Speedometer Error

I changed my tire size to 165R15 which is a smaller diameter that the Beetle. Consequently my speedo isrunning slow. Is there a gear available to correct this or another fix for both speed and odometer?

It can be corrected by having a instument man adjust the spring inside it .

I think VW's came with 155 tires. If you install 165's you're going a bit bigger. Actual vehicle speed should be around 66.5 when the speedo reads 65MPH Or, when the actual vehicle speed is 65 MPH, the speedo should read somewhere around 63.5 MPH

I assume you're measuring you speed via a handheld GPS???
Most of these speedo's don't read correctly anyway.

Speedometer shops make a simple gear reduction or gear increase device that fits between the speedo head and the cable and will make the speedo read correctly. They have equipment that will determine which gears they need to use by rolling the car across a measured distance

Thanks Barry and Larry
My odometer is off by 0.2 miles/mile driven. I can adjust the spring myself but that won't correct the mileage just the speed. I thought perhaps with so many Speedster kits out there that someone might have made a replacement gear for the front wheel. There are no speedometer shops around my area.

VW used bias ply 5.60x15 tires on 4" rims up thru at least 1971 models ('72-76 went to 6.00x15 on 4.5" rims). The originals were 25.5 inches tall --- 165/80x15 radial tires are 25.4 inches tall. That's not much of a difference so effect on speedo would be negligible. The 165mm was never a good fit in the Bug spare tire well so many used 155mm especially on 4 or 4.5" rims - they measured 24.8" tall. Best popular tire size now seems to be 195/65x15 which measures 25" tall.

Reference -
http://www.miata.net/garage/tirecalcold.html

The SAMBA archives has sales specs for all years

Wolfgang
I will have to measure the dia. of my tires. When I purchased them I was told they were smaller in dia. than the original bias tires. I don't remember if they said how much. I was not thinking about a change in speedo readins as I was looking for something smaller to lower the car. As you know the beetle looked very high off the ground. Thanks for the info.

This takes a moment to load, but it tells everything you need to know about any tire size.
http://www.rims-n-tires.com/rt_specs.jsp

Charles,
There is really no "gear" in the front wheel. Any changes need to be internal to the speedometer. It has to be re-calibrated to change the "count" of rotations per mile...so to speak.

Jeff Parsons
I see what you mean by NO GEAR as I look at my manual. I assumed it went to a gear drive.

I do know how to tune a Vw speedo there is a brass pin inside you can pull out that lets you pull the end of the spring up or down in it.

On them you can adjust it by as much as 4 miles per hr. But they have limits.

I have no idea how the speedster speedo is set up.. And I have no desire to tear one appart.

Barry Hudson
You are right about the pin and the hair spring for adjusting tension. I repair watches and they are adjusted in a similar manner. I can adjust the speedo but am trying to figure out why my odometer is off. It is mechanicaly driven and should only be affected by tire diameter. My tire is 25.4" in diameter and is supposed to be close to the VW bias tires. Thanks for your help.

Wolfgang
When you bought Speedster from CMC did you buy their VDO gauges. If so have you noticed any error like I am experiencing. I measured my tires and they were 25.4" dia. I also called the tire dealer and they confirmed the size as 165/80/15.

Diameter measurement is good, but maybe not so easy get just right. Better you should take your wife's sewing tape measure (flexible cloth style) and wrap it around the circumference. Then you know exactly how much distance is gained by one turn. Divide by pi (either apple or blueberry) and you have the diameter PFC.

PS: the Tire Rack website usually has all sorts of data on tires, like revs/mi, so if you can find your make/model in there, you can do it all from your keyboard. revs/mi is good cause it's a pretty big number, usually an integer, and you can easily calculate (ratio) the slight % differences between tires and hence % diffs in speedo readings you will get w/ one vs. the other. I'm just sayin' . . .

