"84 Fleetwood Brougham"

I just changed tires from 215:75R15 to 235:75R15. At 75 the speedometer indicates 60 mph.

I now have (after about 2000 miles) serious grinding noises and vibration from rear area when accelerating with more than 1/2 throttle.

Is the larger tire size to blame?

The driveshaft rear u-joint has just been changed (it was going out!) and will change front u-joint next week).

I plugged the tire sizes (new and old) into a tire diameter calculator. It shows that the tire height increased 1.18" and that you should be at 67.7mph when speedometer shows 65mph. I'd have to say that the problem you are having is not related to the tire change. Had you gone with a 70 profile tire instead of 75 you would have been only .25" more than stock ride height which is the way I would have done it just to keep the height the same.

I doubt very much if the tires had anything to do with the failure. The loads changed are negligable. While some may increase, others decrease. Net load change would be zero.

I have seen my old 76 Delta 88 Royale eat a CV Joint (rear, actually just a double cardon, but often called a CV) and supposedly the long term vibration wiped the rear end pinion (we were on a 500 mile trip and it buzzed the whole way). I suspected the rear was ok, but 'they' (the adults who "knew") said it was bad. $600 later we had another rear in the car from a boneyard.

My 89 Brougham d'Elegance reads about 9-10% low on the needle. My car has 58000 original miles and when the needle says 60 I am actually travelling at around 65-66MPH. This error also passes on the the odometer. I have on the correct tire size(brand new Goodyear Regatta 2) and the error was the same with the previous tires. I have thought about getting the speedometer calibrated but it is easy to figure out the difference for now.

Ryan- Jupiter FL

Cars that have ultra low gears, like 2.56's with OD are more suceptable to speedo error. Even my 91 Lumina when new was off a few mph. I put on P215 75 14's on it and it made up for it! Considering it had 195 75 14's, that was a big jump in tire size to correct the speedo.

I honestly haven't checked my 94 yet. Getting it calibrated isn't like going in with a screwdriver and tweaking a screw. It is either now, a software change, or a gear change on the trans/speedo sensor. Most cars nowdays it is set in ECM software, so all trans are the same. But back in 89 they likely still had some gear changes. My 85 Cutlass did, didn't matter if you had a THM200 4R or THM200C. You can tweak it and get it closer. Get a FSM and look at the available gears. Then you need to work with a dealer to get the right ones.

I also doubt it was the tire size change that caused the noises. From what you describe, it sounds like the rear end.