Problems with speedometer reading

[dab67]

CRG need help again>

I recently changed the speedo gear in my powerglide because my speedometer read 10 mph less than what i was actually traveling.

I have a 3:08 rear end. Everything I have seen states I should have a 21 tooth gear in the tranny for a a 3:08 or 2:73 rear end. So I put the gear in and it still runs 10 mph faster than the speedo reads!!!!!! What's up with that? This is constant thru the speedometer range. When it reads 40 I'm, doing 50, 50 I'm doing 60 etc etc. What gear should I have in the tranny? It is a 350 V8, and the tires are nothing unusual 15". Do I go with more or less teeth? Could there be something amiss with the gauge itself??

dab67

[dutch]

   I'm not up on the speedometer gearing - but as far as I'm aware the only first generation Camaro that had 15" tires was the Z 28 model.
   Again I'm not quite sure how much difference that tire upgrade in diameter would make as translated into actual mph but it obviously would be a constant amount throught the speed range...That would be in addition to any obvious error built into the original gearing for it - which all gear driven speedometer / transmissions have to greater or lesser degree.   --Randy

[sebastien]

Dutch is right the height of the tire will make a difference on your speedo indication.14 to 15 and also depending on what size tire you have.check this link it may help you.

http://www.miata.net/garage/tirecalc.html

[lakeholme]

OK, I have 15 tires on my 68 with powerglide.  Assuming the same gearing (in other words, the only variable is tire size), according to the math, a 14 to 15 switch should only result in an approximate .07 mph increase in the speedometer per mile per hour.  In other words at 10 mph, I'm actually going 10.7, at 20mph/21.4, etc.  If you are experiencing a consistent 10 mph difference as you increase speed, then there has to be another variable.  The tire size alone does not seem to add up to the total problem.

[dab67]

The tires are not standard size. you are correct Dutch, they are 15" on the back and 14" on the front. So which way do i go with the gear? Less teeth to make it rotate faster or more teeth to make it rotate slower. If I read the Calculator the Sebstein showed the link to (Thanks by the way) The speedo is running to slow. Is there a gear with less than 21 teeth?

dab67

67 SS Camaro (Clone)
350, 2sp auto w/console and gauges, Royal Plum w/White Vinyl roof, white stripes, PS,PB (check it out next month on CRG)

[lakeholme]

Dab67,
Was this happening before you got your "passing gear" straightened out?  I'm just curious, because I have yet to get mine up on the rack to adjust it.  As I said above, it's got to be more than the tire size change.

In looking at the miata site that Sebastien mentioned, I also found these two: http://www.4lo.com/calc/geartire.htm
http://www.mysporttrac.net/calulator.htm
These sites also talk about gear ration, etc.

Let us know what you find out.
I guess I ought to be thanking you for finding these problems about two months before I do!

[dab67]

Lake:

Yes it has always been off 10 mph since we bought it.

[lakeholme]

OK, my Chevelle friends say it has to be that speedometer gear.  But no one seems to know which one (# of teeth) it should be.  They all agreed on two things: 1. Call a couple of Chevy parts departments and see if one of them can figure it out. 2. If you don't get lucky with #1, then pull the present gear and use the formula.

http://www.novaresource.org/speedo.htm
http://www.tciauto.com/tech_info/speedo_gears.htm
 

[RJ_RS_SS_350]

I agree with Lake, it probably is something else, maybe a faulty speedo or something.  The wrong gear would cause it to be off by a percentage of actual speed.  So when you are going 10 MPH, the speedo still reads zero?

[dab67]

Lake and RJ

The formula states I should use the gear for a 3:08 (20 tooth) which I have. RJ, I really didn't check to see if it reads zero when it is actually 10. I will check next time I have the car on the road. Knowing that it is off by 10 mph is not a real concern, annoying? to some degree but nothing I can't deal with. If push comes to shove, next year I'll go back to 14" tires all the way around and see how much of a difference it makes.

Thanks for all the input and suggestions CRG!!!!!!!!!!!!!!

Dave

[dutch]

  Just for anyone's information - a few tire size calculator / comparison tools to compare the effects of different sized tires and I'm sure there are better ones available out there!
 
 The third one is kida neat visually - stolen from my son's favorite S10 v8 site that he frequents all the time. It's amazing the difference for example a 265/70/15 to 215/60/14 would make... Doesn't solve anyone's speedo woes - just a fairly good set of tools for comparing the effects of swapping sizes.

http://www.1010tires.com/TireSizeCalculator.asp
http://www.miata.net/garage/tirecalcold.htm
lhttp://www.rgp.nl/bird/misc/wielen.htm

                                                                                                        --Randy