28" high tires vs. 26" high tires

[wmerrell]

How will going from 26" tires to 28" tires effect the speedometer mph?
How do you correct that difference?
Had 16" rims - tire choices are either MT 255/50/16 or M & H 275/55/16.
Which combination will have better traction?

 

[Thumper]

26" to 28" mine changed 12 mph, but easily corrected with a 27 tooth driven gear instead of 30 tooth factory. The 27 is a little hard to find but are still avail.

Thanks
Bill

 

[pacecarta]

really
no way its off 12mph unless you drive over 150mph

26" = 82" roll per rev
28" tire = 88 " per rev


your speedo will be off about 7%

at 50 on speedometer your actual speed will be 53.5 , at 100 it will be 107

 

[wmerrell]

really
no way its off 12mph unless you drive over 150mph

26" = 82" roll per rev
28" tire = 88 " per rev


your speedo will be off about 7%

at 50 on speedometer your actual speed will be 53.5 , at 100 it will be 107

I am asking this because I am ordering a new chip from TurboTweak. The speedo must be within 5 mph accurate.
So at 50 mph, it will be ok. However at 100 mph it will not be ok.
Guess it looks like a speedometer gear change is needed.

 

[turbojd]

My speedometer is off by about 2 mph off with a 27" tire on the rear (255-60-15) When my speedo is reading 50, I'm really doing about 52 ish mph.

 

[BIGPSI]

I am asking this because I am ordering a new chip from TurboTweak. The speedo must be within 5 mph accurate.
So at 50 mph, it will be ok. However at 100 mph it will not be ok.
Guess it looks like a speedometer gear change is needed.

i have 275/55/16 M&H's with turbo tweak chip. no need to worry about speedo being off a little,won't hurt a thing

 

[GNICETRY]

I have 255/60 15's (about 27" tall) and they make the Scanmaster read about 3mph less. I confirmed this by flanking a newer Civic that has the digital speedometer readout.

 

[Thumper]

My 12,ooo mile car was not checked to verify speedo until I had the 28" tall M/T drag radial (275-60-15) with 25 psi, so some may be it was just off to begin with. Using a gps with the factory transmission with orange drive gear 11 tooth and blue 30 tooth driven and 3.42 factory rear gears with the 28" tall tires it shows 12mph difference. At the race track I was seeing 1/8 trap speeds with powerlogger at 85 when track showed 96mph at least 7 different times, a 11mph difference. I installed a 27 tooth driven gear now to find a 1 mph difference with the gps. I guess someday I will put the 30 tooth back in and factory tires back on and check to see how accurate the factory setup was. This IS using gps not estimates or theories which may only get you close, but then again this was my experience as others may not be the same. How accurate is the gps method????


Thanks
Bill

 

[pacecarta]

your situation is what it is
but you cant argue with the math ...26 vs 28 tire cant magically show more than the difference in circumference(aka rollout) percentage

 

[Thumper]

And the math is only 1 side of the equation not taking into consideration that the factory may be 3 or 4 mph off. But the question was what is the diff of 26" and 28" tall tires so ya got me there. I was just trying to help with getting a more accurate speedo reading.