Kelly Frazer
The circumference measure was my next step but it's been so cold and nasty I don't feel like jacking the car up to do it. The car bunks on the carport.Bummer ! Garage is next project ! I have already calculated the RPM(revs/mi)for the diameter. I will try to find the Tire Rack site.
Thanks

I found this useful:

http://74.125.95.132/search?q=cache:sxCgDtM-LXcJ:www.356registry.org/tech/tires.html+vredestein+sprint+classic&cd=48&hl=en&ct=clnk&gl=us&lr=lang_en

My CMC came with the Vintage gauges not quality VDO gauges. After I cut the dash for the larger Vintage gauges I decided not to use the them (argh). Even wear on tires would make a small difference - new they might be 11/32 while 1/2 worn they are 5/32. I just think your expecting old analog guages to be too accurate. VDO does make an electronic speedomenter that uses an adjustable pulse control unit - that could be fine tuned if you want it dead on.

http://www.egauges.com/vdo_send.asp?Sender=VW_GM_Cable

Calculating your tire rev/mile is more complicated that just using the tire size or measuring the circumference, neither of which will give you the right number. The tire compresses when it has weight on it giving a smaller effective diameter and it also expands when it's spinning. This formula should get you within 1.5% of actual value.

OR = Overall Radius, this is measured from the center of hub to top of tire

SLR = Static Load Radius, this is measured from the center of hub to ground while the car is sitting on a level surface.

Deflection = OR - SLR

EF = OR - (0.8 * Deflection) = Effective Radius, this is the "in use" measurement that you need to use.

Rev/mile = 63360 / (PI * EF)

or Rev/mile = 63360 / (PI * (OR - (0.8 * (OR - SLR))))

Pls note two things:

You can roll the tire over the tape measure, then pull the tape over the top and take your measuremrnt w/ out jacking the car. Just a thought.

Second: my bet is that the circumference is the circumference no matter what and unless the tire is slipping wrt the pavement, once around is once around and the distance is pretty fixed. Certainly, the tire will flex (sidewalls) w/ weight on it, so a radius measurement would be subject to some error on account of that. But does the circumference actually srtretch when it rolls? I could not say, but would tend to doubt it. But if so, then not enough to matter?? I suppose one could also roll the car down a straight road and count ten (or some conveneinet number) of tire revs, then measure the distance traveled. That should capture everything going on. Personally, I'd prefer using the web, and looking it up. So let's see, this was all about speedometers, right? How about just putting the top down and feeling wind in your hair -- that seems pretty accurate to me ;-)

See that's why Cory never bother with a speedo in da Hoopty! First he rubbed all hair off his head thinking about tire circumference then he got a headache thinking about speed over distance and MPG (with that 5 gallon fuel cell). Finally he just said "don't need no stinkin' speedo."

For long-time followers of the Hoopty-Saga, some will remember that at one time, I had a full set of gauges from a donor car. The tach couldn't be adapted to my engine, and the spedmeter couldn't be made to work with my left front axle.
Sooooo ... I sold all three to pay for the Autometer gauges I have now.
It was only after running out of gas that fourth time that I added the fuel gauge.
I did get advice from North Hollywood Speedometer about driving a known distance. I think it was 100 feet or so, in order to calibrate the 356 speedo.
Now, thanks to Larry 'the Genius' Jowdy, all I do is double the tach and add a zero. Works pretty accurately, as luck would have it! I had the right combination of R&P and fourth gear.

KELLY FRAZER
I agree with you about just driving and have been doing that for 17 years. It just bugs me that I have put several thousand extra miles on the car and can never run a road rally w/o buying lots of extra instuments($). I am also running out of hair ! How do you put a gauge in a fuel cell ? wont the cell material interfere w/ the float ?
I am still working on all the suggestions. Even if I never get it figured out I am enjoying all the members jousting on the forum !

Yeah remember those old Halda Rallye Instruments (Heuer had them too) way before the advent of GPS (even for the military!) I remember running few miles of Rally in reverse because I had made a wrong turn. I know it didn't turn the odo back but at least it didn't go forward.

