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Speedometer accuracy w/31" tires - did dealer lie?

Hello,

Got a new '04 Wrangler X in September. I plan to use this jeep for "fun" driving on-road in warm weather, occasional snow driving (on-road) and mild off-roading (no serious mudding, rocks or hill-climbing). It is not a daily driver.

The vehicle is a stock Wrangler X with I6 engine and 3.73 gears. The only mod is a set of 31" AT tires (Dayton Timberlines, not BFG) and black wheels with faux beadlock-looking attachments - obviously for appearance only.

I plan to gradually outfit this jeep with more trail-friendly gear, although I don't plan to lift it or use larger tires.

My only question at this time is about speedometer accuracy. When I took delivery, they made me wait for about 45 minutes while the salesguy said they were making an adjustment so the speedometer would read accurately with the (dealer-installed) 31" tires.

I happen to have a GPS, and, according to it, the speedometer reads slow by between 4 to 6 mph.

Is a GPS a good way of measuring speedometer accuracy? I'm not sure if the dealer just gave me a run-around, did something wrong, or if my GPS is just not an accurate device for measuring actual vehicle speed (I use it for geocaching, and it's been very accurate for that).

Thanks for any advice or comments. I enjoy reading this group.

PLM

Reply to
Patrick Mills
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Reply to
Mike T

That's what I figured. Any comments from the group on a brand new Wrangler speedometer reading off by 4-6 mph? Is this to be expected after "adjustment" at the dealer's to compensate for different tire sizes? (I expect not, but I'd like others' comments to use at the dealers' to prevent them just saying "that's a normal amount of error" when it's probably not.) Has anyone else measured the accuracy of their setup after adjusting for larger tires?

PLM

Reply to
Patrick Mills

It is completely normal for a speedo to read *high* by a few MPH, not low. The manufacturers would prefer to err on the safe side. Sounds like they di not do anything and the larger diameter tires would explain the low reading.

-Fred W

Reply to
Fred W.

GPS is accurate to tenths of a mph, when you drive at a steady speed for 10 seconds or so.... assuming the antenna has a good wide view of the sky. Set the cruise control for 60 mph on a dead level road out in the open and watch the readout (carefully).

Get the dealer to fix the speedo drive so it reads within 2-3 mph, preferably on the high side. If you go a little larger on tire size it will read close to dead-on and you won't have to do this again, and you will be less likely to get a speeding ticket in the meantime.

John Davies Spokane WA USA

Reply to
John Davies

I believe it's possible to adjust the speedo "gearing" on the '04 electronically. You could go back to the dealer and ask them to do that. Way easier than changing the little gear on the t-case in earlier models.

They may want to chargea minimum labor fee, even though it's about 5 minutes work with a terminal.

I changed out the stock 30's for 31's on my '97 Sport and now the speedo seems more accurate, based on the displays around schools that the cops here wheel around.

Doug

Reply to
Doug

It is important to be driving in a straight line when checking speed with a GPS. The accuracy will be off when going around a curve in the road. I find both of our cars (one Jeep, one car :-) ) to read 3 to

5 mph high when checked against my Garmin III+.

Dick

Reply to
Dick

My Garmin EMap unit is dead on for speed.

I 'think' that 31" tires are right in-between two spedo gears for accuracy.

MY CJ7 came to me with 31's and my spedo was reading fast of all things. About 4-5 mph fast.

I went to 33's and my spedo is now dead on.

Same for our Cherokee. It reads dead on with P235's. Stock tire would have the spedo reading fast.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

Patrick Mills wrote:

Reply to
Mike Romain

Yeah, I get the same offset - 31's run about 4mph faster than the speedo on my TJ

Reply to
Jason Killian

Reply to
L.W.(ßill)

My '02 TJ speedo with 31's is almost dead-on accurate according to my GPS. It came with 30" tires from the factory and with those the speedo read a few MPH fast.

Tom

Reply to
mabar

Same reason I got my used 1994 YJ with 191K now on the clock.

Generally there is an adjustment to the speedo (gear) to make it read correctly. This is what I read.

Which GPS are you using? Is it accurate for mobile work? I guess you are having no problems with geocaching or are you off on where the cache is too?

Reply to
Peter Parker

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