speedo gear

crazy4ink · Post by **crazy4ink** » Thu Aug 18, 2005 9:02 pm

I have a 1988 BII with a V6 and auto tranny. It has 4.10 gears already and I'm getting ready to put some 31" AT tires under it. Can someone please give me the part number of the speedo gear I need? I looked on TRS but couldn't make heads or tails as far as which gear I need.

Any help would be greatly appreciated.
Rob

Post by **Nobody** » Thu Aug 18, 2005 9:26 pm

Go ahead and put your tires on first. Then get on the freeway and reset your trip odometer at one of the mile markers. Drive for 5 miles(mile markers) and see how far you are off.

You'll find that for every mile you go, your odometer will read a little slow. ie:

Miles ------------Odometer reading
1-----------------0.9
2-----------------1.8
3-----------------2.7
4-----------------3.6
5-----------------4.5
From the above, you can figure the following...
10----------------9.0
20----------------18
30----------------27
40----------------36
50----------------45
60----------------54

You can read the above chart as MPH. So when your speedometer reads 54mph, you are actually going 60mph. (remember this is an example)

In order to speed up your speedometer, you need less teeth on your speedometer gear. Pull your current gear and count the teeth. In the above example, one less tooth would do the trick. I believe each tooth is good for about 10mph at 65mph.

This is really the best way to adjust your speedometer. All those formulas and calculators you see are assuming tires are the size they claim This is rarely true. Different brand tires of the same size vary in actual size quite a bit.

crazy4ink · Post by **crazy4ink** » Thu Aug 18, 2005 10:42 pm

Ok cool, that helps clear it up quite a bit, thank!

crazy4ink · Post by **crazy4ink** » Sat Aug 20, 2005 4:54 pm

Well, I threw my 2" bodylift in and the 31s on. Here is what speeds I was getting according to my GPS.

Speedo Reading GPS speed
15 15
20 24.5
30 36
55 66

So using the little table you posted above, I looks like I need to reduce my tooth count by 1. Does this pass the logic check with you??

Post by **Nobody** » Sat Aug 20, 2005 6:05 pm

Take your GPS and throw it out the window. Do it the way I described using mile markers.

crazy4ink · Post by **crazy4ink** » Sat Aug 20, 2005 6:34 pm

Ok, I'll go for another test drive and make notes of what the speedo says.

Post by **Nobody** » Sat Aug 20, 2005 7:58 pm

Your GPS is probably accurate enough, but if you're trying to make the choice between 1 tooth or 2, you don't really have any hard data to work with.

DBrown · Post by **DBrown** » Sun Aug 21, 2005 5:58 am

I would probably use the data from both and then see how accurate they are. the gps should be accurate with in 30 ft. but how it determains the speed of movement probably varries between units. I am not sure how accurate the mile markers are but if you use both you will deffinatly have a better idea of what is realy going on.

Post by **Nobody** » Sun Aug 21, 2005 6:23 am

Out here we actually have speedometer check sections on the freeway that run for 5 miles. I've found the standard mile markers to be accurate as well. By driving for 5 miles or more, it makes any inconsistencies less significant.