jerseymike

hello all,

had the car out today on the highway for a trip over an hour long using GPS for directions and noticed that my speedometer was off (compared to GPS speed) at speeds over 60mph.

at 60-63 only off about 2mph over but

at 65 and higher off as much as 6-7 mph. driving 70 on GPS puts me over 76 on the speedo.

my GPS has usually been very accurate on my other vehicles within 2mph of speedo w/ GPS usually to the + side of speedometer.

I have the TH400 and 2.75 gears w/ 15" tires.

can a speedo be re calibrated?

I guess now I know why cars are flying by be when I thought I was doing 70-75

thanks,

mike

MDchanic

That's a typical amount of inaccuracy for an American speedometer of that era (European, too, actually).
They intentionally made them read a bit fast so that 1. You'd be amazed at how smooth and quiet the car was at that speed, and 2. You'd be less likely to get tickets.

A speedometer that reads low is a dangerous thing.

By the way, the speedo on my '73 Delta is dead-accurate, but the one on my '98 BMW reads high by a few MPH at 65. The one in my '90 GMC diesel pickup reads LOW, and before I figured this out, I was always wondering why everyone else was going so slow on the highway. While I was towing .

- Eric

4speed455

There are two gears on the transmission side of the speedometer cable. They can be changed to calibrate the speedo. Each gear is a different tooth count and color. Depending on what you have and what you need you may need to change one or both gears, and possible the gear housing. Your rear end gears and your rear tire height (not rim height) will determine what gears to use. Their are calculators on TCI website and they sell most if the gears that are still available.

Fun71

Yes, the speedo can be recalibrated. This can be achieved by changing the drive/driven speedo gears in the trans, installing a recalibration box inline with the speedo cable, or by changing the rear tire diameter. (edit: on your car it may be the front tire diameter)

FYI, the amount the speedo is off is a percentage, so the amount in MPH it is off will increase with vehicle speed.

joesw31

The speedometer is not accurate as mentioned, however, find a shop that does calibration in your area if you want it within 98 to 99 percent. I have had it done before.

rustyroger

If you know your true speed compared to the speedometer reading and the needle doesn't annoyingly flicker I would leave well alone.
You can get it recalibrated but it won't come cheap.
Speedometers will vary in accuracy by a small amount due to factors like incorrectly inflated or worn tires.

Btw I've enjoyed reading the magazines you sold me last year - thanks!.

Roger.

hookem horns

Speedo cable goes to the front wheel on those cars, so I have no idea if they use the same drive/driven gears like in the transmission tail on most cars. What size tire are you using? Most modern radials have shorter sidewalls than the bias plys that came on the car which would make it read a little fast like you mentioned. I would live with it unless that front wheel speedo gear is an easy swap.

MDchanic

There is no gear in the front wheel.

The speedo cable is driven directly by the wheel bearing cover and goes straight into the speedo.

Naturally, any speedo will be inaccurate if the tire size has been changed from the original.
Has it?

- Eric

joe_padavano

If your tires are only 15" in diameter, no wonder your speedo is off...

On the other hand, if you meant that you have 15" WHEELS then that information is useless since what matters is the outside diameter of the tire. Tires for 15" wheels can vary from 22" O.D. (155/60-15) to over 29" O.D. (275/60-15). How the diameter of your current tires compares to OEM is what's important for speedo calibration.

jerseymike

hello all,

thanks for all the input. the wheels are 15"...my tires are 215/70/15

as far as I know these are stock.

info from sticker on door re; tire info reads;

8.85 x 14 std

8.85 x 15 optional

225/R15 Radial Ply optional

as I said in my original post, the speed only seems off above 60 according to the GPS. all speeds below that the GPS is pretty much right on w/ the speedometer.

one of my concerns is that when driving on the highway and if the speed is showing off by 7mph is it recording the miles correctly, i'm assuming no it is not, so the faster I drive and longer the trips the more tenths i'm adding up. I guess to know better i'll have to get back out on a highway and check mileage against mile markers

MDchanic

The diameters of the old-style designations can be looked up, but I don't recall where right now.

Your 215/70x15 has a circumference of 84.4".

The optional original 225/75x15 has a circumference of 88.8"

That's a difference of 5%.

How much did you say your speedo was off again?

- Eric

jerseymike

speeds up to 60 are right on w/ GPS speed.

once speeds go over 60 that's when the difference shows up.

(GPS)60-63 about 2mph higher than GPS speed.

(GPS)65+ reads 7mph higher on speedo than GPS reading.

driving 70 according to GPS reads 77 on speedometer.

75 on GPS puts me in the 82-83 range on the speedometer.

MDchanic

So it looks like you're reading 9% high.

Subtract the 5% error from the tires and you've got 4%, which is well within the normal range of accuracy.

Your speedo's fine.

- Eric

jerseymike

makes me wonder how many "extra" miles have been recorded over the years from this.

MDchanic

No need to wonder.

From the speedo (probably for the life of the car): 4%

From the tires (as long as they've been on the car): 5%

Since most cars have inaccurate speedos, most cars have an error of this magnitude in their accumulated mileage.

Also remember, there is a built-in error in all consumer (civilian) GPS units, which is varied over time, so the GPS is never accurate either.

- Eric