Dealer adjusted tire size-Question.

[Verne]

Hi,

Dealer charged me to reprogram the computer and change out my tire size. Honestly, at the price they offered it was worth it to me to just to have them mess with it and not buy a tazer etc. Im not super confident in my abiilities to not brick my Jeep. Nonetheless, I went from the Falken MTs that come on the Mojave to the Maxxis Rzr AT 35s. The shift points do seem much better, especially going from 7 to 8 on the Hwy but if I use a GPS speedo, it reads about 2-3 mph too fast at 70, about 2 mph fast at 65, and close enough at 50 and below. The dealer set my size to 33.5 at 37-38 psi. I did use a level and a tape to measure myself and I would have probably gone up to 33.75 (if even possible). Are GPS speedometers accurate enough provide you a specific speed?

Thanks for your insight

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[Riverdog]

I believe GPS is more accurate than others. When I had my 35's with stock settings I was off by 2 at 55 and 3 at 65 etc....I set mine with the jscan app for 35" tires and it was off just a tad, so I changed it to 34.75 and now it's 100% accurate according to my phone's GPS speedo app and the police radar signs on the road.

 

[Janster]

I personally wouldn’t worry too much about the speedo being off by 2 or 3mph. Any vehicle I’ve ever owned, was off by that much from the factory. It could be off simply by the position of the needle on the dash readout.

Personally, if that were me…….I’d be checking the odometer. Find a stretch of road for a 1 mile comparison. GPS and/or another vehicle to get different data points.

 

[Maximus Gladius]

Do you know what tool they used to change the tire size? Mine uses the AlphaObd2 to set my 315/70/17 and the GPS app test was bang on with the odometer. With having my dealership do it, it’s a 6 month contract thing with FCA. If for any reason somethings off, they’ll redo it, even if I change tire size again.

 

[Verne]

I personally wouldn’t worry too much about the speedo being off by 2 or 3mph. Any vehicle I’ve ever owned, was off by that much from the factory. It could be off simply by the position of the needle on the dash readout.

Personally, if that were me…….I’d be checking the odometer. Find a stretch of road for a 1 mile comparison. GPS and/or another vehicle to get different data points.

Good call. Personally, I am not worried about the speedo for myself. I'm more concerned about my son driving the jeep and (being young) if he thinks he's going 70 in a 65, he's really going 73 and getting a ticket. Kids...

 

[Verne]

Do you know what tool they used to change the tire size? Mine uses the AlphaObd2 to set my 315/70/17 and the GPS app test was bang on with the odometer. With having my dealership do it, it’s a 6 month contract thing with FCA. If for any reason somethings off, they’ll redo it, even if I change tire size again.

I don't know what they used. My agreement is like that too. It was a contract that I had to sign through Jeep that is now married to the VIN. So changes, etc. are covered. Can I even ask them to increase the size a little bit? I mean would they be, "allowed," to do that?

 

[Hootbro]

Do you know what tool they used to change the tire size? Mine uses the AlphaObd2 to set my 315/70/17 and the GPS app test was bang on with the odometer. With having my dealership do it, it’s a 6 month contract thing with FCA. If for any reason somethings off, they’ll redo it, even if I change tire size again.

The dealership used a wiTech 2.0 scan tool. For them to do a tire size change, they have to pay for a one time subscription that give them a code that will unshadow the function in their wiTech. Of course that cost is passed on to the customer.

Problem with the dealership wiTech tool vs. Jscan or similiar, is that it is usually a one and done programing and you run into the issues the OP has. With Jscan or similar, you can play up and down multiple times with the tire size to fine tune the actual speed plus also have the option of programming a error adjustment (%) to the speedometer signal on the cluster.

Problem of not getting it dead nuts accurate is you will occur a faster odometer count. 3 MPH fast over 60K miles means you will incur that mileage count almost 1800 miles quicker. Not a big deal if already out of warranty, but could be an issue if in warranty and you lost those margins to letting the speedometer read fast.

