Will bigger tires help me go faster?

[BeastlyJimmy]

Here we go, I just got a hold of a 78 gmc Jimmy.
4 inch lift
400, recently rebuilt
Th350
Np203 with the part time conversion
Dana axles, 12 bolt full float 4:10 gears
4 wheel camper

Here is my first question, the new to it axles, limit my speed to 55 if I am pushing it (rpms @ 3,000) I currently have 30s mounted on her, but have another set of 33 inch tires I am going to put on, I am curious if that will help my speed problem? Also my speedometer is reading 10 mph faster from what I am going. Any imput will help greatly!

 

[Beosuun]

Measure the circumference of both tires and compare. BTW, I can guarantee if you drive it over a steep cliff (clear shot to bottom) you WILL be going faster at the bottom with those 33s :smiley_drive:

 

[ramiromunoz]

You will only go about 7 to 8% faster, that is the spedo difference with the change in tire size...

Sent from my HTC One_M8 using Tapatalk

 

[surlydiesel]

You can go any speed you want with any tires, well within the ability of the motor and the road you're on. If you put on Taller tires, you will effectively change the gearing in your axles and at speed, reduce the RPM that the truck is turning. It sounds like you would need 37's to really help you lower your RPM's at 55. You might see 2500 rpm if nothing else about the truck changes. But I think that is an unrealistic way to handle your problem. If you put on the 33's that you have, it will help slightly but every tire size you go up, you add weight and drag. This will make the motor work harder and you will most likely loose gas milage but gain a more comfortable "top" speed. I hope this makes sense. Cool truck and camper!!!

-jorge

 

[BeastlyJimmy]

Awesome, thanks for the info! That helps a ton! so other that swapping transmissions, is there anything els I can do? I commute on a hwy that is 65, and I think I am ticking off more than one driver...

 

[digitaldelay]

33's will:

- lower your RPM @ 55mph to approx. 2700.
- partially correct your speedo (may still read around 60 @ 55mph)
- IMO, add very little strain to a 400ci. motor
- look way better.

Jason

 

[digitaldelay]

I commute on a hwy that is 65, and I think I am ticking off more than one driver...

65 - I'm assuming this is at least 2 lanes per direction. People should have nothing to say. They are more than welcome to pass you.

Jason

 

[BeastlyJimmy]

Oh awesome, that is better than I thought it would do. Too be honest I do not think it would mess with the 400 at all. This thing likes to move, and drink, so fuel economy I am not worried about eather, gettin 8 mpg currently lol!
You know I completly agree with you, to hell with them all!
Thanks a ton for the help!

 

[redthies]

Simple math... 30" to 33" = 10% difference. 55 mph x 10% = 60.5 mph (give or take).

 

[FCM]

Everyone else pretty well summed it up but here is my simple answer.
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Bigger tires will help you go faster if you have the power to turn 'em. This is what I've come to know as a "redneck overdrive".
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I will also add this, you might want to look at your overall gearing selection and make changes there. The tires will only add a small percentage of speed, like the other guys already said, but a ring and pinion swap might give you better results.

View attachment Book1.xlsx

The link is a spread sheet that you might find useful.

 

[superbuickguy]

what is said above is right - but simply because the tire is taller and harder to turn doesn't mean you'll get worse mileage. Your 400 needs to turn slower to get better economy, so larger tires will actually help you (presuming it's a solid motor, and the transmission isn't slipping - something to be wary of on a turbo 350 with big tires). What about 3000 rpm concerns you? too loud? excessive wear? I experienced this first hand - I had a pickup with a 4 speed automatic, and burned up the 3/4 speed clutches.... in grapevine California. I lived in Seattle. I drove the truck in 2nd gear at the speed limit to Portland (parents and reputable transmission shop was there).... and got the same mpg as I was getting with the overdrive. Fuel consumption can be better at higher rpms when you're driving at a steady state.

And honestly, I've had lots and lots of trucks with that very combination and rarely got less than 10 with a full sized camper on top....

and yes, my neck is red because back in the day it was perfectly acceptable (and useful) to go one or two sizes taller in a tire to get better economy.

 

[redthies]

and yes, my neck is red because back in the day it was perfectly acceptable (and useful) to go one or two sizes taller in a tire to get better economy.

Still perfectly normal and acceptable today too!

 

[FCM]

Still perfectly normal and acceptable today too!

most certainly... I use oversized tires to help supliment my gearing selections to help maintain my highway speed and to give me the ground clearance that I am going for.

 

[BeastlyJimmy]

I really want to go with 35s, but currently not in the funds, so the 33s I have will work for now. I can not wait to see it with the 33! Thanks for the help!

 

[red EOD veteran]

most certainly... I use oversized tires to help supliment my gearing selections to help maintain my highway speed and to give me the ground clearance that I am going for.

Same here. running 37"s with 4.56 (or 4.88 gears not sure). That and the tires are cheap lol