since im running 31x10.50's and my '80 came stock with 225/75 (i think..)..would my speedo be reading fast...or would it read slow..in other words...if it says im goin 60..am i actually goin 70...or actually going 50...thanks for the help...people around here dont know..some say it reads slow..some say fast...

 

With larger than stock tires, the speedo will read slower than you are actually traveling, assuming stock gearing is used. Why? The speedo cable is driven by the t-case output shaft and thus it measures drive shaft speed. A taller tire will travel farther for each revolution than a shorter tire, so for a given drive shaft rotation rate, the tires are moving father if they are taller. Find out exactly how much here:
- http://www.4crawler.com/4x4/4LoCalc.shtml

 

mine reads almost exactly 5mph slow with the "34 inch" LTB's

 

according to that caculator, if your spedo reads 60, you are really going 65

 

yea, taller tire = travel further = going faster. I think with 31s mines about 3-4 mph over. at least according to those radar things they put out on the side of the road...

 

Wow, according to the calculator when my speedo reads 100km/h(62mph) im actually going 75mph or 125km/h, Ive had the speedo reading 115km/h before which means I was really going 145km/h.. I didn't think my Toyota could go that fast!

Another funny thing is I cruise at about 60km/h on the speedo on most back roads/side streets, The calculator says im actually doing 75km/h.. I havnt got a ticket yet!

 

ok..that helps...so instead of going 60 all the time..i'll cut back to about 55...or maybe even 45...that helps....is it off in town...because it seems to be REALLY close...like if the speedo says 25..am i going 30...guess it dont matter...i go 20 (by the speedo) in town anyway...lol

 

Pick up a cheap GPS or try a stop watch

 

theoretically since it is a gear driven speedo it would be off by the same amount no matter what speed you are going?

either way I find this to be true using a gps. reads 5mph fast with the "34 inch" LTB's at street pressure. So in a 30mph zone I'll just go 25 on the speedo. never been pulled over in this truck

 

The error will be a off by the same percentage at all speeds, so if it were reading say 10% slower than actual, at an indicated 60MPH, you would be travelling 66 and at an indicated 30 you would be doing 33. It won't be off a fixed number of MPH.

 

Ahhhhh! gotcha. Thanks