My 92' 244 reads 3-4mph faster than what I'm actually going. It has the stock tire size 185/70-14 (according to tirerack.com). I have some Virgos on the way and the stock tire size fitted with Virgos was 195/60-15 (according to tirerack.com). So, then according to http://www.miata.net/garage/tirecalc.html the speedometer will still read 3-4mph too much. If it matters, the 185/70-14s currently on the car are !extremely! bald. Will being bald affect the speedo that much, seems logical? Other that that, how can I correct the speedometer?

Thanks,


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Let's consider this whole situation.

First of all, how do you know you're speedo is reading 3-4 mph too high? Are you comparing your speed to another car? Did you use a stopwatch on either a measured quartermile set of lines, or mile-markers, on the highway? Or did you use a GPS?

Also, at what speed did you observe this error. I ask this not only because a 6-8% error (assuming you're only going ~50mph -- the error would be less if you're going 80mph) is pretty high, and unusual if you're using the stock tire size of 185/70-14. But also, if instead you're claimed error is observed at 75 mph (rather than 50 mph), your error is quite less, only about 4-5%. We have to know at what speed you're seeing a 3-4 mph error, in order to calculate the speed correction needed.

Second, assuming you have the aforementioned error, the only way to fix this is with a change in tire size; the speedo cannot be adjusted, nor can you swap a gear in the tranny because your speedo is controlled by a toothed wheel in the differential that sends electronic pulses (the days of driven cables are gone).

If you want to use the Virgo rims (i.e., use 15 inch tires), we'll calculate the size you'll need to cancel your speedo error. Yes, the 195/60-15 tires that are OEM with the old turbo'd cars' Virgo rims are virtually the same diameter/circumference of the standard 185/70-14 tires. So here goes:

Both the original 185/70-14 tires and 195/60-15 tires calculate to the same diameter -- either
(185 * .70 * 2) + (25.4 * 14) = 614.6 mm
or
(195 * .60 * 2) + (25.4 * 15) = 615.0 mm

Because your car, for some reason, is showing a 7% higher than actual speed, you need to increase your diameter (or circumference) by that amount:
614.6 * 1.07 = 657.6 mm
This will make your tires turn 7% more slowly for any given actual car speed, and you'll see a concomittant reduction in the speedo's reading.

205/65-15 tire would be [(205*.65*2)+(25.4*15)], or 647.5 mm, a 5% increase. This might be quite sufficient if your error was at closer to 75 mph than at 50 mph -- again, we need more info to advise.
On the other hand, 205/70-15 would be [(205*.7*2)+(25.4*15)], or 668 mm, an 8.6% increase, which would be what you'd need if your 3-4 mph error was seen at 50 mph or below.

Otherwise, you can do what I do. I have 205/55-16 tires on 16x6.5 Hydra wheels, a 632 mm tire with a 2.8% error, and I have a GPS more or less permanently sitting on my dash to tell me exactly how fast I'm going.

I don’t think you can make a minor adjustment to the speedometer or the pickup, at least inexpensively. A few MPH over is standard for most cars.

I have Virgos with KUMHO KH16, P195 60 R15 in a 92 244. I thought I was running a little slow but according to a Garmin, the speedometer was right on at 35 and 65 MPH.

The variance you see may be in the actual diameter of the tire you have. If you notice in Tirerack, different makes, and tires types will vary in the outside dia for the same size tire.

Look on the rear edge of the RF door for a label showing the stock size.

That's where my 240's have it, but since my newest is a 1988 244GL, yours may differ.

The speedo system is set up to be accurate with the stock size mounted on the rears. Actually the factory set-up may be to read a tad faster than reality. Better to err on the safe side, I suppose.

If you are running stock tires, and are still getting a speedo reading 3%-4% higher than reality, maybe a re-check of that actual speed is in order??

If there is a way to calibrate the speedo, I've not come across it.

Good Luck,

Bob

:>)

I'm thinking the cop was for sure full of it on my last speeding ticket. Stock tires and reading the odemeter at 3-4 mph below what he said plus the error to the speedo being too fast? Man that judge and prosecutor are lame. Using the whole thing against me about how one guy getting mad for getting popped for weed while the neighbor's doing coke thing was lame to tell me too. What can one do when a large truck is speeding up on your tail? Volvo pride that what.

After getting ticketed with a new to me Chevy S 10 with 235 75 R 15 tires ... stock are 205 75 R 15 ... I checked one of the tires sites and found that the 235 tires have a circumference 6.02% larger .... I then bought a TomTom and found that when traveling an indicated (speedometer) reading of 100 KPH the TOMTOM indicated 106 KPH ... I am currently home to Nova Scotia for the summer and am using that TomTom on my Bertone .... it is telling me that at 100 kph the speedo reading is actually about 103 kph.. which bears out your point about the factory having the speedo reading slightly high. I am planning on replacing the tires on the S 10 with 235 65 R 15 tires .... but apparently this size is only manufactured by GOODYEAR and was stock on the DAKOTA SPORT pickup. However, they are the same circumference as the stock 205 75 R 15 tires that came from the factory .... the circumference of the 65 series tire is approximately 6% smaller than the 75 series ... which makes sense and should bring the speedo back to what it was when it left the factory.

Ole Reliable is wearing 205 75 R 14 tires and the circumference worked out to within 2 revolutions per mile of the stock 6.00 15 tires.
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Brett Sutherland & the 1.5 million mile 122 CANADIAN --- WINDSOR, Nova Scotia the birthplace of HOCKEY www.ecvintagevolvo.com