going to school in a while(45mile trip), and there's cops everywhere. i have my bros car for a week but i dont know how fast im really going. car isnt programmed for his gears and i dont want to get stopped. the car had 2.73's and now has 3.73's i just need to know how fast should i go to reach 30-45-55mph? my brother told me the speedo is way off and i shouldnt trust it. thanks for any help

 

3.73 is about 40% higher than 2.73. So when your speedo reads 40, you're going about 56.

Dave

 

Going to higher numerical ratio makes your driveshaft turn faster for any given road speed. The speedo will read higher than actual speed. Your true speed will be about 3/4 of speedo indication..... speedo say 40, you are only going 30.

 

so your saying that my speedo is off by 40%.

so that means if i want to go 55mph my speedo should read 77mph, right? pretty sure i calc. everything right.

the guy who sold my bro the car told me that when he installed the gears the speedo would be off by as much as 27 mph after 60.

thanks for the help guys

 

Thanks for the correction, Fred. I had it upside-down.

Dave

 

3.73 / 2.73 = 1.366

True - Speedo

30 - - - 41
45 - - - 61
55 - - - 75

 

Another way is take it on any Interstate and go say 10 miles, compare the roadside mile markers with your odometer start and finish, and then compute the % difference in them[that will also be the speedo correction %]
Using Fred's figures, the odometer should show 13.66 mi for 10 'roadside' mile..