Have any of you put larger rear tires on a 123 chassis, it would be nice to lower the high rpms at 80mph, my 300tdt likes to go 80 but the motor is at like 3500rpm which is high, Ive had the car up to 110mph at like 4400 rpms and it is smooth but loud, so I was thinking of a quick and dirty fix like bigger tires and what would anyone recommend, thanks Jimmy

the relationship between your speedometer and tachometer readings will still be the same, except that your speedometer reading will now be pessimistic (i.e., a reading of 80mph occurs at an actual vehicle speed of more than 80mph).

Alternatively, your car can actually be going 80mph, but you wouldn't know it, based on a speedometer reading lower than 80mph, and (and a commensurately lower tachometer reading).

What you will feel though, are the effects of higher gearing and higher load.

i don't know if your car has ABS, but if it does, the larger rear tires' rolling diameter will throw off the ABS's calculations as well.

rpm = (mph x gear ratio x 336) / tire diameter - this formula assumes 1:1 final gear, manual tranny.

Dunno what your gear ratio is, so lets use mine - 3.46.
Tire info - 195/70R14=24.8" DIA.

80mph x 3.46 x 336 / 24.8" = 3750 RPMs

now throw a 225/60R16 on there thats 26.6" diameter, think of it as a little less than an inch larger per side.

80mph x 3.46 * 336 / 26.6" = 3496 RPMs a difference of 250 RPMs


Yup, your speedo will now be slow, and hopefully your top speed will be higher... (get a cheap GPS to get your true speed)


good luck,
Robert

__________________
Cheers,
Robert