I took my '76 242 out on the highway for the first time since I bought it this morning. Judging by the normal traffic speeds on my commute, the speedometer seems to be reading 10% to 15% higher than it should. The car has an M46, which I assume to be original, and the tires ar 195/60R15. The rolling diameter of the tires may be a little different than the original 14" tires, but it doesn't seem like it would cause such a large error.

Any thoughts?

Thanks,
Eric
--
'66 122 Coupe 110k '97 850 Wagon 130k

You can always change the driven speedo gear in the tranny (the one at the end of the cable). Your speedo is spinning too fast, so you'd need to slow it down by increasing the number of teeth on the driven gear.

Those gears typically have about 20 teeth, so going to a 21-tooth gear will decrease its speed by 5%, a 22-tooth gear by 10%, and so on.

BTDT back around '77, or so.
--
Don Foster (near Cape Cod, MA)

Don-
That might be my fix - thanks. The error seems too great to be tire-related. I'll have to check it with the GPS.
Are those gears a dealer part?

Eric
--
'66 122 Coupe 110k '97 850 Wagon 130k

A fellow Volvo lover in LA sent me the following link for a guy who is selling gears on the internet. http://www.odometergears.com/
--
Michelle B. -'85 240 auto 128K; '86 240 auto 144K; '85 240 manual 118K

Michelle:
Those gears are replacements for the one inside the speedometer. They are an excellent reproduction (I have one). But the gear that Foster mentioned is actually located at the rear of the gearbox or, in this case, the OD unit.
--
'80 DL 2 dr

"Are those gears a dealer part?"

Twenty-five years ago, yes. Now, who knows?
--
Don Foster (near Cape Cod, MA)

Eric -

Later model 240s have a sticker label on the rear edge of the RF door that shows the stock tire size. Dunno about your model. It might me 175R-14 or 185R-14. With no aspect ratio number, those two are like "80's", thus a 185R-14 would be close to a 195/75R-14 or a 205/70R-14 in revs per mile.

Once you have the stock size, go to this site to compare what you have against stock. It will give you the speedo error.

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


Good Luck,

Bob

:>)

B.C.s right, most of those vehicles came with the old metric sized tires (175R14) and you're running the new P-metric sizes (which have an aspect ratio digit in the middle, 195/60R15, etc).

On your '76 242, you should see a tire placard on the lower trailing edge of the driver's side door. If it's not there, check the passenger side (where they're located on most of the newer vehicles). But, I'm going to guess that it tells you that you should have 175R14 tires on your car, or if you have a non-US market vehicle it may read DR78-14. Either way, for rough aproximation, you can assume that your 175R14 should be a 175/80R14 or a 175/82R14 (however the 82 is not a valid aspect ratio on the new system). Use the tire calculators as previously listed to find out how far off you are.

Doing some rough math, I'm guessing that your speedometer is reading fast by around 3.5% to 4.5%. For example, if you're going 55 miles per hour acording to your speedo, you're actually going about 53.5 mph or so.

God bless,
Fitz Fitzgerald.
--
'87 Blue 245, NA 223K

Eric,
you don`t say what size the original tires were, but if you Click Here you will find a calculator which will give you the answers.
Bear in mind though that speedos often over-read >5% From new.
Colin.