new tires..speeo off

You need a new gear on the speed sensor.

Generally Jeep speedometers get more accurate with larger tires.

How did you get your numbers?

got my numbers by my son following me in his wrangler, me in my GC. I know my speedo is about 1 mph off (in the GC) because of the radars that display speed on the side of the road used by our local police. We compared our speeds by cell phone. I know not the most accurate way to do this....

cal

Several gears to choose from....how do you determine which one you need?

cal

Got any reasonably lightly travelled highways in the area? Run at steady speed past 10 of the numbered markers along the highway using a stopwatch for timing. Poor man's VASCAR. GPS would work as well.

Cal proclaimed:

Bill, Thanks for the chart link! Looking at the chart, what is R&P? I understand that the top grey line is the tire size (we used 31" tires.) But i don't know what the R&P (the numbers in gray from top to bottom of chart?) represents. thanks for your help!

cal

R&P = Ring and Pinion gear ratio. This is the gear ratio for the differential. If you don't know what yours is, there should be a metal tag under a couple of the cover bolts on the differential cover plate.

Art

Cal wrote:

R & P means ring and pinion, which is referring to your gear ratio.

Chris

I would recommend you get a GPS or do a measured mile to see for sure. My Jeeps speedos became accurate with larger tires. Tests against a stock tire doesn't mean much for that reason.

Mike

You need a new gear in your tcase.

Before you blow a gasket, the gear costs a few bucks, and you can put it in yourself.

Ring & Pinion.

I understand

You have 3.07s or 3.73s. Well, if you have a 4 cyl, you have 4.10s.

Say what?

He needs a small plastic gear that takes about 5 minutes to install.

Jeff, 31's put the Wrangler speedo right on usually. The smaller tires are the ones that are off. The next gear up will set the speedo for

Same for the Cherokee. The top size listed for mine is 225's. When I went to 235's, my speedo became right.

Mike

Agreed.

I think the OP's testing method leaves alot to be desired. The only way to find the accuracy of a speedo is to find a measured mile, and run it at a fixed speed while using a stop watch, then doing the math -- 3600 (or multiples, depending on how many miles are involved) / time (in seconds) to travel the distance = speed.

Of course, a GPS makes this much easier, but most of us already have a stop watch and a calculator.

My guess is that the pace car he is using is off, and the Jeep with the new tires is closer to being right.

I also agree. Putting 235s on my 2002 TJ put the speedo spot on. Tomes