kmgar99 Posted December 13, 2005 Share Posted December 13, 2005 Yesterday on a flat stretch of freeway here in SoCal I checked the accuracy of my spedo by looking at the digital speedo of a Prius next to me. Mine was showing almost exactly the same....The Prius was showing 80, we were side by side, steady on the throttle and my spedo was showing halfway between the lines of 79 and 80? With 32" tires you would figure my spedo would be off a lot more than 1/2 MPH? I also decided to used those freeway mile markers to check my odometer and after 5 miles my odometer was showing less than 1/10 of a mile behind...More like barely even noticable..... Can anyone explain this. Does the computer compensate? I don't know how it could without me telling it i installed 32's. Anyway......Off to work I go..... Link to comment Share on other sites More sharing options...
Precise1 Posted December 13, 2005 Share Posted December 13, 2005 Odd... but then many speedo's are inaccurate to begin with. Perhaps your was reading fast before ?? (I'm assuming 31" tires) Also, tires aren't exactly the size they say on the sidewall. For example, I believe the BGF AT is about 30.5 mounted. The width of the rim could influence it. Perhaps your 32" tires are 31.6 or something. Here's a fun little tool. You have to play with the metric sizes to get the desired diameter, but you'll figure it out... http://www.miata.net/garage/tirecalcold.html B Link to comment Share on other sites More sharing options...
kmgar99 Posted December 13, 2005 Author Share Posted December 13, 2005 My original tire size (Stock) was 29.6" tall x 10.4" wide on a 15" stock SE model Rim. My now tires are BFG AT KO's 32x11.5x15 (Actually 31.7"x11.4") on American Racing rims with 0 offset. So if my math serves me right...usually doesn't!...My new tires are 2.1" Taller and 1 inch wider than stock..... So maybe those two inches don't make enough difference with the speedo or odometer? Then my calculating gas mileage using the odometer and the gas pump is correct.....Yikes! That bites! I'm really getting 13MPG! Ouch! I need to slow down... Link to comment Share on other sites More sharing options...
Filthy Luker Posted December 13, 2005 Share Posted December 13, 2005 My speedo reads 9% lower than actual speed with 32" MTs and about 4% lower with the stock 31" ATs. GPS verified. Link to comment Share on other sites More sharing options...
Precise1 Posted December 13, 2005 Share Posted December 13, 2005 Well, according to your #'s and if your speedo was correct with the smaller tires, it now reads 7% slow. Or 60 mph indicated is actually 64mph traveled. No real biggie. B Link to comment Share on other sites More sharing options...
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