as to how big of tires I can run. I may be new to the forum, but I'm not new to mechanics. My question had absolutely nothing to do with what tire to run with what lift, just if anyone had checked into which would be more accurate for speedometer readings. Considering the tire wear, not completely accurate sizing, and margin of error that is always associated with speedometers, I'm going to run the 33 tooth first. It's recommended to go with a 32 tooth, but I think 33 will be more accurate for a majority of the tire life. Thanks for all the input though.

I agree on the tires being different sizes. I also see that it can only get so accurate. That having been said, I am one of those people that feels that if there I'm going to do something, it will be 100% all the way. No half-assing allowed. If I can get it closer, I will.