Speedometer driven gear question

[smurfvet]

My speedometer is off 10 mph, I am traveling 60 mph with 70 mph indicated. I verified this with a friend's radar gun. The current tires are the correct diameter to replace the stock G70-15 ones, and I verified the diameter of each on the tire calculator.

I found a source for the new gears, but the question is this- how many teeth do I increase the new gear to drop 10 mph indicated speed? A Chevy parts guy told me that "each tooth is 3 mph, more or less.

Anybody verify this is true?:confused

I'd rather do it right the first time....

 

[wicky88s1975]

mines off by @+5mph I was told to drop one tooth. Haven't done it yet due to the fact that it had to ordered. I'm waiting til I decide on the final wheel and tire configuration before I go any further with it. Hope that helps a little.

 

[Ala75Vette]

I've done this very thing on my car in the past 6 months (replace speedo driven gear). My chevy parts guy said basically the same thing -- each tooth is 3-5 mph, more or less. I was reading too low by about 10 mph, so I got a gear with 2 more teeth, and it put me 'right on' (well, by about 2 mph or so). I'd recommend going with 2 less teeth on your driven gear & you should be good to go.

btw, just depending, it may or may not be any big deal to replace. On mine, I have one of those 90 degree adapters between the driven gear [housing] and the speedo cable. My biggest challenge was getting it loose. Getting the driven gear out was no big deal.... there's one little screw holding a retaining arm that keeps it 'in' the transmission. Loosen it, swing the retaining arm 90 degrees out of the way, then pull the driven gear assembly (about the size/shape of a shotgun shell) out of the transmission. Replace driven gear (a plastic thing, which has never ceased to amaze me), put it all back together again, and you're in business.

 

[born2saw]

Ok, if you know, please tell me why so many Vettes seem to have speedos that are off. I have an original tranny & correct size wheels, but I'm off about 10 mph at 70 mph!!

I asked about how to correct the gears & was directed here.

I have not gotten around to changing the gears yet (maybe a spring project when I take her out of storage) so if any of you do it using this formula, let me know how it works!

Hey Ala75Vette, do you know anything about the 1975 roadster registry? I'm at work or I would have their web address to give you. It is an guy up in Canada & I was curious if you had heard anything about him or any other 1975 sites.

 

[beige81]

speedo gears

just a quick note: 20-30 years ago, I switched these gears more than once to get out of speeding tickets. would put a gear that would throw off the speedometer enough to beat the ticket, have a test done at independent place in Charlotte that would put the car in gear w/ the wheels on rollers to show the speed. Of course, I would ask the people doing the test to fix the problem, which of course, meant having the original gear put back in. with the receipts and tech report showing the "problem", I'd go to court and get excused. what a crazy world. Incidentially, AAANY ticket was a big deal to me back then, ...at one time Lloyds of London was the only place I could get insured...$3000/year!! in the '60s !! regards.

 

[Ala75Vette]

born2saw.... haven't heard of the '75 roadster registry. If you've got any info, please post/send it to me.

And after taking a look at Ken's speedo formula info at the link in your post, it looks like he's got the definitive answer for determining the number of teeth needed for speedo gears! <wish I'd seen that when I was working on mine> His answer is more satisfying and complete for my analytical side (ok, I'll admit it, I'm an engineer). Thanks for including that in your post.

 

[smurfvet]

Thanks born2saw- that is what I was looking for.

Thanks Ala75vette for your info.