 

[Light]

Hi,

Dealer charged me to reprogram the computer and change out my tire size. Honestly, at the price they offered it was worth it to me to just to have them mess with it and not buy a tazer etc. Im not super confident in my abiilities to not brick my Jeep. Nonetheless, I went from the Falken MTs that come on the Mojave to the Maxxis Rzr AT 35s. The shift points do seem much better, especially going from 7 to 8 on the Hwy but if I use a GPS speedo, it reads about 2-3 mph too fast at 70, about 2 mph fast at 65, and close enough at 50 and below. The dealer set my size to 33.5 at 37-38 psi. I did use a level and a tape to measure myself and I would have probably gone up to 33.75 (if even possible). Are GPS speedometers accurate enough provide you a specific speed?

Thanks for your insight

My spedo is off too but I get an average of 1 mph slower

 

[audibahn]

You should ask them to increase the tire size setting at no charge. I've changed my tire size via jscan numerous times for different tires until it was dialed in. For context, the original 245/75/17 tires on mine had an actual measured tire height of 30 3/4 and the setting was set to 31.93. My 33" Toyos measure 32 1/4 actual height and I have the setting at 33.00.

 

[Janster]

Good call. Personally, I am not worried about the speedo for myself. I'm more concerned about my son driving the jeep and (being young) if he thinks he's going 70 in a 65, he's really going 73 and getting a ticket. Kids...

There are ‘add on’ throttle controllers you could buy to help prevent that.

Years ago, I had a ‘Pedal Commander’ on my previous truck. It had an Econo mode - Tested it, but never used that mode myself.

You could secretly install it and put it in *Econo* mode. It‘d make the truck into an instant TURD!! He’d never want to drive it again.

 

[Chapol]

Since the day I got my Mojave, the needle is about 2+/- mph faster than the digital readout. I haven't been able to verify which is accurate. I've only ever had that happen on recent used vehicles I bought that came from Canada. Where the dealer here has had to swap the dash gauges from KPH to MPH.

So, having my brand new vehicle having the same issue, is annoying. Maybe it's just a setting that can be changed.... otherwise, not sure what the fix is. Other than taking to a speedo shop?

 

[Greg_L]

The ECRI unit uses your own manual measurements in addition to a GPS speedo to calculate your actual rolling tire size and calibrate the speedo. Mine's pretty much dead-nuts accurate with 37s and verified against other GPS methods.

 

[Overland_Dreamrz]

Since they are charging you to do it, I wouldn't accept "good enough"......your speedo should match GPS. I've got my Tazer set at 34.98" for my 35" AT4W's and it is dead on.

 

[Splenda]

Hi,

Dealer charged me to reprogram the computer and change out my tire size. Honestly, at the price they offered it was worth it to me to just to have them mess with it and not buy a tazer etc. Im not super confident in my abiilities to not brick my Jeep. Nonetheless, I went from the Falken MTs that come on the Mojave to the Maxxis Rzr AT 35s. The shift points do seem much better, especially going from 7 to 8 on the Hwy but if I use a GPS speedo, it reads about 2-3 mph too fast at 70, about 2 mph fast at 65, and close enough at 50 and below. The dealer set my size to 33.5 at 37-38 psi. I did use a level and a tape to measure myself and I would have probably gone up to 33.75 (if even possible). Are GPS speedometers accurate enough provide you a specific speed?

Thanks for your insight

How much did the dealer charge you to change the tire size? I recently bought the ECRI and did it myself using the ECRI's GPS calibration, and my 37x13.5x17 tires at 39 psi came out to 35.5". My speedo now matches my phone and Android Auto, but my Uniden radar detector always reads about 1 mph faster. I ended up spending about $135 for the ECRI as I was unwilling to pay $350 for the Tazer.

 

[Josh00333]

I'm anal abought my speedo. Took me a few settings changes to get it right.

The BEST way to set it is on your next road trip. Set the trim odometer and a gps trip, do your drive. Check and change, then double check and adjust on you way home!

You can spot check the speedo and gps driving down the road but you'll always be off a little.

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