33 inch tires and speedometer accuracy

[noir1975]

I have 33 inch tires. Is it true that what your speedometer is reading is acutally 10mph slower than you are going? I got pulled over last week and the guy insisted that i was going faster than my speedometer was reading.

[JDPascal]

The larger tires roll further during each revolution so at the same engine rpm you are traveling faster than you were with the stock tire sizes.

How much faster depends on the difference in the loaded radius of the tires.

there is a calculator at this website that will tell you the percentage difference in speeds.

http://www.1010tires.com/TireSizeCalculator.asp

JD

[stangfreak7]

well im running 32" tires right now and when my spedo says about 60 mph im actually running about 71 mph or so the sheriff told my a while back when he clocked me, hehehehe. Didnt get a ticket though, but that should give you an idea of what it will do to the accuaracy of the spedo...

[goinbig]

Originally Posted by stangfreak7

well im running 32" tires right now and when my spedo says about 60 mph im actually running about 71 mph or so the sheriff told my a while back when he clocked me, hehehehe. Didnt get a ticket though, but that should give you an idea of what it will do to the accuaracy of the spedo...

Umm, I don't think your speedo is that for off from the slightly larger tires. There has to be something else wrong with it, or he clocked you at a different time then when though he did.

[jeeper61]

assuming stangfreaks tires are 32/11.5/15 tires the calculator in the link above shows 71.5 mph compared to a 215/70/15 stock tire.

[jeep_cj4x4]

easiest way to get an idea of what your doing is follow a buddy around the block or somewhere and have them drive the posted speedlimit. Take note of where your speed is reading on the gauge and remember it. I'd take note of the most common speeds in your area....25, 35 and a highway speed 55 or 65.

Another fairly accurate way not to get pulled over is dont pass anyone up (go with the flow). If every one else is doing 70 and your doing 68, you should be pretty safe.

ps. if you have a GPS, you can check your speed yourself.

[Blazernomore]

I've got 33" tires on my 89 Wrangler YJ. I saw a local cop running radar and asked him to check my speed. Results were something like this:

30 speedo = 34 radar
40 = 45
53 speedo = 60 radar

Its about 15% off best I can tell. Don't forget, your odometer is running slow also. My odometer says 200 miles to tank of gas when its really about 230 miles.

Next time you see a radar cop just pull over and ask him to check you if he's not on a real busy street. I've done this on lots of cars, mostly after changing tire size, and they always seem glad to help.

[fredjacksonsan]

Yup, and some shops will run your car on a dyno to calibrate the speedo for you, gives readouts for all speeds. I got out of a ticket by having mine calibrated.

Newer cars can have the electronic speedo recalibrated for the larger tires.

[Rail-Driver]

I also run 33'' tires, with 4.56 gears. I was just about ready to opt for a aftermarket speedometer adjuster when I stumbled on a company called Advance Adapters in California. I called the tech department, and explained my situation. Low and behold MOPAR makes re-placement speedo drives just for this purpose. The tech checked my ratio's, then gave me a part number for MOPAR. I went to my local dealer and ordered it up for less than 15 bucks. It came in a few weeks later, and took about 30 minutes to install. Speedo now reads correct, and the ECU is still doing it's job correctly.

[CanaMark]

I have been thinking about getting a GPS unit to install as a digital speedo. because Im canadian eh, my jeep is in miles. I go by km/h. Be alot of money to get a digi speedo. So why not up it one by buying a GPS. I like digital speedos, cuz what it says is what your doing. Your mind cant lie to you when your doing 67, and you read a analog dash speedo, your mind will say 60. Also I will be installing a cruise control soon. These 2 things on my vehicles save me on fines for speeding.