Save result of a Firebase and Ionic calculation

I'm trying to save a result of a simple sum in Firebase but I can not retrieve the value and insert. Where am I going wrong?

Home.html

<ion-content padding class="content">
  <form [formGroup]='form' class="principal">
    <section class="index">
      <ion-list>
       <ion-item>
          <ion-label floating> Km inicial </ion-label>
          <ion-input formControlName="kminicial" [(ngModel)]="kminicial"></ion-input>
        </ion-item>
        <ion-item>
          <ion-label floating>Km Final</ion-label>
          <ion-input formControlName="kmfinal" [(ngModel)]="kmfinal"></ion-input>
        </ion-item>
      </ion-list>
    </section>
  </form>
  <section class="botaoCalc">
    <button ion-button round outline (click)="calcular()"> Calcular </button>
   </section>
</ion-content>

Home.ts

@Component({
  selector: 'page-consumo',
  templateUrl: 'consumo.html',
})
export class ConsumoPage {

  title: string;
  form: FormGroup;
  resultkm: number;

  constructor(
    public navCtrl: NavController,
    public toastCtrl: ToastController,
    public alertCtrl: AlertController,
    public navParams: NavParams,
    private formBuilder: FormBuilder,
    public provider: Provider) {
    this.consumo = this.navParams.data.consumo || {};
    this.setupPageTitle();
    this.createForm();
    this.consumos = this.provider.getAll();
  }

  private setupPageTitle() {
    this.title = this.navParams.data.consumo ? 'Alterando Consumo' : 'Novo Consumo';

  }

  createForm() {
    this.form = this.formBuilder.group({
      key: [this.consumo.key],
      kminicial: [this.consumo.kminicial],
      kmfinal: [this.consumo.kmfinal],
      resultkm: [this.resultkm]
    });
    console.log('entrou no create form');
  }

  calcular() {

     this.resultkm  = this.consumo.kmfinal - this.consumo.kminicial;   
    console.log(this.consumo.result)
    this.provider.save(this.form.value)
      .then(() => {

        this.alertCtrl.create({
          title: 'Resultado',
          message:
            'Abastecimento: ' + this.consumo.dataAbs +
            '<br>Re-abastecimento: ' + this.consumo.dataReAbs +
            '<br>Total km rodados: ' + this.resultkm,
          buttons: ['OK'],
        }).present();

      })
      .catch(() => {
        this.toastCtrl.create({ message: 'Ocorreu um erro', duration: 2000 }).present();
        this.navCtrl.pop();
      })
  }
}

}

Provider

@Injectable()
export class Provider {

  private PATH = 'consumo/'

  constructor(private db: AngularFireDatabase) {
    console.log('Hello ** consumo **  Provider');

  }


  save(consumo: any) {
    return new Promise((resolve, reject) => {
      if (consumo.key) {
        this.db.list(this.PATH)
          .update(consumo.key, {
            kminicial: consumo.kminicial,
            kmfinal: consumo.kmfinal,
            resultkm: consumo.result,
          })
          .then(() => resolve())
          .catch((e) => reject(e));
      } else {
        this.db.list(this.PATH)
          .push({
            kminicial: consumo.kminicial,
            kmfinal: consumo.kmfinal,
            resultkm: consumo.result,
          })
          .then((result: any) => resolve(result.key))
      }
    })
  }

 remove(key: string){
   return this.db.list(this.PATH).remove(key);
 }

 getAll() {
  return this.db.list(this.PATH, ref => ref.orderByChild('dataAbs'))
    .snapshotChanges()
    .map(changes => {
      return changes.map(c => ({ key: c.payload.key, ...c.payload.val() }));
    })
}

getOne(key: string) {
  return this.db.object(this.PATH + key)
    .snapshotChanges()
    .map(c => {
      return { key: c.key, ...c.payload.val() }
    })
}

}