I did some searching and found numerous calculators etc. for helping with a speedo correction with different tires and gears. Nothing seems to be what I need though.

Can anybody help me figure out a calculation for speed knowing the following info:

Stock tire size 28"
New tires size 33"
Stock gears 4.30
New gears 5.29
NO Tach

Before anyone says anything the 5.29 gears were for 35's which I replaced yesterday with 33's. I don't have $$$ to go to 4.88's and don't want the 4.30's back so the 5.29's will stay for now.

I like to try to stay close to the speed limit so as not to give the police anymore reason to want to pull me over.

Thanks for any help.

 

http://www.discounttire.com/dtcs/infoTireMath.dos

 

my brain is swimming, but I think ~67 indicated mph = 65 actual mph

 

bigger tires lower speed then speedo shows

 

Well, based on the numbers given your speedo should be about 4% high, as in when your speedo says 68 you're going about 65.

Here's the kicker though: do you know if your speedo was accurate with stock tires and gears? My '92 4Runner theroretically came with 225's as stock tires, but my speedo always read too high. Turns out when I put 31's on the speedo was dead on (it has 4.30's).

So, my 0.02 is to find a friend with a GPS and do a measurment. That or a speed trap

 

bigger tires mean you're going faster than the spedo shows, however the regear drops it back down.

without the regear 55 indicated = 65 actual, and the gears drop it down to about 65 indicated = 67.6 (I think thats the number I got) actual

 

Here's what I did, it seems like it's correct- but it's important to note it might not be correct because your speedo may have come calibrated for 31" tires since your truck has 4.30's...

28" Tires / 33" Tires = 17.8% Increase in diameter, speedo reads 17.8% slow (65 indicated is 76.6 actual, Indicated / Actual = 0.848).

5.29 gears / 4.30 gears = 23% increase in gearing, speedo reads 23% fast (65 indicated is 52.8 actual, Indicated / Actual = 1.23).

So, to combine them:

(Gears Ratio) * (Tires Ratio) = Final driveline ratio change

1.23 * 0.848 = 1.043

Ratio's Use:

Indicated / Actual = 1.043

Actual = Indicated / 1.043

But, if your speedo is calibrated for 31" tires

Indicated / Actual = 1.155

As can be seen, it makes a big difference. If you want to be sure, like sure enough to drive by a cop with his radar gun, I say GPS it.

 

See:
http://www.4crawler.com/4x4/4LoCalc.shtml

Borrow a GPS or try a speedo check station on a freeway or run ~5-10 miles with good mile markers and time the transit for an actual reading.

 

Thanks for the help all.

At least I now have some ball park figures to get me close. I definitely like using the ratios to figure it out.

I honestly don't know how accurate the speedo was. The tag says the truck had the 28" tires but when I bought it the previous owner had closer to 29's. I do remember it being off slightly when I had 31's (speedcheck thingy) but that was a while ago.

Maybe Santa will bring a GPS for christmas and I'll never have to worry about it again.

Thanks again