Replacement Tyres for F250's

Brian Your spare will not fit unless deflated. This is an absolute fact. For anyone living in Qld visit your local Ambulance Center & look at the tyres on the F250 or F350 therein - the ratings will be well below that recommended by Ford. As a matter of interest if you discsuss the GVM with the Ambulance Guys you will find that with all of the gear fitted this (GVM) is well above the plate before adding the Ambulance Personnel let alone a patient. Andy

Tyres under ADR cannot be more than 10% larger in diameter. Forgetting about the legalities the manufacturer tuned the running components from the engine through to the differential and most importantly braking components for the standard size tyre fitted to the vehicle. With you towing you are going to create excess strain factors with larger size tyres as you are increasing the gearing and your effective braking distance is going to be increased. Increasing tyre size from the manufacturers recommendation should never be considered without full respect and impact for the results. Locally they have in stock Bridgestone D694 (have not checked load rating) for $238 fitted and balanced, and there is no shortage of supply to their knowledge. This graphic is from the Bridgestone website.

Thanks for your comments. I have fitted the Bridgestone D693 285/75R16 and they only increased the diameter by about 2cm and the circumference from 2600 to 2680. The speedo with the original tyres was out compared to GPS but now with the new tyres at 80km/h is now accurate in comparision to the GPS. I haven't checked at 100km/h yet. The only down side is that they make a bit more noise than the Goodyears. Brian

The F250 is load index legal with a tyre rated at 121 or above. The rear axle limit is 2817kg and the 121 index tyre is 1450kg. Fitted as standard after the Firestone tyre drama in the USA is a 123 index tyre rated at 1550kg or 3100kg coverage on the rear axle. A 122 load index is perfectly acceptable as in the 265/75-16 Bridgestone above. Here are some stats comparing the 256/75R-16 that Brian intends replacing with a 285/75R-16 ..... 265/75R-16 : Sidewall 198.8mm Radius 402mm Diameter 803.9mm Circumference 2525.5mm Revolutions Per Kilometre 396 285\75R-16 : Sidewall 213.8mm Radius 417mm Diameter 833.9mm Circumference 2619.8mm Revolutions Per Kilometre 382 As per above the new 285 width tyre will turn 3.7% less revolutions per K than the original ... giving at 100kph indicated on the speedo a road speed of 103.7 kph. As far as I recall the transport regs only allow an increase of 25mm in tyre diameter before a "Mod Plate" can/has to be issued ??? Dont quote me on this .... it should be checked via Qld Transport for the latest regs ??? The new tyre being 30mm approx bigger in dia. technically makes this tyre illegal if the 25mm rule is still applicable. Mod plates for tyres might be a distant memory ... once again check latest regs. As for speedo error in the F250, the OBD-11 diagnostic software for the ECU (powertrain control module, engine control unit) has provision for adjusting Revolutions Per Mile/Kilometre. Reduce this by 3.7% .... the speedo will be accurate again and autotrans shift points will be as standard. Have a chat to a Ford Dealer techy mechanic that likes F250's and they should know what you're on about .... On the gearing side of things .... the XLT's run 3.73 diff ratio and the new tyres would lower this to around 3.60 which is minimal. The next ratio is 4.10 so compared to the 3.60/3.73 change, it is far away. The high stall speed of the F250 torque converter and huge torque of the engine will swallow this small change taking off from standstill. As for overstressing brakes ... the 3.7% increase in diameter technically puts more strain on the brakes ...but ... as the F250 and F350 share the same brakes and the F350 carries a 1000kg more ... in my opinion you'd have more to worry about from being at GVM (F350) than a 3.7% increase in tyre diameter. Going on F350 GVM the brakes on the F250 can handle 25% more weight than a F250 can carry ....but I aint no brake engineer .... Regards Anthony Explore this Great Land ...Do it Easy ...Tow a Bushtracker

Brian Further to my e-mail, after puzzling over the question as to why we could not fit the larger tyre into the spare position it has finally come to me. We have a Trailmaster towbar & the configuration of this is such that the standard spare just fits. Other brands of bar probably do not create this (potential) problem. Andy

My Hayman Reese towbar has this problem also. I'm almost positive Hayman Reese has swallowed the Trailmaster/ Trailboss brand. Maybe they are the same bar dimensionally ??? I was thinking that my spare could go in the rear and the aux. fuel tank could go where the spare is now ? It would give about a 120 litre space where the spare was and make it very simple to run lines to the standard tank. A little draincock at the rear would be good for filling the Webasto tank .... now that the boss has ok'ed one ! [formsInTriplicateSmile] This would be a reasonable trade off for the people with a canopy but as I only have the rear tub with a decklid I'm loath to put the spare in the back. Life is a compromise huh ? [wink] The stock F250 tyres are listed on the Goodyear Aust site ...are they in short supply or expensive ... or both ? Anthony Explore this Great Land ...Do it Easy ...Tow a Bushtracker

Thanks for the detailed report Anthony. From the posting above From: <NOBR>OscarVII</NOBR> Sent: 16/04/2004 9:49 AM Tyres under ADR cannot be more than 10% larger in diameter. It appears that we are well within the ADR's with the minor increase in size. The speedo readings as against the GPS are 60 Speedo = 58 GPS 80 Speedo = 80 GPS 100 Speedo = 103 GPS 110 Speedo = 114 GPS So the speedo error is only minor and well within the 10% tolerence that manufactures of speedo's have to comply with. I am very happy with the performance of the new tyres on wet bitumen, and I think people like Jay and Angie who have F250's and have a BT on order would probably be wise to specify these tyres for their Bushtracker and then they would be legal if fitted on their F250. One question Anthony, what are your thoughts on have one Goodyear (Current spare) on with the larger Bridgestones. Is the difference in size enough to worry about? Brian

Brian, The diameter difference is too large to run the different sized tyres on the same axle. It will wear the differential mechanism/limited slip components prematurely because one tyre is turning slower than the other effectively keeping the differential in a permanent turn. Excess engagement of the LSD clutches will wear the friction material and overheat the components. On a modern differential I'm even loath to put a new tyre on the same axle with a well worn one( the difference there in dia. can be 20mm) because of differing tread depth. Of course in an emergency ..... you can always slip them on the front as we have free wheeling hubs. You could still engage 4x4 for short periods at slow speed with them on the front. Change them back to free wheel as soon as practical. Regards Anthony Explore this Great Land ...Do it Easy ...Tow a Bushtracker

Thanks Anthony, I had a funny feeling that you would say that. Looks like I had better find the most worn tyre on the caravan and have a Bridgestone fitted to that and the current new Goodyear spare from the vehicle can go onto the caravan. It is only money and they say it is made round to go round. Brian