Speedometer way off
 

got the car pretty well done changed an 82 240D to an 82 300DT, changed the engine, trans, rear end, and speedometer everything came out of the same 300D but when driving the speedometer is close to 10 MPH off I didnt check the rear end ratio before I put it in but if I need to Ill get it up in the air and look at the tag, but the speedometer number is 123 542 63 01. I think thats the right speedometer for an 82 300D turbo, the tires are not the correct size but should not make a 10 mph difference if anyone has any ideas as to why its so far off it would help allot

what size are they? 10mph is not unbelievable. I think this is about what mine is. How are you verifying the speed offset?

the tires are 195/70R14's, I checked the speed the best I could without a radar gun or a GPS, I took an average after driving behind friends with the cruise set to dead on 60

for information purposes........
 

not exactly relevent here. but for info......

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

this be for those who are lazy.

it's pretty possible for the odo to be from any of the other 123 vehicles out there, they are so simple to change out. what year was the donor car?

240 is a 82 and 300 was an 82, didnt know if you could tell what that speedometer is supposed to fit by the part number off the face

what MPH does the speedometer go to?

I think 85 I would have to check to 100% but thats what I remember

85 is the correct answer. so the speedo is right, it must be the tires, or the diff. time to crawl under there and find out what diff you have. my bet is that it's a 2.88 from an 85 model.
is it off over or under the indicated?

hard to say what had been done to the car that I got the parts from it was in a junk yard, your going faster then the speedometer says

yup, classic 2.88 dif symptoms... time to find an 85 speedo... good luck, there aren't very many out there.
on the plus side, you will have a 125MPH speedo then.

well at least I know whats going on, so does anyone had a 3.07 rear diff, or a speedometer for an 85 with a 2.88

Hold on. I dont think its a 2.88 diff. A 3.07 speedo should be 110mph or so. and 85mph speedo is indicative of a ~3.48 rear.

not true.
the 3.07 speedo from an 82 turbo 300D would be 85MPH as per the stupid fed rules.

the only way to accurately check your speedo that I have used is by driving an indicated 60 on the interstate and time a mile. One mile equals one minute at 60 mph. If it is less than one minute you are going faster than indiacted, and vice versa.

Tom W