Wolfgang
I ran rallys years ago and did a lot of backing up as it did back off the miles on the old cars. Only problem was you had to go like hell to make up the time. Scared myself sometimes. It did work though as we came first and second often. It was also the hard way with stop watch and tables.

Charles, I'll take a stab at the fuel gauge question for you -- but there are honest-to-goodness mechanics here who are much smarter about these things than I am, so double check my assumptions. (I know a lot about one car; if you're doing what I did, this may help.)
First, what kind of fuel cell or tank are you using?
Generally, putting a gauge and sending unit in is as simple as it sounds. There needs to be power put through a fuse, a wire to the sending unit's positive side, and then a wire to the gauge.
The amount of fuel will provide buoyancy to the float. The float will dictate the amount of current which passes to the gauge, and the needle will indicate the amount of current over a measured series of ticks on the face of the gauge.
There's not much more to it, unless the gauge needs to go to ground afterward to expend the leftover current.
The trick is to make sure the gauge corresponds directly with the calibration of the float. I went to a speed shop and bought the gauge that was called for by my sending unit.
The foam cellulose (whatever it's made of) in my cell does not interfere with my float because the float lives in a tube. The tube lives inside the deepest part of my fuel cell, not quite touching the bottom.
It is important to preplan where you want the sending unit, buy the right gauge, sending unit, and correct gauge of wire -- but then you're in business.
Let's hear what you've got on hand, and maybe we can get you off on the right foot.

CORY
I apologize for directing that question to Kelly. I got a little confused as to who I was send a response. I guess I was thinking only only about the swing type floats and not a vertical tube. I thought of fuel cells on several ocassions but heard they have less capacity for a given size compared to a conventional tank. I'm not sure I would want forego those extra gallons especially since there is only 12 or 13 to start with.

JUSTIN
i'm in the process of working on your formula. I measured the rolling distance of my tire and compred it with the simple circumference calc. and came up about 1 1/2" short on the measured distance as to what I calculated and TIRE RACK listed in their published data. I admit I only measured one revolution but I don't think several would be that much closer.

No worries, Charles.
Most threads here aren't specifically aimed at any one reader; they tend to wander around a bit and folks answer up with opinions when they have them.
I have a ten-gallon cell in my car. The Hoopty's been modified a little from stock, so it worked for my application.
I also have two different sizes of idle jets, and can improve my fuel economy by switching them out. Usually, I leave the larger ones in and fill up at the three-quarter mark. I get the smaller jets out for road trips.

CORY
"A LITTLE FROM STOCK" ?

Maybe a little. ;)

Another point of view:

My gauges are all metric, from a Euro 944 I think. The speedo and odometer read in kilometers. I suppose I could multiply each reading by .621, but screw that! I just stay with traffic. When there's no traffic, I just watch the tach and "drive the road".

Hey Charles, not to burst your bubble, but most Speedsters/Spyders have stock VW type1 tanks. I can only get about 6.5 USABLE gallons in mine(Vintage Spyder) because of the mounting angle. Speedster owners get about the same or slightly more with the same tank. I believe it is a 10 gallon tank in a Bug.

"Ok! Ok! I must have, I must have put a decimal point in the wrong place or something. Shit. I always do that. I always mess up some mundane detail."

Yeah, that formula above is messed up. Forgot to multiply by 2 for the diameter instead of radius. I wrote it down wrong but managed to calculate it right every time I tried.

Rev/mile = 63360 / (2 * PI * EF)

DANIEL PIP
You are right about the capacity being 10 gals. I don't know where I came up with 12. I don'y think I have ever filled it up from empty. I just put some in when It gets low. My exhaust(car) stinks especially at idle and a mechanic told me once to burn premium in it. Has anybody had that problem ?
Looking at all the info everyone contributed and making a run today with the GPS about .085%.Comparing all the tires and sizes indicates a change to 185/65-15would correct most of my error. I appreciate all the advice.

ALL
I meant to say my speedo error is about.085%.