tire size for correct spedo reading

[FestYboy]

OK, I've tested it twice with radar and once with a rolling road (chassis dyno) and both say I'm dead on between indicated speed and actual speed.

tire size: Kumo 185/60/14s

they run about .75" taller than the stock 155/12s so i did the math and found that they are about 3% different. so right now my correct mileage figures are 37.67 mpg as of the last fill up. i have a 1.6 block and head (ported/polished) stock cam, intake, and carb, and full 2.25" 1.6L header back exhaust with cat resonator and muffler.

dyno numbers came out to 89.9 lbs.ft torque @3120 and 66.7hp @ 5100. don't have a scanner to post the sheet at the moment but I'll be home in 3 weeks so i can post then. Run was made in 4th gear.

[25Horseplay]

Use this tire calculator to figure out how far off the speedo is.

http://www.net-comber.com/tirecalc.html

[FestYboy]

that's all well and good if you're changing your tire size with the assumption that the speedo is correct with the original tire size. this, however, is false. the speedo is NOT accurate with the stock tire size from the factory. actual vehicle speed will be LOWER than indicated speed with stock tires, this is done by the manufacture to help reduce speeds and increase calculated fuel mileage.