ok so I may have done it - I put 23575R15 on the 88 and the speedo reads approx 3 mph low i.e. reads 60 GPS says 63

pretty much consistent from 30 - 80

did I put too big a tire on?

 

No, you just need to change the speedo driven gear. Actually, 3MPH off is darn good.

 

easy for you to say! where do I get one? what do I ask for? so these tires are a bit too big huh?

same size on Toro and CC and speedos are exact on those

 

Why are you even concerned with a 3mph error? That's plenty close enough for me.

 

only because it is not erring on the side of caution and I am getting more and more forgetful lately (52) and the last thing I need is to get stopped for speeding when I know better but forgot which (of the 3) cars I am in

you are right 3mph is no big deal unless you are going 74 in a 70 zone accdg to the speedo (not bad) but are really going 77 accdg to the truth (not good)

 

http://speedometersolutions.com/ http://www.transmissioncenter.net/sp...n_______va.htm here's a couple of places.

 

awesome thanks! so I guess I will run it by the speedo shop before it goes to bed for the summer thanks a million!

 

Here is the best Tire size calculator I've found on the net. Visually shows you what the differance is.
http://www.miata.net/garage/tirecalc.html

 

pretty cool I think I went from 225 75R15 to 235 75R15 and accdg to that website



ok, closer

 

Hmm... I thought the 71-76 B&C bodies were supposed to run 235/75 R15. Isn't that the closest size to the old J 78s that were on it from the factory?

3mph is pretty darn close. I can see the desire to be dead on though.

 

I though the Bs were HRs I will look at the glovebox sticker when I get home and report

 

[quote=Olds64;173739]Hmm... I thought the 71-76 B&C bodies were supposed to run 235/75 R15. Isn't that the closest size to the old J 78s that were on it from the factory?

I think that I have to agree with you, my dad had 235/75 R15's on his '71 98, and I commented that they were the size used on pickups!

 

Let us know what you find out Jeff. Now I am wondering...

 

ok

75 88 Conv = HR15 = 19575R15 right?

if right, then speedo @ 60 = really 65.3 which is more realistic, I am @ 64ish

 

I don't know about today, but back 'round 1970 the factory set up every car/trans/rear/tire combination so that the speedometer would read 1 - 2% fast, usually closer to 2%. (unless you ordered the calibrated police speedometer option)

 

The 88's had HR78x15=225R75x15
The 98's had JR78x15=235R75x15

 

AHA!

 

I am not convinced HRs were 225

I think they were 195 or maybe those were G

So long ago so many brain cells ago

:cheers: