Front vs. Rear Tire Pressure

[Bill E.]

Hello All:

I just looked at a ton of old posts about tire pressure. The view seems to be 32psi all around for modern radial tires is "best" or a "good starting point". I just had four new Firestone Firehawk Indy 500 tires installed on my '74 (the B.F.Goodrich tires I wanted have been backordered for months). I'm trying to find the best pressure for my style of driving which is back country
at 35-55mph and freeway at 65-75mph. I live in New England where summertime temperatures can be high but usually I don't drive if it's over 80 degrees.

I remember in the old days that a lower tire pressure was recommended for the front tires something like 28-30psi in the front and 32psi in the rear. I didn't see any posts addressing the front to rear pressure other than drag racing and heavy engines where higher front pressures were used. Is there some reason why lower front pressures are not recommended any longer? Thanks, Bill

[Duke W.]

Re: Front vs. Rear Tire Pressure

IMO equal pressure front and rear is best for SBs, but BBs will probably work better with a little more front than rear to offset the additional inherent understeer due to the added front weight bias.

OE tire pressures are usually established to minimize ride harshness and ensure plenty of understeer. For example, my '88 190E 2.6 has 57/43 weight distribution and the OE recommended pressure for the 185/65VR-15 tires was 26/29. As you can imagine it understeered like an old pickup when pushed. I ended up running 42/36 (max placarded pressure was 44) and I run the currently installed 205/55ZR-15s at 38/34.

The general range of best pressure will vary with the tire load relative to tire capacity. A highly loaded tire as in the above case (front) may require near the maximum cold pressure. Since Corvette tire loading is relatively low relative to weight, they should not require near maximum cold pressure.

The recommended tire pressure for the OE 6.70-15s on my SWC was 24, but I ended up running them at either 32 or 36. I ended up running the 6.70-15 Michelin X radials I installed in 1964 at 50/45 (There was not maximum placarded pressure back in those days), but when I installed 70 series radials 32/32 appeared to be the best setup. This is an example of how different tire design can affect optimum pressure.

The only hard and fast rule is to not go below the minimum OE cold pressure (assuming the installed tires have AT LEAST as much load capacity as the OE tires), and don't exceed the maximum cold pressure placarded on the tire sidewall. Everything in between is fair game.

Ideal tire pressure will vary with driving conditions, temperature, tire construction details, and your individual preference for vehicle response.

Start with 32/32 cold and experiment from there. Very sensitive drivers may be able to detect a difference of 2 psi and most should feel a differnce of 4 psi. Put together a test plan of varying pressures and pressure bias and do some testing. What is "best" for me or anyone else may not be best for you.

On a racetrack you try to find the ideal hot pressure, then measure it cold when the tires cool down to get your starting pressure for the next event. Racetrack hot lapping can increase cold pressure by 8-12 psi, and the tires will have different characteristics in the morning than in the afternoon when the sun heats up the track surface,so you are constantly chasing a moving target.

Typical street driving in warm to hot weather will increase pressure 4-6 psi or higher if you are running very high speed or working a twisty road.

Duke

[Richard D.]

Re: Front vs. Rear Tire Pressure

Bill,

Put a set of FireHawks on my 68 on a recommendation from a mechanic friend who did a lot of resto work on the car, and he also had a set of them on his 76 that he liked a lot (and therefore recomended that I give them a try). He also had a buddy in the tire business that got them for me relatively cheap.

So far, I've run 32 psi all around, and have been pretty happy with them (235-60, 15" all around on 8" Rallye Wheels).

I drive mine in pretty much the same fashion as you.

Regards,

Rich

[Bill E.]

Re: Front vs. Rear Tire Pressure

Duke and Rich:

Thanks for your comments. The tire shop inflated the tires to 38 psi and the ride was too harsh. I set them at 32 psi and it's much better and seems to tramsmit less road vibration to the steering wheel. I was wondering if lowering the front pressure another 2 to 4 psi would further reduce the vibration or just increase tire wear, rolling resistence, and screw up the handling. Bill

[Duke W.]

Try it and see what happens *NM*

[Wayne P.]

Re: Front vs. Rear Tire Pressure

Factory sticker on the door of my 75 says 20 psi front and rear. I running 30 at this point.

[Bill E.]

Re: Front vs. Rear Tire Pressure

That's what the sticker on my car says too, but that was for a GR70-15 back in 1974. I wonder if use of a lower pressure tire was ment to soften the ride and that if today's higher pressure tires produce a harsher ride. Anyone with a background in suspension design have an opinion on this? In the mean time, I'll follow your lead and drop the pressure to 30 psi and see if I notice a difference. Thanks for your comments, Bill