[rufcar]

I love all the rim sizes but, on all my other cars from 1935 to 2007 even the slightest change in radis size screws up the speedo, the tripo and everything associated with that gets screwed up as well! On old cars you just change out the gear in the tranny once you calculate the loss! How does one fix that on XLR's when you chage out to larger rims? jim

 

[cubby558]

I believe the dealer can get into the onboard computer and change that for you. I know when I changed sizes on my 2004 Ford F150 the dealer did exactly that. I would think GM can do the same for their vehicles.

Cubby558

 

[rufcar]

readjust

Cubby, I figured that someone should be able to recalculate the computer to do that. I guess its just finding the right person at the dearlership whos already done one ! I was hoping a chip change in one of the computers would do it too? Thanks jim

 

[rufcar]

rims

Hans, Ill ask tire rack and see. The easiest way is to drive thru any posted radar speed signs near construction zones like we have in PA. I just look down at say 65 and hold it then read the sign. I put original size nylon tires on my 59 Rolls cloud I and that combined with a cloud II rear changed my speed 8% and my mileage about 10-12% maybe worse. If I put the car on a dyno the type that you drive your car on to 2 moveable drums so the rear can spin in gear and tie down the front end tight. It will calculate the exact difference.Then I just change the plastic gear in the tranny where the speedo cable hooks up. jim

 

[Caly]

I have a tuning computer for my Corvette that will adjust the car's computer for wheel size and rear-end gearing changes. I suspect that the C6 version of that will also work on an XLR... but getting any reliable information beyond trial and error is hard to do.

Mine downloads the original program before changes are made... you can always restore if something doesn't work out.

(Hypertech - about $400)

 

[rufcar]

rims

Caly, Is that $400 item, just for your vette? Can you use it on other cars? I would guess you cannot but if not, can you then purchase other chips to put into it? Like the hand held ones snap on makes?

 

[Kahuna]

Hans, Ill ask tire rack and see. The easiest way is to drive thru any posted radar speed signs near construction zones like we have in PA. I just look down at say 65 and hold it then read the sign. I put original size nylon tires on my 59 Rolls cloud I and that combined with a cloud II rear changed my speed 8% and my mileage about 10-12% maybe worse. If I put the car on a dyno the type that you drive your car on to 2 moveable drums so the rear can spin in gear and tie down the front end tight. It will calculate the exact difference.Then I just change the plastic gear in the tranny where the speedo cable hooks up. jim

When doing a +1, +2 or other rim size change you should also change the aspect ratio on the tires so that you end up with as close as possible to the same rolling radius as stock. This does NOT change the ride height if done correctly. This also is the reason that Hans sees no difference with the V wheels on his car. In theory new vs worn tires with give some difference in speedo readings, but usually not enough to be concerned about. It may be that the problems you had with your '73 RS had more to do with wheel width and offset than rolling radius unless the wheel/tire combination was not done correctly.

Jon

 

[rufcar]

tires

Jon, The Porsche problems are similar but because I used to race them it gets complicated because I ask for more than can be given. But you are exactly correct I forgot about the aspect ratio! I didn't think you could do pull that off for an 18" to a 20". I know the tires on my RUF almost resemble rubber bands now! It is also getting more and more difficult to find shops that have the equipment and know how to even change some of those tires. Jim

 

[Caly]

Caly, Is that $400 item, just for your vette? Can you use it on other cars? I would guess you cannot but if not, can you then purchase other chips to put into it? Like the hand held ones snap on makes?

I'm sure there are shop versions available, but the common "home" type will juggle only one vehicle at a time... it stores your VIN number and original codes, you must put it back to stock in order to do another vehicle. 1 tuner = 1 car.

Oh, and that's a good point about the aspect ratio... the bigger rims usually use a thinner tire so the overall circumfrence doesn't really change much.

 

[Kahuna]

Jon, The Porsche problems are similar but because I used to race them it gets complicated because I ask for more than can be given. But you are exactly correct I forgot about the aspect ratio! I didn't think you could do pull that off for an 18" to a 20". I know the tires on my RUF almost resemble rubber bands now! It is also getting more and more difficult to find shops that have the equipment and know how to even change some of those tires. Jim

Jim, you might want to check out Steve Hammond's Auto Camp in Camp Hill. Steve races a Corvette and is very knowledgeable on wheel and tire combinations, alignment,etc. He corrected a tire wear problem on my XK8. I have known Steve and his partner Vince (who drives a vintage Porsche) for over 20 years and have received excellect advice on tires, alignment issues and related problems. Jon

 

[rufcar]

rims

Jon, Thanks, Is that vince like in Gladfelter? If its he. we used to autocross a lot. jim

 

[Kahuna]

Jim, I think it might be, but to be honest I don't know his last name. Jon

 

[Trentonso]

Good day all, I had a question regarding recalibrating the speedometer on my 2005 XLR. Basically, I wanted to change the rear end gears to a higher ratio for better 0-60 and 1/4 mile times, but I need to know if I can recalibrate the speedometer. Can it be done?