what is everyone doing about speedo calibration after some of the semi radical tire size changes? where are you getting the speedo gears if that is what you are doing.
Great! wasn't sure they were still available. Next question is how do you determin which one you need? do you just order them by # of teeth? if so is there and easy way to figure which one you need after a tire size change?
you would have to have several inches taller tire before the would a noticeable difference like four or five inches if you have a inch or two taller tire maybe if your speedo says your going 60 you might be going like 62
It doesn't take much to throw your speedo off esp. at highway speeds The main concern here is this is going to be my sons first car. He doesn't need speeding tickets because of miscalibration.
heres a few Calculators might help http://www.mgbexperience.com/service/tirecalc.php http://www.wallaceracing.com/speedo-calculator.php http://www.crownvic.net/liquid/speedo_gear.html
http://www.transmissioncenter.net/speedometer_calibration_______va.htm this site told me what size gear I needed. use the calculators, go to the Ford dealer, and ask for the gear with the number of teeth you calculated. Pretty easy. I did it when I swapped my 2.79 to 3.80 gears, but still couldn't get the 22 tooth gear I needed. Got the biggest Ford makes, 21 I think. Puts me within 5mph at 70mph. My next tires will be a little bigger, and may even things out a bit. I think I paid $12. PS--scroll way down to find the ford gears on that web-link.
I seriously doubt anyone's gonna get a ticket for going 1mph over. Taller tires make the speedo read slower. Shorter tires make it read faster. They would have to be a LOT taller (or shorter as the case may be) to throw the speedo off enough to get you into trouble. Speedo gear changes are for rear-end gear swaps. You're not gonna find a speedo gear that will correct a tire size difference...
For once, I agree with TL (but this is not a sidepipes thread!). A slight increase in tire size will not affect your speedo too much. A gear change will. My stock 74 with original tires was still about 5% off. Now, with 3.80 gears replacing my 2.70, and 245/60R15 on the back, and the biggest speedo gear Ford makes, I am about 10% off. Luckily, I can subtract 10% off my speedo while driving, without pulling out the laptop or calculator.
TL, are you sure you don't have it backwards? A taller tire (larger circumference) means the car is traveling further for every rotation then the speedo thinks. (At least with my logic, this may be flawed) but all your answers have gotten me to do some math. According to one of the calculators in the above posts the diameter of 245 60 15 is 25.774803 to get circumference we multiply by Pi (3.14 to be simple) = 80.93288142” we’ll round to 80.9” the 195 78 14 on the car now have a diameter of 25.176378 x pi = 79.05382692” Rounded to 79.05 There are 63,360 inches per mile. @ 1000 rpm on the motor= 366.3 rpm of rear wheels with a 2.73 rear. The 245 60 15 tires would travel 29,633.67” per 1000 rpm. The 195 78 14 would travel 28,956.015” per 1000rpm 29633.67 -28956.015 = 677.655” further per 1000 rpm To get the percentage of MPH off I divided 677.655 by 63,360 = .010695312 @ 1000rpm @ 3000 rpm the difference is .032085937% @ 60 mph = 61.925…….MPH My math may be flawed and please tell me if anyone is bored enough to do the math. But you guys are right it’s too small a difference to worry about. Thanks for all the help with this
T.L. is correct ... Larger tires means the tranny is turning less revolutions to move the same distance ... Speedo drives off the tranny ... The mileage is affected as well ... Larger tires will rack up less miles over time on the odometer ... Smaller will rack up more ...
I misread TL's post. I thought he was saying if the speedo says 60mph you were actually going slower. I should have known better then to question TL. Thanks DM
