110 mph on the highway! (not really)

I've got a strange problem with my '79 mgb. It thinks it's going a lot faster than it really is... In fact it thinks it's going exactly 1/3rd faster than it's actual speed. at 30 it tells me I'm doing 40. At 60 it says 80. Acording to my poor addled spedometer it thinks I regularly hit 110 just keeping up with traffic.

I understand that with after market tires I'm likely to be a bit off but the discrepancy seems a mite extreme.

I knew my speedo was incorrect but I found the exact discrepancy by comparing my odometer readings to the mile markers I passed. After 30 miles, my odometer had counted off exactly 40.

I purchased this B used and non running. After much blood, sweat, tears, and money it now humms happily along the road, but I can't figure out this little quirk. I have no idea which componants may be transplants, after-market, retrofitted, etc, with the exception of the transmission which came from another MG organ donor out there somewhere. Meanwhile my odometer is racking up an unearned mile for every three I pass. Any thoughts or suggestions?

It sounds to me like your car originally had overdrive but now it doesn't, you just need a non-OD speedo.

Or I could have that backwards! :)

I have a 77 B with no OD and it figures about the same (showing 80 doing 60). Its okay, keeps my wife's speed down :-) My insurance and state inspection allows so many miles a year so that aspect is disappointing. Someone had suggested calibrating the speedometer off enough show it shows 60 at 60. Seems dumb to me. Most solutions you will get here seem to complicated for me but they will be good solutions. I figure Ill just a get different spedometer when I go back to inspection. I have 14" tires.
You might post your transmission information and also the numbers on the spedometer at the base of the face as the people that will know are going to ask that next. Mine says "5373/00 1000"

I recently put a 1971 MG BGT transmission with O/D into my non-O/D 1979 Tourer. When at highway speeds the tachometer tells me I'm travelling 65 mph but the speedometer indicates 85+. The orginal instrument for a 1971 is a 1280 tpm speedo while a 1979 has a 1000 tpm speedometer. Perhaps you have a similiar situation.

I plan on getting that calibrated as soon as I start driving the car regularly. Sure feels odd to look down at the speedometer and see the needle of the north side of 80.

Mike, side note, what yellow is that, nice, not "school bussie"

Hi John

Welcome! Try doing a search, there have been many posts about this in the short time I been here.

Green 79B in Woodstock

Pat

draymond Wrote:

First it makes no difference if you have overdrive or non-overdrive. MGB's since 68 use the same turns per mile speedometer for OD or non OD cars. You mentioned your transmission has been replaced from a donor car. Does it have a fill plug on the side? It would be on the right side of the transmission. If not it probably has a dip stick fill on top for filling. I don't know if rubber bumper cars have a dip stick access hole behind the console. Your car should have a 1000 tpm speedometer. Unfortunately you probably need a 1280 tpm speedometer but they aren't the same size. Your best alternative is to get a speedometer shop build a converter. Re-calibrating would require some internal changes for the odometer.

If you aren't sure which transmission you have this easy check will help you determine which one you have, I think you already know based on the milepost markers. Mark off a section on a level parking lot or drive exactly 52.8 feet (1/100 mile) between the marks. line up one of the wheels with the first mark. Disconnect the speedometer cable at the speedometer and place a marker or flag on the cable so you can count the turns. Roll the car to the second mark and count the turns the cable makes. Stop the same wheel you started with on the second line. If the cable turned approximately 12.8 turns you have a transmission needing a 1280 tpm speedometer (12.8 X 100)=1280. Of course, if you have a transmission needing a 1000 tpm speedometer(I don't think you do) the cable will turn 10 turns.

Clifton

kencampbell Wrote:

Here is a link to show how changing tire size affects your speedometer, its cool:

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

The easiest way to check your speed is just put a GPS in the car and take it for a drive.

A GPS will tell you if you have an error, but it will not tell what is causing the error.

Clifton

Where around Bradenton can you be clear of traffic to do 80. I used to live in Sarasota back in the 70's, nice then, but now just one big traffic jam. Same in West Palm, Jupiter, one big parking lot. Ft. Lauderdale syndrome has spread out a lot.

Well my 69 had a 4 speed with dip stick, speedo 1280. Just fine, right on. Dropped in a side fill overdrive and the speedo is off. Add 65 series tires and it gets beyond figuring out. I just use the tach and traffic flow as a guide. The lower speeds are pretty much spot on. Really accurate at a stop light.

comart45 Wrote: