Speedo gears. What did I do wrong?

[subtle71]

I thought I followed all the right steps to make sure my speedometer would read the correct speed. Somehow I am reading about 10 mph slower at highway speeds. Any help would be appreciated. Here is my info:

truck lowered 4" front, 5" rear
Tire size: 275/40zr 18
Tranny: 700r4 old style
Rear end: 4.11 stock

[70GMCer]

Try this out......

http://700r4.com/speedoCalc/speedocalc.shtml

[subtle71]

That is the calculator that I used. It was recommended by the guy that built my tranny as well as several other board members. This calculator states to us a 15 tooth (gray) Drive gear and a 45 tooth (light green) Driven gear. That is what I used and am still coming up with my speedometer showing 65 mph when I am actually going 75 mph. I verified this with my with pacing me on the interstate in her 2005 Altima that is still running the stock wheel/tire package.

Any other suggestions?

Dean

[greenhorn male]

What I used to verify my speed was a basic handheld GPS I got from Walmart.
I know my company truck, which is all stock is off by 3 mph when compared to road speed monitors. When the monitor says I'm going 32 mph, the speedometer says 35, FWIW.

[pjmoreland]

Are you absolutely certain you have 4.11 gears?

[70GMCer]

maybe 3.73's

[subtle71]

If I switched to the 42 tooth, which the 3.73 calls for, would that little bit make the difference? I hate to keep buying parts and "throwing darts" at it! Speakin' of parts, I've got a few other mistakes, I mean new parts for sale:

http://67-72chevytrucks.com/vboard/s...=1#post1644296

[rodnok1]

Usually you can go to a tranny/speedo shop and show them the old parts(including holder) and they can tell you what you need based upon speed/distance reading. And yes a couple teeth can make a big difference..

[Smitty]

Check with BowTie Overdrive. I think 700r4.com!

[pjmoreland]

If your speedometer says you're going 65 when you're actually going 75, that's pretty far off.

65/75 = .86666666 (Your speedometer is telling you you are going 86.6% of what you are actually going)


Your speedometer gear ratio is 15:45 right now.

15/45 = .33333333 (for every three revolutions of your driveshaft, your speedometer cable spins one time)

.33333333/.86666666 = .3846 (this is the speedometer gear ratio you need)

15/39 = .3846
17/44 = .3863 (this would allow you to use your existing 40-45 housing)

I'm thinking you might have a 3.42 rear end. That 700R4.com calculator says 17/43 speedo gears for a 3.42 rear end with your tires and 17/43 = .3953, which is pretty close to the calculation based on 75MPH/65MPH.

My recommendation is to try a 17-tooth (red) drive gear and a 43-tooth (purple) driven gear.

[pjmoreland]

Before you buy any gears, I'd recommend taking your truck back out on the highway for another speed test. Drive at a constant speed, like with your speedometer pointing steadily at 75MPH, and use a stopwatch to time how long it takes you to go 5 miles according to mileage markers on the side of the highway. Assuming you do a good job of keeping your speed steady, and assuming the markers are spaced accurately, you will be able to calculate your actual speed:

Speed = 5 miles x 3600 / time elapsed in seconds

[Green Machine]

The method I use is to use the mile markers also, but note the odometer reading when you pass one and then drive 10 miles and see if your odometer put on 10 miles. Do the calculations from there.

[pjmoreland]

Originally Posted by Green Machine

The method I use is to use the mile markers also, but note the odometer reading when you pass one and then drive 10 miles and see if your odometer put on 10 miles. Do the calculations from there.

That sounds way, way easier.

[subtle71]

Originally Posted by Green Machine

The method I use is to use the mile markers also, but note the odometer reading when you pass one and then drive 10 miles and see if your odometer put on 10 miles. Do the calculations from there.

So, in theory, if I were to drive what is shown as 60 mph on my speedo...note the odometer when I reach a mile marker. Drive 3 or 5 miles, should take 3 or 5 minutes and the odometer SHOULD read 3 or 5 miles greater than when I started.

Getting back to pjmoreland's idea: Can I swap out to a 42 tooth that I have and this make up the difference that I need? (Since I have a spare one that is new?)

Dean

[Green Machine]

Yes, 60 mph is 1 mile per minute, but I wouldn't worry about the speed part right now. ASSUMING that the speedo is in correct calibration, if you get the odometer to click off exactly or real close to 10 miles for every 10 miles you drive the speed should be right on. Also I would drive more than 3-5 miles, I usually drive about 15 miles with a helper to write down the miles on the odo at every marker and then kind of average the readings. I am not sure that the mile markers are always exactly one mile apart due to various physical things in the way of placing the markers or just human errors. Usually given the tooth combinations it is hard to get it to read exactly spot on. If I have a choice between 2 different driven gears I will get the gear with the smaller tooth count, since that will error to show you are running a little faster than you really are, say it would then read 60 mph but you are really only going 59 mph. (helps that gas milage figures too ).

[BubbaGreen]

by chance is your speedometer bad? If you have worked out the math for your gears and tire size and you are correct, i would suspect your speedometer may be the culprit.

[chuck1960]

Doe's anyone know which drive & driven gear set works for the 3.08 or the 3.73 in a turbo 350, such as colors or number of teeth. Subtle71, when you get yours straightened out would you post your info for future reference. I would appreciate it. I have a 700r4 that I may swap in later. Thanks.

[pjmoreland]

Originally Posted by chuck1960

Doe's anyone know which drive & driven gear set works for the 3.08 or the 3.73 in a turbo 350, such as colors or number of teeth. Subtle71, when you get yours straightened out would you post your info for future reference. I would appreciate it. I have a 700r4 that I may swap in later. Thanks.

What's your tire size?