Oversize tires?

[boblichty]

Hi

Has anyone had any experience with this. I recently got a car that took 650x15 tires. I had a brand new set of 750x15 tires still in the wrapper. I put them on, they look great, the car drives fine, but friends of mine tell me this will really damage front suspension quickly and I should go back to 650 to 670x15's asap.

[joe_padavano]

Hi

Has anyone had any experience with this. I recently got a car that took 650x15 tires. I had a brand new set of 750x15 tires still in the wrapper. I put them on, they look great, the car drives fine, but friends of mine tell me this will really damage front suspension quickly and I should go back to 650 to 670x15's asap.

Keep the tires, find new friends.

[Guest DeSoto Frank]

As long as the 7.50s aren't rubbing on the chassis / suspension parts, I'd use them.

[keiser31]

I agree with DeSoto Frank....Cars were designed for a certain tire geometry, but a size or two over the original tires will more than likely be o.k. to use.

[Marty Roth]

I agree with Desoto Frank and Keiser31,

I will frequently go one or two sizes larger for the extra capacity, the better ride, and just for the look.

Where you want to lower engine revs for driving at closer to highway speeds in earlier cars, this is a cheap and easy way to make your car more driveable without non-reversable modifications. Depending on the size differential, you can lower engine revs by anywhere from 2-1/2 to 10% or more.

There are cars which do not lend themselves to this mod: Our 1937 Buick is an extreme low-mileage original, having served as New York City Mayor Fiorello LaGuardia's Parade Car, and having transported many dignitaries in "Ticker-Tape Parades", such as President Franklin Delano Roosevelt, and Vice-President Harry S. Truman, with General Douglas MacArthur and General Dwight David Eisenhower. The car came with 7.00-16 tires, and I thought about upgrading to 7.50-16, but it appears that the side-mount fenders and the metal covers for the sidemounts are a limiting factor. Sure, I could put the bigger tires on the ground, and smaller ones in as spares, but then in the case of a flat (Yeah, we do a lot of touring) we would have to run some considerable distance on mis-matched tires - if you are 10 miles out on a 200 mile day, this can be disconcerting, and absolutely not the best thing for the differential.

[keiser31]

Just as a side note, I used to work at Firestone Tire Company and that is where my opinion stems from on the tire size issue. Now, if you were to put radically different sized tires on opposite sides of the rear axle, that MAY cause a problem with one wheel turning faster than the other. You know on those temporary space-saver spare tires where it says TEMPORARY? They mean it.

[Guest DeSoto Frank]

Good point about staying consistent with tire-size all the way around the vehicle...

Mixing tire-size on the same axle is not advisable, especially on a drive-axle: it will cause increased wear on the differential gears. It will also cause erratic braking performance, with the vehicle tending to pull toward the side with the smaller-diameter tire.

Having different size-tires between the front and rear axles will change the alignment geometry slightly, and can cause peculiar handling / odd tire wear.

My '41 De Soto De Luxe was originally furnished with 6.00 x 16 tires. The upscale De Soto Custom came with 6.50 x 16 tires. Same car, same chassis, different trim & pricing.

When I bought new tires for my car, I decided to "go oversize" and put 6.50s on my car; the additional tread width being the main advantage.

I am toying with the idea of using 7.00 or even 7.50 x 16 tires in the future, but will definitely keep the same size all the way around the car.

[boblichty]

Thanks everyone, good advice. Of course these are all four identical matching tires I bought from Coker as always. I have been buying my tires, both radial and bias from Coker for 35 years and have never had anything but the best service and when a problem arrose they stood behind their product. Corky and crew are good folks.

[Marty Roth]

Manufacturers built cars "TO A PRICE", and the everyday motorist would not stand for having to carry two different sized spare tires.

That said, the tire size can dramatically affect handling.

Hotrodders in the 50s and 60s went big/little, putting taller and wider tires on the back for more "bite" going fast from a standing start and going straight down the road. Unfortunately this did not do much for cornering, and we learned about oversteer (Volkswagen/Porsche) and understeer (just about any big front engine - rear wheel drive car - think Ford/Plymouth/Chevy and their upscale siblings).

Another way to think about it is this: When you approach a curve too fast, the understeering car goes through the fence nose-first; the oversteering car goes through the fence tail-first.

When you vary the tire size between the front and the rear, it is possible to get the car more toward a "neutral-handling" vehicle. This sometimes results in an "unconventional" appearance.

Back in the late 70s and into the early 80s I was still into Autocross and SCCA Racing, but no longer had the jazzy little sports cars I had grown up with. My Citroens all had a Tear-Drop-Shaped Body with the wheel track (distance between left and right) that was 8" wider in front than in back. These cars had a great reputation in the European Rallyes. While the USA models had equal size tires all around, the European spec. cars had 2-3 size differential with the larger size on the FRONT, and these were Technically a mid-engine, front-wheel-drive car which should have been really nose-heavy.

I translated the idea of the technology to our '69 Pontiac Tempest Custom"S". It came with little black doughnuts: 8.00-14 as I recall. Going all-out, I found that the '72 Grand-Prix still used the same smaller 4-3/4" bolt circle, and bought a set of Rallye Wheels out of a junk yard at $50 for a set of 5 because one had a gouge on the edge (my new spare). Then came the Firestone 721 Radial tires which were on the set of wheels I bought --- I went to a Firestone dealer to see about new tires for the rims and they told me that the 721 Radial was under a RECALL ! When I started to explain that I hadn't bought them new, he said it did not matter, that I was entitled to a free replacement for each of the 5 tires. I got 3 of them in the 215-15 Radial size, as was on the rims, and got 2 (for the front) in a size 235-15 Radial for an extra $5 each with size upgrade. The next day I went back to the junkyard for another rim and any size "721" I could find, and got even luckier - a matching rim and "correct 721 Firestone recalled tire" for another $10. That plus the $5 upgrade cost, and I now had 6 new tires (including a spare for the front 235-15 and another for the 215-15 rear. Six tires and rims for $75.

The big plus was that the Tempest Custom "S" originally ordered with the bigger "Pontiac 350 V8", F-41 ride and handling package, variable ratio power steering, and Power disk brakes was now acting like a GTO in 4-door tempest clothing.

Best of all, that thing would corner like it was on rails. The bigger front tires, wider rims, radials, -- everything added up to a major improvement in handling and safety. A grocery-getter now commanded some respect in "Sedan-Class" weekend events, and took home a decent sample of trophies as well.