Speedo Correction for tire size change

[Tim]

Has anybody got any experience with devices to correct the speedo for changes in tire size? Usually I guess 4x4 owners would be looking to increase tire diameter for ground clearance but the knock on effect, apart from changing the gearing, is the speedo under reads.

I'm looking at using one of these (called Truspeed)

http://www.sleeoffroad.com/technical/tz_truspeed.htm

It's for electronic speedos. Has anybody used one? Or do people not bother? I guess I could rely on the GPS speed readout.


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[GoingCommando]

My rig is in the shop now getting a 4" lift & 33" tires. The outfitter strongly engouraged me to get a "flash pack" that communicates with the vehicle OBII computer. It is programmed for the new tire size in order to correct the speedometer, mileage, even auto-transmission gear changes. I don't know what the make and model is. It cost close to $500. I'll let you know how it performs. Whoever does your install for your tires should be familiar with these kits. If they aren't, consider a different installer.

 

[Tim]

My rig is in the shop now getting a 4" lift & 33" tires. The outfitter strongly engouraged me to get a "flash pack" that communicates with the vehicle OBII computer. It is programmed for the new tire size in order to correct the speedometer, mileage, even auto-transmission gear changes. I don't know what the make and model is. It cost close to $500. I'll let you know how it performs. Whoever does your install for your tires should be familiar with these kits. If they aren't, consider a different installer.

That is one my concerns, could cause a lot of issues if it doesn't work properly. Unfortunately I don't have the OBII option... my rig is too old.


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[GoingCommando]

I'm no mechanic, so don't take my word for it, but I would think if your vehicle is old enough to not have OBII it probably isn't as vulnerable to changes like tire size. If it's an automatic, it may not rely on computer electronics the way newer vehicles do. You might find some useful info on this web site. https://www.sizemytires.com/calculators/compare. It has a calcutor that tells you speed variance. Also warns that "you should never go beyond 3% of the original tire, otherwise you run the risk of brake failure."

 

[Tim]

I'm no mechanic, so don't take my word for it, but I would think if your vehicle is old enough to not have OBII it probably isn't as vulnerable to changes like tire size. If it's an automatic, it may not rely on computer electronics the way newer vehicles do. You might find some useful info on this web site. https://www.sizemytires.com/calculators/compare. It has a calcutor that tells you speed variance. Also warns that "you should never go beyond 3% of the original tire, otherwise you run the risk of brake failure."

Thanks I'll check it out. I've gone up an inch in tire diameter which according to the gps means the speedo under reads by about 7%



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[britz]

My Rubicon has a Bullydog GT tuner installed that recalibrated the speedo for me (35's/ 3.5" lift), in addition to other goodies. A LEO buddy clocked me and it is right on target. I've never calibrated my 2dr from stock to 35's since rarely does it leave the mountain. But sure enough, they got a new Marshall down in the Valley and I forgot. Now I know when my speedo says 25, I'm doing 38. Now i'm looking at the AEV Procal for calibrating., only costs a few dollars more than that damned ticket.

 

[vegasjeepguy]

I'm running a 4" lift with 33s on an 04 Rubicon and have about a 3 mph variation. I know this because I have a GPS with the speedometer data. In fact I use my GPS as my primary speedometer which sits on the dash.

 

[Tim]

My Rubicon has a Bullydog GT tuner installed that recalibrated the speedo for me (35's/ 3.5" lift), in addition to other goodies. A LEO buddy clocked me and it is right on target. I've never calibrated my 2dr from stock to 35's since rarely does it leave the mountain. But sure enough, they got a new Marshall down in the Valley and I forgot. Now I know when my speedo says 25, I'm doing 38. Now i'm looking at the AEV Procal for calibrating., only costs a few dollars more than that damned ticket.

Yeah, have to keep an eye on mine when I don't have the GPS in the car for the reason.


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[Kelly]

There's a small gear at the Transmission end of the speedometer cable. You can change that gear to correct your speedometer.

 

[vegasjeepguy]

There's a small gear at the Transmission end of the speedometer cable. You can change that gear to correct your speedometer.

Every 4x4 shop around hear usually has a bucket of these in "pretty colors" specific for the tire size.

 

[Dave Decker]

I haven't bothered changing the speedometer or the odometer. It was just easier to figure out the multiplier and use that to figure out distance and speed. I drove on the freeway and watched the mile markers for 100 miles, my odometer read 93.5 miles, so I just inverted that to get the multiplier of 1.07 (for every mile on my odometer I actually travel 1.07 miles, for every mile per hour on my speedometer, I am going 1.07 mph faster, e.g. at 60 mph I'm actually travelling 64.2). Geeking out? YES! But it works for me and cheaper than buying a chip programmer.

 

[NightCrawler]

My jeep is old as well and I had to change out the speedo gear in the transfer case to get an accurate speed reading when I went with larger tires...I will have to change it back when I re-gear. For awhile I was using an app called speed box to check on my MPH. Once I got the new speedo gear in...I checked how accurate it was and its dead on.

 

[Neil Q Smith]

In Norway, lots of towns have radar-LED speed panels, telling you haw fast you travelling, just before entering a town.
Checking these values displayed, with the dashboard speedo, I see that there is only 2-3 MPH difference.
So, each time I enter a speed restriction area, or known speed trap area, I just drop down a few MPH, and I always seem to be in the clear.
However, this is not an exact science, and I appreciate the threads above, offering advice on how to correct this properly.
Thanks :)